july 2-4, 2018 - · 2018. 6. 27. · taran driver imperial college london [email protected]...

Harvard-Smithsonian Center for AstrophysicsITAMP

MS-14, B-32660 Garden Street

Cambridge, MA 02138 USA

Joint ITAMP/UCL workshop:

Organizers:

Agapi Emmanouilidou (UCL)Paul Corkum (University of Ottawa),Matthias Kling (Ludwig-Maximilian University,Munich & MPI for Quantum Optics)Hossein Sadeghpour (ITAMP, Harvard-SmithsonianCenter for Astrophysics)Jonathan Tennyson (UCL)

University College London

https://www.cfa.harvard.edu/itamphttps://eventbooking.stfc.ac.uk/news-events/afels-2018

Abstracts, Program, Participants

ITAMP ia funded by the National Science Foundation

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ITAMP

Attosecond and Free ElectronLaser Science

July 2-4, 2018

Roberts BuildingTorrington Pl, Greater London WC1E 6BT

Lecture Theatre: G06 Sir Ambrose Fleming LT

INDEX

AbstractsKasra Amini .............................................................................43Andre Bandrauk........................................................................12Henry Banks..............................................................................13Dieter Bauer..............................................................................14Boris Bergues.............................................................................15Francesca Calegari......................................................................18Dimitrios Charalambidis.............................................................20Paul Corkum..............................................................................22Reinhard Dörner .......................................................................23Dave Dunning............................................................................24Brett Esry...................................................................................25Eleftherios Goulielmakis..............................................................26Hugo van der Hart......................................................................27Georgios Petros Katsoulis.............................................................29Margarita Khokhlova...................................................................30François Légaré...........................................................................32Eva Lindroth...............................................................................34Jon Marangos..............................................................................35Andrew Maxwell.........................................................................36Nimrod Moiseyev........................................................................38Linda Reichl...............................................................................39Jan Michael Rost.........................................................................40.Arvinder Sandhu.........................................................................41Anthony F. Starace......................................................................45Andre Staudte.............................................................................47Richard Taieb.............................................................................49John Tisch..................................................................................51John Travers...............................................................................52David Villeneuve.........................................................................54Linda Young...............................................................................55Amelle Zair.................................................................................57

Synopsis ..............................................................................1Participants .........................................................................2Program ..............................................................................7

65

Notes

Synops is

Ultra-Short and Intense Laser pulses offer the means to observe, control and probe multi-electron effects during ionization and break-up of strongly-driven atoms and molecules. Moreover, Free Electron Laser facilities is a route to delivering XUV and X-ray pulses with intensities orders of magnitude larger than those provided by conventional synchrotron radiation sources.

Ultra-fast and intense X-ray pulses open-up new horizons for probing and controlling the attosecond motion of inner-shell electrons in multi-photon multi-ionization processes.

This conference aims to bring together theorists from the FEL and Attosecond-Strong Field Science communities to discuss and present recent advances in theoretical techniques developed to tackle multi-electron effects in ionization of atoms and molecules.

Another goal of this meeting is to draw together theorists and experimentalists in order to identify the most interesting challenges that both communities will face in the future.

Organizers:

Agapi Emmanouilidou (UCL)Paul Corkum (University of Ottawa)Matthias Kling (Ludwig-Maximiians University Munich and MPI for Quantum Optics)Hossein Sadeghpour (ITAMP, Harvard)Jonathan Tennyson (UCL).

164

Notes

Par t i c ipan ts

Kasra AminiInstitute of Photonics Sciences [email protected]

Timur AvniImperial College [email protected]

Ruth AyersImperial College [email protected]

Ya BaiShanghai Institute of Optics and Fine Mechanics Chinese Academy of [email protected]

Andre BandraukUniversity de [email protected] Henry BanksUniversity College [email protected]

Jonathan BarnardImperial College [email protected] Dieter BauerUniversity of [email protected]

Federico BelliHeriot-Watt [email protected]

Jakub BendaThe Open [email protected]

Boris BerguesMPQ/LMU [email protected]

Diego I. R. [email protected]

Christian BrahmsHeriot-Watt [email protected] Francesca [email protected]

Lawrence CampbellUniversity of [email protected]

Dimitrios CharalambidisUniversity of Crete FORTH & [email protected]

Heloise ChometUniversity College [email protected] Paul CorkumUniversity of [email protected]

Jan Marcus DahlstromLund [email protected]

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Notes

Par t i c ipan ts

Simone Di MitriElettra Sincrotrone [email protected] Reinhard DörnerUniversity [email protected] Taran DriverImperial College [email protected] Helena DrueekeUniversity of [email protected] Dave DunningSTFC Daresbury [email protected]

Agapi EmmanouilidouUCL [email protected]

Brett EsryJ. R. Macdonald LabKansas State [email protected]

Carla Figueira de Morisson FariaUniversity College [email protected]

Douglas GarrattImperial College [email protected]

Iulia GeorgescuNature [email protected] Jimena GorfinkielThe Open [email protected] Vitaliy GoryashkoUppsala [email protected]

Eleftherios GoulielmakisMPI of Quantum [email protected] Daniel GreeningImperial College [email protected]

Teodora GrigorovaHeriot-Watt [email protected] Antonis HadjipitasUniversity College [email protected] Kathryn HamiltonQueens University [email protected]

Hugo van der HartQueens University [email protected]

Sebastian JaroschImperial College [email protected]

362

Notes

Par t i c ipan ts

Uzi KaldorTel Aviv [email protected]

Maria KanellopoulouUniversity College [email protected]

Georgios Petros KatsoulisUniversity College [email protected]

Ursula KellerETH [email protected]

Anatoli KheifetsAustralian National [email protected]

Margarita KhokhlovaImperial College [email protected]

Matthias KlingDept. of Physics / LMU [email protected]

Premysl KolorencCharles University Faculty of Mathematics and [email protected]

Dietrich KrebsCUI / [email protected]

François Légaré[email protected]

Athanasios LekosiotisHeriot Watt [email protected]

Eva LindrothStockholm [email protected]

Alan MakUppsala University [email protected]

Jon MarangosImperial [email protected]

Gilad MarcusHebrew [email protected]

Zdenek MasinMax Born [email protected]

Andrew MaxwellUniversity College [email protected]

Brian McNeilUniversity of [email protected]

Jiawei MiUniversity of [email protected]

Nimrod [email protected]

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Notes

Par t i c ipan ts

Jenny MorganUniversity of [email protected] Florian OppermannLeibniz Universitat Hannover Institute for Theoretical [email protected]

Linda ReichlUniversity of Texas at [email protected]

Jan Michael RostMax Planck Institute for the hysics of Complex [email protected]

Marco RubertiImperial College [email protected] Hossein SadeghpourITAMP [email protected]

Peter SalenUppsala [email protected]

Arvinder SandhuUniversity of [email protected] Armin ScrinziLMU [email protected]

Anthony F. StaraceUniversity of [email protected]

Andre StaudteNational Research Council of [email protected]

Richard TaiebSorbonne Universite/[email protected]

Jonathan TennysonUniversity College [email protected]

John TischImperial College [email protected] John TraversHeriot-Watt [email protected]

Vasily TulskyUniversity of [email protected]

Arnau VilaUniversity College [email protected]

David VilleneuveNational Research [email protected]

Muxue WangPeking [email protected]

560

Notes

Par t i c ipan ts

Bruce WeaverImperial College [email protected]

Benjamin WillenbergETH Zurich Ultrafast Laser [email protected]

Rachel WonNature [email protected]

Jack WraggQueen’s University [email protected]

Linda YoungArgonne National [email protected]

Amelle ZairKing’s College [email protected]

6 59

Notes

Program

Joint ITAMP/UCL workshop: Attosecond and Free Electron Laser Science

July 2-4, 2018University College London

Torrington Place, WC1E 6BT - G06 Sir Ambrose Fleming LT

Monday, July 2, 2018 8:45- 9:00 am Agapi Emmanouilidou Welcome/Introductory RemarksSession IChair: Matthias Kling

9:00 -9:35 am Eva Lindroth “Many-electron eects on attosecond time delays”

9:35 -10:10 am Ursula Keller “Linear momentum transfer in multiphoton strong-field ionization with subcycle time resolutions”

10:10 - 10:45 am Richard Taieb “Photoionization dynamics: Transition and scattering delays”

10:45 - 11:15 am Coffee Break

Session IIChair: John Tisch

11:15 - 11:50 am David Villeneuve “High Harmonics to Photoionize Molecules”

11:50 - 12:25 am Armin Scrinzi “Correlation in attosecond dynamics”

12:25 - 2:00 pm Lunch Break

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Notes

Program

Session IIIChair: Paul Corkum

2:00 -2:35 pm Andre Staudte “Ultrafast Dissociation of Metastable CO2+ in a Dimer”

2:35 -3:10 pm Andre Bandrauk “Circularly Polarized Molecular High Order Harmonics -Generation and Applications in Attosecond Science”

3:10 - 3:45 pm Reinhardt Dörner “How do electrons exit the tunnel?”

3:45 - 4:15 pm Coffee Break

Session IVChair: Linda Young

4:15 - 4:50 pm Eleftherios Goulielmakis “En Route to a Valence Electron Crystallography”

4:50 - 5:25 pm Fransesca Calegari “Ultrafast stabilization of adenine following ionization by XUV attosecond pulses”

5:25 - 6:00 pm Wine & Food Reception

6:00 - 7:00 pm Jon Marangos Public Talk “Science with X-ray Pulses: New frontiers in exploring the science of matter at the microscopic scale”

Tuesday, July 3, 2018

Session IChair: Agapi Emmanouilidou

9:00 -9:35 am Paul Corkum “High harmonic generation with structured light beams ”

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Long quantum-path in High harmonic generation

Amelle ZairKings College London

The long-quantum path can lead to high harmonics generation under very specific phase matching conditions. Yet, this trajectory was not considered further to reveal ultrafast dynamical process. However, this trajectory does share the same spectral bandwidth has the short trajectory and if properly phase- compensated could lead as well to attosecond pulses.

In the last few years I have concentrated one of my research line on the characterisation and control the long-quantum path which I believe is a good candidate for attosecond pump-probe experiments. I will present results obtained in this particular direction which comprises the temporal characterisation of the long quantum path, the control of its yield and a route to compensate for its attochirp.

Program

9:35 -10:10 am Georgios Katsoulis “Nucleus-assisted non-sequential double ionization as a gate to anti-correlated two electron escape”

10:10 - 10:45 am François Légaré “High harmonics source for probing ultrafast demagnetization”10:45 - 11:15 am Coffee Break

Session IIChair: Fransesca Calegari

11:15 - 11:50 am Linda Young “Ultrafast x-ray probes of inner- and outer-shell electron dynamics”

11:50 - 12:25 am Dimitris Charalambidis “Multiple, multi-XUV-photon ionization by table top high harmonic generation sources”

12:25 - 1:00 pm Linda Reichl “Arnold Diffusion in Molecules and Lattices Driven by Time-periodic Radiation”


Session IIIChair: Tennyson

2:30 - 3:05 pm (CCPQ session) H. van der Hart 3:05 -3:20 pm (CCPQ session) Henry Banks 3:25 - 3:55 pm (CCPQ session) David Dunning 3:55 - 4:15 pm (CCPQ session) Kasra Amini

4:15 - 4:35 pm (CCPQ session) M. Khokhlova

4:35 - 6:30 pm Posters Beer & Pizza956

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the hydrated electron [6].

[1] A.A. Lutman et al., Phys. Rev. Lett. 110, 134801 (2013).[2] A.A. Lutman et al., Nat. Photon. 10, 745 (2016).[3] A. Picón et al., Nat. Commun. 7:11652 (2016).[4] A. Al Haddad, G. Doumy, C. Bostedt, et al. to be published.[5] L. Young et al., Phys. Rev. Lett. 97, 083601 (2006).[6] E. Gouliemakis et al. Nature 466, 739 (2010).[7] J. Li et al., J. Phys. Chem. Lett. 4, 3698 (2013).

* This work was supported in part by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

7:00 pm Conference Dinner

Wednesday, July 4, 2018 Session IChair: Andre Staudte

9:00 -9:35 am Dieter Bauer “Harmonic generation in linear chains: size-dependence, topology, and robustness”

9:35 -10:10 am Boris Bergues “Two photon interaction of 100-eV attosecond pulses with inner-shell electrons in Xeon”

10:10 - 10:45 am Nimrod Moiseyev “Effect of exceptional point in the spectrum of helium atom in laser fields on its photo induced dynamics”

10:45 - 11:15 am Coffee Break

Session IIChair: Eva Lindroth

11:15 - 11:50 am Antony Starace “Doubly-excited state effects on two-photon double ionization of helium bytime-delayed, oppositely circularly-polarized attosecond pulses” 11:50 - 12:25 am Jon Michael Rost “Adiabatic passage to the continuum”

12:25 - 1:00 pm John Tisch “New attosecond light sources and applications”


Program

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Ultrafast x-ray probes of inner- and outer-shell electron dynamics

Linda Young1,2, S. H. Southworth1, A. Picón1,3, A. Al Haddad1, G. Doumy1, C. Bostedt1,4,5

R. Santra6,7, J.E. Rubensson8, Z.-H. Loh9

1Argonne National Laboratory, 2The University of Chicago, 3Universidad Autónoma de Madrid, 4Paul Scherrer Institute, 5EPFL, 6CFEL/DESY, 7University of Hamburg,

8Uppsala University, 9Nanyang Technological University

X-ray free electron lasers potentially have the pulse duration and intensity to probe electron motion on the intrinsic timescales found in atoms and molecules, as characterized by the Bohr orbital period of ~150 attoseconds. While sub-orbital period dynamics for deep inner-shell electrons may still out of reach using direct time-domain probes, readily available few femtosecond x-ray pulses are well suited to characterize the relaxation pathways of the highly energetic hole state created by x-ray photoionization, i.e. the competition between the atomically localized Inner-shell decay (Auger emission, fluorescence), intramolecular charge dynamics and Coulomb explosion.

For studies of molecular inner-shell dynamics, we have taken advantage of accelerator-based developments at LCLS [1,2] that engineer two femtosecond x-ray pulses with adjustable duration, wavelength and time delay to probe the first steps following an inner-shell photoelectron ejection event [3,4]. Inner-shell dynamics in two prototypical molecules have been studied using the x-ray pump/x-ray probe methodology: XeF2 via recoil-ion spectroscopy, and, CO via photoelectron spectroscopy, demonstrating both the potential power and limitations of these methods. For studies of outer-shell electron dynamics, we use the more standard optical pump/x-ray probe configuration. Here a strong-field optical pump laser pulse generates a distinctive ion signature by impulsively removing an electron from a high-lying orbital to open an isolated resonance characteristic of the valence hole which can then be used to track its subsequent dynamics [5]. We will report on an XFEL-based experiment that addresses the origins of the long-lived electronic coherence in strong-field ionized water previously observed in all-optical experiments that track

Session IIIChair: Hossein Sadeghpour

2:00 - 2:35 pm Brett Esry “Attosecond nuclear time delays in strong-eld photodissociation”

2:35 - 3:10 pm Arvinder Sandhu “Controlling attosecond transient absorption with tunable, non- commensurate light fields”

3:10 - 3:45 pm John Travers “High-brightness vacuum ultraviolet generation through ultrafast soliton self-compression in hollow capillary bres”

3:45 - 4:15 pm Coffee Break

Session IVChair: John Travers

4:15 - 4:50 pm Amelle Zair “Long quantum-path in High harmonic generation”

4:50 - 5:10 pm Andrew Maxwell “Controlling below-threshold nonsequential double ionization via quantum interferences”

5:10 - 5:20 pm Concluding Remarks

Program

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High Harmonics to Photoionize Molecules

David VilleneuveJoint Attosecond Science Laboratory

National Research Council and University of Ottawa, Ottawa ON Canada

Despite the availability of high harmonic sources of extreme ultraviolet radiation in university-scale facilities, HHG has not been commonly used in photoionization experiments. One of the reasons is that molecular photoelectron kinetic energy spectra become congested due to the 3 eV spacing between harmonic orders.

We have instead used the third harmonic of our Ti:Sapphire laser system, at 266 nm, to generate harmonic orders separated by 9 eV. This permits the excited states of the nitrogen cation (X, A, B) to be resolved without overlap.

By using a VMI detector and impulsive molecular alignment, along with a sophisticated mathematical procedure, we are able to determine the partial-wave contributions to the photoelectron angular distributions of nitrogen in the molecular frame.

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Circularly Polarized Molecular High Order Harmonics -Generation and Applications in

Attosecond Science

Andre D Bandrauk,PhD,O.C.,FRSC,FAAAS Faculte des Sciences,Universite de Sherbrooke,Que,Canada

MHOHG,Molecular high order harmonic generation is a highly nonlinear nonperturbative response of molecules to ultrashort(fs) intense(I>1014 W/cm2) laser pulses leading to multiphoton ionisation and laser induced electron recollisions in linear polarisation[1].MHOHG is suppressed with intense single circularly polarized pulses but has been shown in 1995 to be generated with co- or counter-rotating pairs of bichromatic ( w1/w2=n1/n2) circularly polarised pulses [2-3] leading to the generation of circularly polarised attosecond(10-18 s) pulses, the time scale of electron motion in atoms and molecules [4].Parallel computer simulations of TDSE,s,Time Dependent Schroedinger Equations coupled with Maxwell’s equations show that molecules are the ideal systems for circular polarised harmonic and attosecond pulse generation due to lower rotational symmetries than atoms. The TDSE simulations confirm the electron-parent ion recollision mechanism in the presence of bichromatic circular pulses and maximum circular polarised MHOHG efficiency is obtained when the net time dependent electric field of the combined pulses is compatible with molecular symmetry.The resulting circularly polarised attosecond pulses are shown to generate in molecules coherent attosecond quantum electron currents from which one can create intense attosecond magnetic pulses [5] for studying ultrafast magnetism and dynamical symmetry in molecules [6-7]

[1] P B Corkum,Phys Rev Lett 71,1994(1993).[2]T Zuo,A D Bandrauk,J Nonl Opt Phys Mater 04,533(1995).[3]S Long,W Becker,J K McIver,Phys Rev A 52,2262(1995).[4]K J Yuan,A D Bandrauk, Phys Rev Lett 110,023003(2013).[5]K J Yuan,A D Bandrauk,Phys Rev A 92,063401(2015).[6]D Baykusheva,M S Ahsan,N Lin,H J Woerner,Phys Rev Lett 116,123001(2016).[7]A D Bandrauk,F Mauger,K J Yuan, in “Progress in Ultrafast Intense Laser Science”,edit K Yamanouchi,vol XIII (Springer,Tokyo,Berlin,2017),chapt 6.

All of the above results can be realistically scaled to the multi-mJ energy, terrawatt peak power, regime, and plans for such scaling will be discussed.

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Multiple core-hole formation in molecules with an x-ray

free-electron-laser pulse

H. I. B. Banks and A. Emmanouilidou

We developed tools to compute photo-ionization and Auger decay rates foratoms and molecules[1, 2]. Using these rates, we set up rate equations to obtain atomic ion yields and electron spectra for molecular nitrogen. We show that our results agree very well with experiment, see gure 1. In addition, we identify all the energetically allowed molecular pathways that contribute to the formation of each atomic ion yield. This allows us to determine the proportion of the ion yield that was formed via a two-site double-core-hole (TSDCH)[3] or single-site double-core-hole (SSDCH) state. Moreover, we also identify the contribution of triple-core-hole (TCH) states to the nal ion yields.

Figure 1: Atomic ion yields for FEL pulses with (a) a total energy of 0.15 mJ, 77% loss and 4 fs FWHM duration and b) a pulse energy of 0.26 mJ, 84% loss and 7 fs FWHM duration c) a pulse energy of 0.26 mJ, 70% loss, and 80 fs FWHM duration. Our results are compared with the experimental results [4, 5]. We also calculated the ion yields with certain pathways excluded and compared with other theoretical results [5].[1] H. I. B. Banks et al. Phys. Chem. Chem. Phys. 19, 19794 (2017)[2] H. I. B. Banks et al. J. Phys. B 51, 095001 (2018)[3] L. S. Cederbaum et al. J. Chem. Phys. 85, 6513 (1986)[4] M. Hoener et al. Phys. Rev. Lett. 104, 253002 (2010)[5] J. C. Liu et al. J. Phys. B 49, 075602 (2016)

High-brightness vacuum ultraviolet generation through ultrafast soliton self-compression in

hollow capillary fibres

John C. Travers , Teodora F. Grigorova and Federico BelliSchool of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh

EH14 4AS, United Kingdom

The vacuum ultraviolet (100 nm to 180 nm) and deep ultraviolet (180 nm to 300 nm) spectral regions are of central importance to science, in particular for applications in molecular spectroscopy, photoemission techniques (such as ARPES), and coherent control. However, these spectral regions, especially the vacuum ultraviolet (VUV), are relatively poorly served by current light sources, which are either very inecient, and of low brightness, or involve expensive large-scale infrastructure such as synchrotrons; restricting the quantity and quality of science which can be achieved. Ultrafast (few and single-cycle) pulse sources in these regions are even rarer still, often involving, for example, inecient low-order harmonic generation.

Here we demonstrate very high brightness vacuum ultraviolet generation through ultrafast soliton dynamics in gas filled hollow capillary bres. We eciently generate tunable VUV pulses with more than 10μJ energy in a table-top system. The generation process is based on the well known resonant dispersive-wave emission from self-compressed solitons. While well known in the context of optical bres, this process has not previously been demonstrated in hollow capillary bres, and has not been scaled to such high powers.

We also demonstrate soliton-eect pulse self-compression in a hollow capillary bre, a process that can, in principle, eliver single-cycle|and even sub-femtosecond|pulses without any need for ultra-broadband chiped mirrors or other dispersion-compensating optics. In fact such pulse compression is an inherent part of the VUV generation process, so we can already infer their existence from the experimental VUV results presented, along with rigorous full-eld and fully spatially resolved propagation simulations. So far we have directly measured compression of 30 fs pulses down to 8 fs, with measurement of further compressed pulses (down to 1 fs) limited by our current characterization system.

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Harmonic generation in linear chains: size-dependence, topology, and robustness

Dieter Bauer,1 Helena Drüeke,1 Kenneth K. Hansen,2

Lars B. Madsen2

1Institute of Physics, University of Rostock, 18051 Rostock, Germany 2Department of Physics and Astronomy, Aarhus University, DK-8000, Denmark

Strong-field physics effects in laser-atom interaction, such as energetic electrons in photoelectron spectra, high-harmonic generation or non-sequential multiple ionisation can be largely understood in terms of classical electron orbits revisiting the ion. This paradigm has been recently applied to non-destructive laser-solid interaction successfully as well, opening up a new era of strong-field physics with (relatively) weak lasers and exciting potential applications in the field of ultra-fast, light-driven electronics and excitonics. The qualitatively new aspect in solids clearly is the band structure. Electronic motion in the conduction band replaces the electron motion in the continuum with simple dispersion relation p²/2m in the atomic case. Further, the usually adopted single-active electron (or hole) approach appears to be less adequate in the solid case.

We introduce a one-dimensional model of a solid slab interacting with a laser field and investigate the time-dependent, non-perturbative dynamics using time-dependent density functional theory in local spin density approximation [1]. Harmonic generation in the two well-known topological phases of dimerised linear chains is analysed. For harmonic photon energies smaller than the band gap, the harmonic yield is found to differ by up to 14 orders of magnitude for the two topological phases. This giant topological effect is explained by the degree of destructive interference in the harmonic emission of all valence-band (and edge-state) electrons, which strongly depends on whether or not topological edge states are present [2]. The robustness of this topological effect against disorder is demonstrated as well as its dependence on the length of the linear chain [3].

[1] Kenneth K. Hansen, Tobias Deffge, Dieter Bauer, Phys. Rev. A 96, 053418 (2017).[2] Dieter Bauer, Kenneth K. Hansen, Phys. Rev. Lett. 120, 177401 (2018).[3] Kenneth K. Hansen, Dieter Bauer, Lars Bojer Madsen, Phys. Rev. A 97, 043424 (2018)

New attosecond light sources and applications

John Tisch1, Dane Austin1, Thomas Barillot1,2, Davide Fabris1, Daniel Greening1, Allan Johnson1,3, Paloma Matia-Hernando1,

William Okell1, Daniel Walke1, Bruce Weaver1, Tobias Witting1,4,Jon Marangos1

1 Blackett Laboratory, Imperial College London, SW7 2AZ, United Kingdom2 EPFL SB ISIC LSU, CH-1015 Lausanne, Switzerland

3 ICFO, Av. Carl Friedrich Gauss, 308860 Castelldefels (Barcelona), Spain4 Max-Born-Institute, Max-Born-Straße 2 A, 12489 Berlin, Germany

The femtosecond barrier was broken in 2001 when the first isolated, attosecond-duration (1 as =10-18s) light pulses were generated via the process of high harmonic generation (HHG) in gases. The current world record stands at 43as (within a factor of two of the atomic unit of time of 24 as). Attosecond light pulses provide scientists with the shortest controllable probes currently available. Such pulses, used as exquisitely sharp temporal scalpels, are allowing previously immeasurably fast electron dynamics in matter to be tracked and, potentially, even controlled at a fundamental level.

A growing number of groups around the world have established attosecond measurement capabilities in their laboratories, and are employing these powerful new tools to conduct innovative experiments in atoms, molecules and condensed phase matter

This talk will describe recent work at Imperial College London to develop new sources for ultrafast and attosecond science, including synchronised attosecond pulses at two different photon energies; attosecond pulse generation into the water window; and enhanced HHG driven by sculpted waveforms formed by multi-colour field synthesis. Some applications of these attosecond light sources will be discussed, including two-photon experiments in He, and attosecond streaking measurements of photoelectrons from surfaces.

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B. Bergues1,2,*, D. E. Rivas1, 2, M. Weidman1, A. A. Muschet1,3, W. Helml4, A. Guggenmos1,2, V. Pervak1 2, U. Kleineberg1,2, G. Marcus1,5,

R. Kienberger1,4, D. Charalambidis6, P. Tzallas6, H. Schröder1, F. Krausz1,2, and L. Veisz1,3

1Max-Planck-Institut für Quantenoptik, Hans-Kopfermann Strasse 1, 85748, Garching, Germany.

2Physics Department, Ludwig-Maximilians-Universität München, Am Couloumbwall 1, 85748, Garching, Germany.

3Department of Physics, Umeå University, SE-901 87 Umeå, Sweden.4Physics Department, Technische Universität München, James-Frank-Str. 1, 85748,

Garching, Germany.5Department of Applied Physics, the Benin School of Engineering and Computer Science,

The Hebrew University of Jerusalem, Jerusalem, 91904, Israel..6Foundation for Research and Technology-Hellas, Institute of Electronic Structure and

Laser, PO Box 1527, GR-711 10 Heraklion, Crete, Greece.*[email protected]

Two-Photon Interaction of 100-eV Attosecond Pulses with Inner-Shell Electrons in Xenon

Time resolving electronic motion in inner atomic shells requires three essential ingredients. The first one is attosecond temporal resolution, needed to resolve the ultrafast electron dynamics. The second one is a photon energy in the upper end of the Extreme Ultraviolet (XUV) spectral range, needed for inner-shell excitation. The third one is a sufficiently high photon density facilitating a measurable interaction of the electrons with two photons. While the first photon initiates the dynamics, the second photon probes the state of the atom after some controlled attosecond delay. Time resolved studies of attosecond inner shell dynamics performed so far have been relying on XUV-pump / IR-probe schemes using an attosecond XUV pulse to trigger the dynamics and an infrared (IR) femtosecond pulse to probe it. One of the main limitations of such a scheme is that the constant presence of the strong IR-field significantly disturbs the dynamics under study. This is why the realization of attosecond XUV-pump / attosecond XUV-probe experiments is one of the long-standing goals of attosecond physics.

Despite the efforts undertaken to increase the intensity of attosecond light sources, the low efficiency of attosecond pulse generation at higher photon

group delay associated with that phase can now be interpreted as a “transition delay”, and how it can be accessed experimentally in a straightforward reinterpretation of the rabbit interferometric technique, initially designed for the characterization of coherent xuv pulses [5,6].

[1] S. Haessler et al, Phys. Rev. A 80 011404 (2009); M. Schultze et al Science 328 1658 (2010) ; K. Klunder et al Phys. Rev. Lett. 106 143002 (2011).[2] J. Caillat et al Phys. Rev. Lett. 106 093002 (2011); R. Gaillac et al Phys. Rev. A 93, 013410 (2016); M. Vacher et al, J. of Optics 19 12401 (2017).[3] H. Park et al J. Chem. Phys 104 4554 (1996). [4] E. P. Wigner Phys. Rev. 98 145 (1955).[5] V. Gruson et al., Science 354, 734 (2016).[6] S. Beaulieu at al., Science 258, 1288 (2017).

16 49


energy has constituted the major obstacle towards this goal. Although nonlinear light-matter interactions with attosecond XUV pulses have been reported in a few pioneering studies [1−5], the photon energies used in these experiments have not exceeded 50 eV, hampering nonlinear excitation of inner electronic shells. Here, we demonstrate that all essential ingredient for attosecond XUV-pump XUV-probe studies on core electrons are now available, and report the first observation of a nonlinear interaction between an attosecond pulse and electrons from an inner atomic shell.

In our experiment, we focused intense attosecond XUV-pulses with a photon energy in the 100 eV region into a xenon gas target, from which we generated different ionic charge states via XUV photoionization. The highest xenon charge states, Xe4+ and Xe5+, where found to result from a two-photon absorption process. We used an ion microscopy technique [6] to record the spatial distribution of the different charge states in the XUV focus. The measured ion distributions, resulting from either linear or nonlinear ionization processes, allow us to characterize the XUV focus and to determine the partial two-photon ionization cross sections for the production of Xe4+ and Xe5+ at photon energies around 93 eV and 115 eV. We observe significant deviations with respect to measurements with femtosecond pulses, which indicates the existence of much faster ionization pathways.

These achievements were made possible by the development of a novel high-harmonic XUV source driven by a 10-Hz multi-TW laser system, the `Light Wave Synthesizer 20’ (LWS-20) [7]. The LWS-20 delivers sub-two-cycle 75 mJ pulses with a central wavelength around 740 nm, from which up to 40 mJ are used on target. This high pulse energy allows us to maintain a peak intensity of 1015 W/cm2 in a focus with a full width at half maximum of about 360 micron. The XUV pulses, which are generated in a neon target gas target, are focused with a multilayer XUV mirror (focal length = 125 mm) into the ion microscope after an expansion length of 14.5 m. We measure XUV energies up to 40 nJ within a bandwidth ranging from 65 eV to the cutoff at 130 eV. This represents a two-orders-of-magnitude upscaling of the XUV pulse energy with respect to comparable sources pumped with mJ-scale few-cycle driving pulses [8].

[1] P. Tzallas et al., Journal of Modern Optics 52, 321–340 (2005).[2] K. Midorikawa et al., Prog. Quant. Electron. 32, 42 (2008).[3] Y. Nabekawa et al., Nat. Commun. 7, 12835 (2016).[4] B. Manschwetus et al., Phys. Rev. A 93, 061402(R) (2016).

Photoionization dynamics: Transition andscattering delays

R. Taïeba UPMC, CNRS, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement,

Paris, France

Resolving electron motion in atoms and molecules on its natural attosecond (as) scale is way beyond the temporal resolution of available detection devices. The techniques developed to achieve such attosecond resolution thus rely on interferometric setups [1]. In fact, the reported times are actually group delays derived from phase measurements, involving coherent photoemission processes. Therefore, the analysis of the experimental data and the related theoretical development ask for rigorous and unambiguous definitions and interpretations of these phases, and of the inferred group delays [2]. It is now accepted that a “scattering delay” [4] affects the dynamics of any photoemission process. However, the simplicity of the underlying physics is not fully recognized yet. Formally, such delays are imprinted in the phase shifts of the photoelectron wave-functions, which are commonly expressed on the basis of incoming waves. In this framework, the “scattering phase” associated to photoemission appears as the argument of the transition amplitude, thus obscuring the significance of the delay - which may be misinterpreted for example as a transition duration. Here, we will present the benefits of working with the continuum wave-functions selected by the transitions (scwf) [2,3], which (i) carry all the information related to the continuum reached by photoabsorption, (ii) are defined independently of the arbitrary basis one chooses to work with and (iii) are real valued for single-photon transitions. They provide a clear-cut interpretation of the scattering delays. In higher order processes, the scwf comes with an additional complex phase, as soon as the transition is resonant. We will show that the

1748


[5] T. R. Barillot, et al., Chem. Phys. Lett. 683, 38 (2017).[6] M. Schultze et al., New J. Phys. 13, 033001 (2011).[7] D. E. Rivas, et al., Sci. Rep. 7, 5224 (2017).[8] M. Ossiander et al., Nat. Phys. 13, 280 (2016).

mixing of orbitals.

When a polyatomic molecule or cluster is mul-tiply ionized, the system can dissociate in two different ways. The chemical bonds can break simultaneously and all fragments repel each other under Coulomb interaction or repulsive potential energy curves. This is a concerted breakup chan-nel. The system can also breakup sequentially, where one bond breaks first and fragments prop-agate to a distance that is long enough for the interaction between them to be neglected before the next bond breaks. Sequential and concerted breakup channels are observed for many differ-ent systems, like CO2 [5], ArCO [6], N2O [7], and CS2[8]. However, the causes for the two breakup channels are unknown. We focus on a simple van der Waals system – the carbon monoxide dimer. We observe both concerted and sequential chan-nel in the dissociation of (CO)2

3+. By comparing the dissociation channels of the dimer and the CO monomer, we study the mechanism of the con-certed breakup channel [9].

[1] L. Cederbaum, J. Zobeley, and F. Tarantelli, Phys. Rev.Lett. 79, 4778 (1997).[2] U. Frühling, F. Trinter, F. Karimi, J. Williams, and T. Jahnke, J. Electron Spectrosc. Relat. Phenom. 204, 237 (2015).[3] T. Jahnke et al., Nat. Phys. 6, 139 (2010).[4] F. Trinter et al., Nature (London) 505, 664 (2014).[5] N. Neumann et al., Phys. Rev. Lett 104, 103201 (2010).[6] X. Gong, et al., Phys. Rev. A 88, 013422 (2013).[7] M. Ueyama, H. Hasegawa, A. Hishikawa, and K. Yamanouchi, J. Chem. Phys 123, 154305 (2005).[8] A. Hishikawa, H. Hasegawa, and K. Yamanouchi, Chem. Phys. Lett. 361, 245 (2002).[9] D. Ding et al., Phys. Rev. Lett. 118, 153001 (2017).

4718


Ultrafast stabilization of adenine following ionization by XUV attosecond pulses

E. P. Månsson1, M. Galli2,3, V. Wanie3,4, S. Latini5, U. De Giovannini5, M. C. Castrovilli6, F. Frassetto7, L. Poletto7, J. Greenwood8, F. Légaré4, M. Nisoli2,3,

A. Rubio5 and F. Calegari1,3

1Center for Free-Electron Laser Science, DESY, Notkestr. 85, 22607 Hamburg, Germany2Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32,

20133 Milano, Italy3Institute for Photonics and Nanotechnologies CNR-IFN, P.za Leonardo da Vinci 32, 20133

Milano, Italy4Institut National de la Recherche Scientifique, 1650 Blvd. Lionel Boulet, J3X1S2, Varennes

(Qc), Canada5MPSD, Luruper Chaussee 149, 22761 Hamburg, Germany

6Inst. for the Structure of Matter CNR-ISM, Area Ricerca di Roma1, Monterotondo, Italy7Institute for Photonics and Nanotechnologies CNR-IFN, Via Trasea 7,

35131 Padova, Italy8Centre for Plasma Physics, School of Maths and Physics, Queen’s University Belfast

BT7 1NN, UKEmail: [email protected]

Ionizing radiation causes mutations and irreparable damage to DNA. However, nucleobases also exhibit relatively high inherent photo-stability. The complexity of these molecules makes it a challenging task toelucidate in detail all the physical mechanisms activated by ionization including fragmentation, internal energy dissipation and electronic correlation effects.

As recently shown, access to the time scale immediately following ionization of a bio-chemicallyrelevant molecule can be gained using an extreme ultraviolet (XUV) attosecond pump in combination with a near-infrared (NIR) few-femtosecond probe [1, 2]. Moreover, attosecond pulses can be produced in the 20–40 eV energy range, which is highly relevant for the biological context. This is indeed the typical energy range of secondary electrons that - by impacting DNA - represent the main source of indirect damage following tissue irradiation [3]. Ionization with 20-40 eV energies is often accompanied by the transition of an outer shell electron to an unoccupied state by a shake-up mechanism. It has been demonstrated that the population of such shake-up states can be probed with attosecond time resolution[4]. So far this investigation has been only limited to atoms and small molecules.

Ultrafast Dissociation of MetastableCO2+ in a Dimer

Xiaoyan Ding*, Marko Haertelt*, Stefan Schlauderer*, Michael S. Schuurman† ‡, Andrei Yu. Naumov*, David M. Villeneuve*,

A.R.W. McKellar†, Paul B. Corkum*, André Staudte* 1

* Joint Attosecond Science Laboratory, National Research Council and University of Ottawa, Ottawa, Ontario, Canada K1A 0R6

† National Research Council, 100 Sussex Dr., Ottawa, Ontario, Canada K1A 0R6‡ Department of Chemistry and Biomolecular Sciences, University of Ottawa, 10

Marie Curie, Ottawa, Ontario, Canada K1N 6N5

Synopsis We use Coulomb explosion imaging to study the fragmentation dynamics of the molecular dimer (CO)2

3+ in a strong laser field. The dimer breaks up sequentially or instantaneously. The different channels can be separated by the momentum of the fragments. Whereas the C+ and O+ fragments from the sequential breakup channel show similar features as fragments from the breakup of the doubly charged monomer, the instantaneous breakup of the dimer has no equivalent in the monomer. Albeit very weak, van-der-Waals, or intermo-lecular electrostatic interactions play a fundamen-tal role in chemistry and molecular biology. Temporary and permanent dipoles create a meV potential energy landscape, which weakly guides molecules towards, for example, catalytic centers, or, an assembly into a larger complex. In addi-tion, electrostatic interactions as they occur in van-der-Waals complexes can have a critical in-fluence on the decay of electronic excitation in one of the cluster constituents. A prominent ex-ample is the interatomic (or intermolecular) Coulombic decay [1-4]. In ICD electronic excitation is extremely efficiently transferred within a clus-ter to allow for a fast relaxation.

Here, we show that the interactions between molecules in a cluster can also lead to efficient de-excitation through a mechanism that does not require the transfer of electronic excitation. We use the example of metastable CO2+ in a dimer to show, that the mere presence of another charge opens up new relaxation pathways through the field-induced

46 19


Here we present a time-resolved study of photo-fragmentation of the nucleobase adenine, one of the key building blocks of DNA, following ionization by an XUV attosecond pulse. Our most intriguingobservation is that a stable dication of the parent molecule can be produced if (and only if) the probing NIR pulse is very briefly delayed from the XUV pulse. Our experimental and theoretical findings indicate that this short delay corresponds to the time required for a shake-up process to occur.

In our experiment, ionization of adenine was initiated by isolated sub-300-as pulses with photon energies between 17 eV and 40 eV, and subsequently probed by sub-4-fs, waveform-controlled NIRpulses. Adenine was evaporated and carried to the laser interaction region by a buffer gas. The produced ions were then collected using a VMI spectrometer operated in the ion time-of-flight mode as a function of the XUV-pump NIR-probe delay. The time dependent yield of many fragments displays a clear step-like increase, in some cases followed by a rapid decay. The most interesting observation is that the formation of the parent molecule dication (67.5 u/e) is delayed compared to the other cationic fragments by about 2.3 fs. It is worth noting that no stable dication of the parent is observed in the XUV-only signal, or in the combined XUV+NIR signal when the XUV energy is below 17 eV.

Theoretical calculations based on the Time-Dependent Density Functional Theory (TDDFT) indicate that direct single (or double) ionization of adenine by XUV pulses inevitably leads to dissociation. This implies that the sub-sequent creation of a stable parent dication requires relaxation of the system. Here we propose that the energy dissipation occurs via ionization of a shake-up state by the NIR probe pulse. Using the GW quasiparticle approximation and the time dependent perturbation theory based on DFT molecular orbitals, we calculated the characteristic time of shake-up and found a good agreement with the measured time delay of 2.3 fs.

[1] F. Calegari, D. Ayuso, A. Trabattoni, L. Belshaw, S. De Camillis, S. Anumula, F. Frassetto, L. Poletto, A. Palacios, P. Decleva, J. B. Greenwood, F. Martín and M. Nisoli, “Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses”, Science 346, 336 (2014)[2] E. P. Månsson, S. De Camillis, M. C. Castrovilli, M. Galli, M. Nisoli, F. Calegari and J. B. Greenwood, “Ultrafast Dynamics in the DNA Building Blocks Thymidine and Thymine Initiated By Ionizing Radiation”, Phys. Chem. Chem. Phys. 19, 19815 (2017)[3] C. Caleman et al, Europhys. Lett. 85, 18005 (2009)[4] M. Uiberacker et al., Nature 446, 627 (2007)

the two ionized electrons in the polarization plane (cf. Fig. 1). Thedoubly-excited states also completely change the structure of fixedenergy,two-electron angular distributions. Moreover, both the fixedenergyand energy-integrated angular distributions, as well as the two electronenergy distributions, exhibit a periodicity with time delay τ between the two attosecond pulses of about 69 as, i.e., the beat period between the (2s2p) 1Po doubly-excited state and the He ground state. Using PT, we derive an expression for an angle-integrated energy distribution that is sensitive to the slower beat period ~1.2 fs between different doubly-excited states as well as to the long timescale ~17 fs of autoionization lifetimes. However, with our current computer codes we are only able to study numerically the time-dependent phenomena occurring on an attosecond time scale.

[1] J.M. Ngoko Djiokap and A.F. Starace, J. Optics 19, 124003 (2017) (DOI:https://doi.org/10.1088/2040-8986/aa8fc0).

This work is supported in part by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Grant No. DE-FG02-96ER14646. Our computations were conducted using the Sandhills and Crane computing facilities of the Holland Computing Center at the University of Nebraska, and as well as the Stampede supercomputer (TACC) under U.S. National Science Foundation Grant No. PHY-120003.

20 45


Multiple, multi-XUV-photon ionization by table top high harmonic generation sources

P. Tzallas1,2, I. Makos1,5, I. Orfanos1,5, A. Nayak1,2, M. Dumergue2, S. Kühn2, E. Skantzakis1, B. Bodi3, K. Varju2,4, C. Kalpouzos1 and D.

Charalambidis1,2,5

1Foundation for Research and Technology - Hellas, Institute of Electronic Structure & Laser, PO Box 1527, GR71110 Heraklion (Crete), Greece

2ELI-ALPS, ELI-Hu Kft., Dugonics ter 13, H-6720 Szeged Hungary3MTA "Lendület" Ultrafast Nanooptics Group, Wigner Research Center for Physics,

1121 Budapest, Hungary4Department of Optics and Quantum Electronics, University of Szeged, Szeged Hungary

5Department of Physics, Univ. of Crete, PO Box 2208GR71003 Heraklion (Crete), Greece

Systematic developments of high peak power high harmonic generation sources [1], [2] emitting attosecond pulse trains and/or isolated attosecond pulses has led in the last 15 years to the observation of several two-XUV-processes [3], exploited in attosecond pulse metrology [4-6] and he study of ultrafast dynamics via XUV-pump-XUV-probe measurements [7-9]. Very recently FO.R.T.H. has developed a new attosecond beamline utilizing loose focusing geometry (f=9m) and dual pulsed gas jets, reaching 20-gigawatt operation in the XUV spectral region (around ~50nm). Focusing these XUV pulse trains tightly in an Argon gas jet we were able to observe Argon ions up to Ar4+, indicative for the presence of highly non-linear interaction processes. XUV intensity dependence measurements of the ion yield of the different Argon charge states show appearance of the next charge state when the previous one is saturated and slopes in log-log scale compatible with the order of non-linearity of the processes involved.The present results provide evidence of the exciting prospect of the under implementation ELI-ALPS research infrastructure. The so called “compact HHG” beamline of ELI-ALPS [10] is designed as an advanced version of FORTH’s beamline with the additional unique feature of operation at 1kHz repetition rate. The pulse energy levels of the present work at 1kHz repetition rate are challenging coincidence experiments in non-linear XUV laser-matter interactions targeting detailed studies of ultrafast dynamics.

Doubly-Excited State Effects on Two-Photon Double Ionization of Helium by Time-Delayed, Oppositely

Circularly-Polarized Attosecond Pulses

J. M. Ngoko Djiokap and Anthony F. StaraceDepartment of Physics and Astronomy, University of Nebraska

Lincoln, NE 68588-0299, USA

We study two-photon double ionization (TPDI) of He by a pair of timedelayed (nonoverlapping), oppositely circularly-polarized attosecondpulses whose carrier frequencies are resonant with 1Po doubly-excitedstates [1]. All of our TPDI results are obtained by numerical solution ofthe six-dimensional, two-electron, time-dependent Schrödinger equationfor the case of circularly-polarized attosecond pulses, and they are alsoanalyzed using perturbation theory (PT). As compared with the corresponding nonresonant TPDI process, we find that the doubly-excitedstates change the character of vortex patterns in the two-electronmomentum distributions for the case of back-to-back (BTB) detection of

Figure 1 Two-electron momentum distributions for TPDI of He produced by a pair of nonoverlapping, right-left circularly polarized, six-cycle, attosecond pulses. (a) The pulses have the resonant carrier frequency ω = 60 eV, duration 413.6 as, and time delay τ = 413.6 as. (b) The pulses have the nonresonant carrier frequency ω = 45 eV, duration 551.4 as, and time delay τ = 551.4 as.

d

und state. ntegrated energy

2 fs between 17 fs of

computer codes we !"#$%&' ( Two-electron momentum

44 21


1. S. Chatziathanasiou, et al., Photonics 4, 26 (2017).2. C. M. Heyl et al., JPB 50, 013001 (2017)3. N. A. Papadogiannis et al. Phys. Rev. Let. 90, 133902 (2003)4. P. Tzallas et al. Nature 426, 267 (2003).5. Y. Nabekawa et. Al. Phys. Rev. Let. 97, 153904 (2006)6. Y. Nabekawa et. al. PRL. 96, 083901 (2006)7. P. Tzallas at al. Nat. Phys. 7, 781–784, (2011)8. P. A. Carpeggiani et al. Phys. Rev. A89, 023420 (2014)9. Y. Nabekawa et all. Nature Communications (2016) DOI: 10.1038/ncomms1283510. S. Kuehn., et al., JPB, 50, 132002 (2017).

In order to detect all ions produced in the Coulomb explosion, we coupled the Pixel Imaging Mass Spectrometry (PImMS) camera5-7, a time-stamping device that records the hit position and arrival time for up to four ions per pixel, with velocity-map ion-imaging. To enhance the effects of momentum correlation in our ion images and to help us probe changes in molecular structure, the molecules were constrained in one- or three-dimensional space8

through adiabatic alignment pulses from a nanosecond Nd:YAG laser.

We compare and contrast the two different fragmentation processes of CH3I and DFIB probed with either 800 nm laser photons or 11.6 nm FEL photons3,4. We then investigate time-dependent effects in UV-FEL pump-probe experiments, in particular comparing and contrasting the onset of charge transfer1,7 in both CH3I and DFIB3. We also utilize the covariance imaging capability of the PImMS camera in a gas-phase structural isomer identification study using Coulomb explosion imaging through a table-top TiSa set-up coupled with covariance analysis10. Here, we identify isomers of DFIB and dihydroxybromobenzene, and we show, for the first time, that two isomers can be identified and distinguished in a mixed molecular beam sample10.

[1] B. Erk et al., Science 345, 288 (2014)[2] R. Boll et al., Faraday Discuss. 171, 57 (2014)[3] K. Amini, E. Savelyev et al., Struct. Dyn. 5, 014301 (2018)[4] K. Amini et al., J. Chem. Phys. 147, 013933 (2017)[5] K. Amini et al., Rev. Sci. Instrum. 86, 103113 (2015)[6] A. Nomerotski et al., J. Instrum. 5, C07007 (2010)[7] J. J. John et al., J. Instrum. 7, C08001 (2012)[8] T. Kierspel et al., J. Phys. B. 48, 204002 (2015)[9] R. Boll et al., Struct. Dyn. 3, 043207 (2016)[10] M. Burt, K. Amini et al., J. Chem. Phys., In Print (2018)

4322


High harmonic generation with structuredlight beams

Paul Corkum, Fanqi Kong and Chunmei ZhangJoint Attosecond Science Laboratory

University of Ottawa and National Research Council of Canada

In the visible and infrared it is possible to transform a Gaussian beam into vortex beams with orbital angular momentum. Such vortex beams are very important for advanced microscopy and for quantum optics. Like spin angular momentum, orbital angular momentum is conserved during high harmonic generation. We confirm conservation of orbital angular momentum and show how it leads to a method for coupling a controlled orbital angular momentum on any harmonic. Our results open a pathway for attosecond science with similarly structured light.

Besides shaping the wavefronts, a Gaussian beam can also be transformed into beams with complex polarization states or so called vector beams. We show that, like Gaussian beams, cylindrical symmetric vector beams can be compressed in hollow core fibers. High power vector beams open a new pathway to creating circular polarized harmonics that we demonstrate experimentally.

Probing Ultrafast Chemical Dynamics by Time-Resolved Ion Imaging at the FLASH FEL

Kasra Aminia, Evgeny Savelyevb, Rebecca Bollb*, Alexandra Lauera, Michael Burta, Lauge Christensenc, Felix Brauβed, Benjamin Erkb,

Cédric Bommeb, Nora Berrahe, Stefan Düstererb, Hauke Höppnerb, Per Johnssonf, Thomas Kierspelb,g, Faruk Krecinicd, Erland Müllerb, Maria Müllerh, Terry Mullinsb,g, Piotr Rudawskif, Nora Schirmelb, Simone Techertb, Jan Thøgersenc, Sven Toleikisb, Rolf Treuschb,

Sebastian Trippelb,g, Anatoli Ulmerh, Joss Wieseb,g, Henrik Stapelfeldtc, Jochen Küpperb,g, Arnaud Rouzéed, Artem Rudenkoi, Mark Brouarda

and Daniel Rollesb,i.

aDepartment of Chemistry, University of Oxford, United Kingdom. bDeutsches Elektronen-Synchrotron (DESY), 22607 Hamburg, Germany.

cAarhus University, 8000 Aarhus C, Denmark.dMax-Born-Institut, 12489 Berlin, Germany.

eUniversity of Connecticut, Storrs, CT 06269, USA.fDepartment of Physics, Lund University, 22100 Lund, Sweden.

gCenter for Free-Electron Laser Science (CFEL), DESY, 22607 Hamburg, Germany.hTechnical University of Berlin, 10623 Berlin, Germany.

iJ.R. Macdonald Laboratory, Kansas State University, Manhattan, KS 66506, USA.

Femtosecond pump-probe experiments provide opportunities to investigate and understand changes in molecular structure and photochemical reaction dynamics in unprecedented detail1,2. Here, we study and compare the UV-induced photodissociation of tightly-bound simple aliphatic molecule,iodomethane (CH3I), with an aromatic molecule, 2,6-difluoroiodobenzene (DFIB), in a pump-probe arrangement using two complementary probe schemes, either an 800 nm (near-IR) Ti:Sa laser or the FLASH free-electron laser (FEL) tuned to 11.6 nm (XUV)3,4. The absorption of multiple near-infrared photons results in strong-field ionization that can occur anywhere in the molecule leading to subsequent Coulomb explosion3,4. The XUV photon on the other hand induces inner-shell ionization and Auger cascade that is initially strongly spatially localized at the iodine atom in the molecule, before the charge is spread to the rest of the molecule and leads to a Coulomb explosion3,4.

42 23


How do electrons exit the tunnel?

S. Eckart, K. Fehre, A. Hartung, J. Rist, D. Trabert, L. Ph. H. SchmidtT. Jahnke, M. S. Schöffler, M. Kunitski and R. Dörner

Goethe-Universität, Institut für Kernphysik Max-von-Laue-Str. 1

60438 Frankfurt, Germany

We will present experiments on strong field ionization of atoms and molecules by one and two colour strong TiSa laser pulses. Questions we will address are:

• Is there a mean offset momentum of the electrons upon exiting the tunnel? • What is the role of the initial state angular momentum in tunnel ionization?• What is the role of chirality in molecular ionization?• What is the role of spin in tunnel ionization?

References:

• S. Eckart, M. Kunitski, M. Richter, A. Hartung, J. Rist, F. Trinter, K. Fehre, N. Schlott, K. Henrichs, L. Ph. H. Schmidt, T. Jahnke, M. Schöffler, K. Liu, I. Barth, J. Kaushal, F. Morales, M. Ivanov, O. Smirnova, and R. Dörner

Ultrafast Preparation and Detection of Ring Currents in Single Atoms Nat. Phys. May (2018)

• D. Trabert, A. Hartung, S. Eckart, F. Trinter, A. Kalinin, M. Schöffler, L. Ph. H. Schmidt, T. Jahnke, M. Kunitski and R. Dörner

Spin-and Angular Momentum in Strong-Field Ionization Phys. Rev. Lett., 120 (2018) 043202

• S. Eckart, M. Richter, M. Kunitski, A. Hartung, J. Rist, K. Henrichs, N. Schlott, H. Kang, T. Bauer, H. Sann, L. Schmidt, M. Schöffler, T. Jahnke, and R. Dörner

Nonsequential Double Ionization by Counterrotating Circularly Polarized Two Color Laser Fields Phys. Rev. Lett., 117 (2016) 133202

• Hartung, F. Morales, M. Kunitski, K. Henrichs, A. Laucke, M. Richter, T. Jahnke, A. Kalinin, M. Schöffler, L. Ph. H. Schmidt, M. Ivanov, O. Smirnov, R. Dörner

Electron spin polarization in strong-field ionization of Xenon atoms Nat. Photonics, 10 (2016) 526

envision that this approach will offer significant advantages in disentangling of the ultrafast dynamics of electrons along the complex manifold of molecular potential energy surfaces.

The work at University of Arizona was supported by the grant DE-SC0018251 funded by the U.S. Department of Energy, Office of Science.

4124


Future potential for attosecond pulses in X-ray Free-electron lasers

David DunningSTFC Daresbury Laboratory

X-ray free-electron lasers have recently been shown to produce pulses with sub-fs duration. With further development they have the potential to deliver high-brightness radiation pulses with duration even shorter than the present leading technique, high harmonic generation (HHG). This would extend attosecond science to probe ultrafast dynamics with even finer resolution. To do so requires breaking below a characteristic FEL timescale of typically a few hundred optical cycles, dictated by the relative slippage of the radiation and electrons during amplification. The concept of mode-locking enables this, with the ‘mode-locked afterburner’ configuration predicted to deliver few-cycle pulses (with theoretical potential to reach pulses as short as ~1 attosecond at hard X-ray). However such techniques would produce a train of closely separated pulses, while an isolated pulse would be preferable for some types of experiment. Building on previous techniques, a new concept has been developed for isolated few-cycle pulse generation and it is presented alongside a comparison with other potential methods.

Controlling attosecond transient absorption with tunable, non-commensurate light fields

Nathan Harkema1, Jens E. Bækhøj2, Chen-Ting Liao1, Mette Gaarde2, Kenneth Schafer2, Arvinder Sandhu1

1Department of Physics, University of Arizona, Tucson, AZ 85721 USA2Department of Physics and Astronomy, Louisiana State University

Baton Rouge, LA 70803 [email protected]

Two features common to most Attosecond Transient Absorption Spectroscopy (ATAS) measurements are the commensurate nature of the XUV and IR wavelengths (the XUV frequencies appearing as harmonics of the driving laser) and the collinear alignment the beams (where any IR-driven dynamics of interest is generally probed against an XUV-only background signal). Theoretical studies of ATAS show that the IR wavelength is a key parameter in ATAS and that tuning the wavelength can reveal a range of absorption and emission features, such as Autler-Townes splitting, delay-dependent XUV transparency, light-induced structures etc. We experimentally combine wavelength control of a range of IR-driven interactions with a collinear, background-free measurement. We implement an ATAS scheme in which an attosecond pulse train, generated by a 780nm pulse, is combined with an IR probe pulse that has been routed through an optical parametric amplifier to obtain synchronized, tunable, long wavelength pulses with wavelengths ranging between 1200 and 1700 nm. By tuning the probe laser we demonstrate exquisite control of the resonant AT doublet around the 1s4p absorption line in helium, and we show how the symmetric doublet transitions to an asymmetric pair of lines associated with the 1s4p state. and a light-induced sideband. The non-commensurate probe laser also allows for the background-free study of the four-wave-mixing in a collinear geometry. We illustrate two different such emission processes with non-trivial delay dependencies, one prompt and the other delayed with respect to maximum overlap between pump and probe lasers. We identify the nonlinear processes underlying the two-IR-photon emission processes by comparing the experimental results to calculations based on solving the time-dependent Schrödinger equation in the single active electron approximation. The tunable, background-free ATAS spectroscopy technique reported here enhances the attosecond science toolkit by providing new control knobs. We

40 25


Attosecond nuclear time delays in strong-field photodissociation

B. D. Esry James R. Macdonald Laboratory, Kansas State University

Without the ready availability of single attosecond pulses with suffcient energy to perform pump-probe experiments, the push to measure electronic dynamics on its natural timescale of attoseconds has enlisted less direct approaches. Some photoionization “time delays”, in particular, have been measured and calculated to be on the attosecond timescale and thus have attracted considerable attention. The ultimate goal of such attosecond-scale measurements is the molecular movie — i.e., making movies of the electronic motion during chemical reactions. It has been universally assumed, however, that any measured attosecond timescales in observables relate exclusively to electronic dynamics, even during a reaction that necessarily includes nuclear motion. I will explore some of the limits of this assumption andhighlight a few specic cases where it fails. I will also explore dierent measurement schemes with an eye towards their analysis. Finally, I will argue that extracting the energy-dependent phases should be favored over the time delays”.

Supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Oce of Science, U.S. Department of Energy.

*In collaboration with G. Armstrong, T. Severt, B. Berry, P. Feizollah, P. Kanaka Raju, B. Jochim, J. McKenna, B. Gaire, M. Zohrabi, F. Anis, D. Ursrey, J.V. Hernandez, K.D. Carnes, and I. Ben-Itzhak.

Adiabatic passage to the continuum

Jan M RostMax Planck Institute for the Physics of Complex Systems, Dresden

Traditionally, control of ionization is achieved in STIRAP with at least two time delayed laser pulses which operate in the NIR.

Here, we will introduce a scenario which allows to enhance or suppress photo ionization in the VUV to XUV regime with a single chirped laser pulse. The only requirement is a two-photon transition (via an intermediate bound state) to the continuum. Opposite chirping leads then two difference in ionization by a factor of 5. Typical laser parameters are about 20 eV photon energy, a one fs pulse length and 1016 W/cm2 intensity.

The mechanism and its sensitivity to the direction of chirp liesin the fact that rapid adiabatic passage creates a superposition of bound states through which the transition amplitudes to the continuum adopt a well defined interference. The strict selection rules of single photon transitions linking gerade to ungerade states limit the participating channels and render the control mechanism efficient.


3926

En Route to a Valence Electron Crystallography

Eleftherios GoulielmakisMax-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1

D-85748 Garching, Germany

X-ray and electron diffraction can probe the atomic and core-electron structure of solids but are insensitive to the spatial distributions of valence electrons. As a result, probably the most essential part of the electronic structure of solids— responsible for their chemical, optical, electronic properties— cannot be directly imaged experimentally. We discuss how extreme ultraviolet radiation emerging by laser driven, coherent electron oscillations in solids can offer direct insight into structure of valence electrons and the direct measurement of the electronic potential with Angstrom resolution.

Arnold Diffusion in Molecules and LatticesDriven by Time-periodic Radiation

L.E. Reichl , Han Hsu, and Y. BoretzThe University of Texas at Austin

The effect of laser radiation on matter has been a topic of growing interest because time periodic forces can be used to control and possiblythermalize matter. Pulses of laser radiation have long been used to control quantum transitions. Carbon nanotubes driven by laser radiation re-emit high harmonic radiation [1], but also break up in the presence of sufficiently strong radiation [2]. There is some evidence that optical doping can induce phase changes in materials [3]. It has been known since Arnold’s classic work [4] that dynamical systems with 2.5 or more degrees of freedom are intrinsically unstable. As a consequence, time-periodically driven systems can experience large excursions in energy [5]. In this talk we show the mechanisms by which time periodic driving of molecular and solid state systems can potentially cause their destabilization.

1. H. Huang, R. Maruyama, K. Noda, H. Kajiura, and K. Kadono, J. Phys. Chem. B 110 7316 (2006).

2. Han Hsu and L.E. Reichl, Phys. Rev. B 72 155413 (2005); Phys. Rev. B 74 115406 (2006)

3. T. Han, F. Zhou, C.D. Maliakas, P.M. Duxbury, S.D. Mahan, M.G. Kanatzudusm and C-Y Ruan, Sci. Adv. 1 e1400173 (2015).

4. V.I. Arnold, Russ. Math. Surveys, 18:6, 85 (1963))5. Y. Boretz and L.E. Reichl, Phys. Rev. E 93 032214 (2016)


Recent progress in time-dependent R-matrixtheory and its applications

Hugo W. van der HartCentre for Theoretical Atomic, Molecular and Optical Physics

School of Mathematics and PhysicsQueen’s University Belfast

Belfast BT7 1NN, Northern Ireland, UKEmail: [email protected]

Over the last several years, we have developed the sector-leading computer code known as RMT (R-matrix with time-dependence) [1] to solve problems of general multielectron atoms and ions in strong laser fields. Recent progress has pushed the capabilities of this code to allow applications to arbitrarily polarized light, molecular targets, and novel attosecond techniques such as XUV-initiated HHG.

R-matrix methods are ideally suited to the description of general, multielectron atoms in short intense laser pulses. This is facilitated by dividing space into two distinct regions: an inner region, close to the nucleus where all multielectron interactions are fully described, and anouter region, far from the nucleus where an isolated electron moves under the long-range potential of the ionic core and the laser potential. On top of this theoretical foundation, the RMT approach employs a mixed basis set/finite difference approach and several layers ofparallelism to harness massively parallel computer architectures and facilitate the solution of the time-dependent Schr¨odinger equation for complex, multielectron targets even in fewmicron-wavelength laser pulses.

RMT has in recent years been established as a leading method at the intersection of atomic and attosecond physics, through applications to HHG [2], strong-field rescattering [3], attosecond transient absorption spectroscopy [4] and, most recently, XUV-initiated HHG [5, 6]. In this talk I will discuss results of stimulated electron-correlation in strong field studies, using XUVinitiated HHG to drive resonant enhancement of the harmonic yield with autoionized electrons in argon.

I will then discuss recent extensions to the RMT code which allow the 2738

Effect of exceptional point in the spectrum of helium atom in laser fields on its photo induced dynamics

Nimrod MoiseyevTechnion-Israel institute of Israel

Exceptional point (EP) in the spectrum results from a coalesence of two metastable states. This non hermitian degeneracy has shown already a dramatic effect on the propagation of light waveguides. Such as asymmetric optical switch and stopping light as it approaches the condition of an EP. Here we will discuss the effect of coalesence of two autoionization states of helium atom on the photo induced dynamics of Helium in high intensity laser fields.

Arnold Diffusion in Molecules and LatticesDriven by Time-periodic Radiation

L.E. Reichl , Han Hsu, and Y. BoretzThe University of Texas at Austin

The effect of laser radiation on matter has been a topic of growing interest because time periodic forces can be used to control and possiblythermalize matter. Pulses of laser radiation have long been used to control quantum transitions. Carbon nanotubes driven by laser radiation re-emit high harmonic radiation [1], but also break up in the presence of sufficiently strong radiation [2]. There is some evidence that optical doping can induce phase changes in materials [3]. It has been known since Arnold’s classic work [4] that dynamical systems with 2.5 or more degrees of freedom are intrinsically unstable. As a consequence, time-periodically driven systems can experience large excursions in energy [5]. In this talk we show the mechanisms by which time periodic driving of molecular and solid state systems can potentially cause their destabilization.

1. H. Huang, R. Maruyama, K. Noda, H. Kajiura, and K. Kadono, J. Phys. Chem. B 110 7316 (2006).

2. Han Hsu and L.E. Reichl, Phys. Rev. B 72 155413 (2005); Phys. Rev. B 74 115406 (2006)

3. T. Han, F. Zhou, C.D. Maliakas, P.M. Duxbury, S.D. Mahan, M.G. Kanatzudusm and C-Y Ruan, Sci. Adv. 1 e1400173 (2015).

4. V.I. Arnold, Russ. Math. Surveys, 18:6, 85 (1963))5. Y. Boretz and L.E. Reichl, Phys. Rev. E 93 032214 (2016)

28 37


description of processes in arbitrarily polarized light fields, and show our first results of so-called electron vortices [7] in a general multielectron target. Finally I will discuss the ongoing expansion of theRMT method including applications to molecular targets, relativistic (spin-orbit) dynamics and double-ionization.

[1] L. R. MOORE, et al, J. Mod. Optics 58, 1132 (2011).[2] O. HASSOUNEH, A. C. BROWN, and H. W. VAN DER HART, Phys. Rev. A 90, 043418 (2014).[3] O. HASSOUNEH, et al, Phys. Rev. A 91, 031404 (2015).[4] T. DING, et al, Opt. Lett. 41, 709 (2016).[5] A. C. BROWN and H. W. VAN DER HART, Phys. Rev. Lett. 117, 093201 (2016).[6] D. D. A. CLARKE, H. W. VAN DER HART, and A. C. BROWN, Phys. Rev. A 97, 023413 (2018).[7] J. M. NGOKO DJIOKAP, et al, Phys. Rev. Lett. 115, 113004 (2015).

distributions in the below-threshold intensity regime, which have been related to direct ionization, may be in fact RESI. Third, since excitations to s, p or d states lead to very distinct shapes in the RESI distributions, different coherent superpositions of channels and events could be used to reconstruct or even control the intermediate state of the second electron. As a testing ground [5], we model experimental data for RESI on Argon from [7], where an increasing laser pulse results in cross-shaped distributions collapsing to

a slightly backto- back correlated electron emission distribution. For very short pulses, the prevalent intermediate (excited) state of the second electron resembles an s state, which leads to cross-shaped RESI distributions. As the pulse length increases, this intermediate state consists of a coherent superposition of p and d states. Estimates for frequency and intensity regions for which s, p or d states are accessed, depending on the pulse frequency and intensity widths, are consistent with this picture. This strongly suggests that below-threshold NSDI may be used for quantum-state reconstruction.

[1] C. Figueira de Morisson Faria and X. Liu, J. Mod. Opt. 58, 1076 (2011).[2] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993).[3] W. Becker, X. Liu, P. J. Ho, and J. H. Eberly, Rev. Mod. Phys. 84, 1011 (2012).[4] A. S. Maxwell and C. Figueira De Morisson Faria, Phys. Rev. A 92, 023421 (2015).[5] A. S. Maxwell and C. Figueira De Morisson Faria, Phys. Rev. Lett. 116, 143001 (2016).[6] X. Hao et al., Phys. Rev. Lett. 112, 073002 (2014).[7] M. K¨ubel et al., New J. Phys. 16, 033008 (2014).

FIG. 1: RESI distributions for argon (E1g = 0:58, E2g = 1:02 a.u.) modeling results from [7]. The specific the excited ionization potential, different coherent superposition and further details on this figure can be found in [5]. The number in the top left represent the pulse length being modeled.

2936


Nucleus-assisted non-sequential double ionization as a gate to anti-correlated two

electron escape

G. Katsoulis, A. Hatzipittas and A. EmmanouilidouDepartment of Physics and Astronomy

University College LondonGower Street, London WC1E 6BT, United Kingdom

At intensities below-the-recollision threshold, in NSDI two electrons escaping opposite to each other along the laser-field direction---anti-correlated escape---has been studied intensely by experiment and theory alike. This pattern was found to prevail, but not substantially, over correlated two-electron escape. It was observed in NSDI of several atoms driven by intense (strongly-driven) long duration laser pulses. Multiple re-collisions, in the context of RESI, were put forth to explain anti-correlated two-electron escape. Electron-electron repulsion was also suggested as a possible explanation.

Here, we show that RESI does not necessarily prevail the delayed pathway, for strongly-driven He at 400 nm. We find another competing mechanism where the still bound electron ionizes shortly past even extrema of the laser field after re-collision. Around these extrema the forces from the nucleus and the laser field are exerted along the same direction on the still bound electron. We label this mechanism as re-collision-induced excitation with subsequent nucleus-assisted ionization (RESNI). We show that anti-correlated two-electron escape is the striking hallmark of the decisive role the nucleus plays in RESNI. Hence, our results suggest that RESNI and not multiple re-collisions accounts for the anti-correlated two-electron escape obtained in previous studies [1].

G. P. Katsoulis, A. Hatzipittas, B. Borgues and M. Kling and A. Emmanouilidou, arXiv:1805.10044 (May 2018).

Controlling Below-Threshold Nonsequential Double Ionization via Quantum Interference

A. S. Maxwell and C. Figueira de Morisson FariaDepartment of Physics & Astronomy, University College London

Gower Street London WC1E 6BT, United Kingdom

Laser-induced nonsequential double ionization (NSDI) is the archetypal example of electron-electron correlation occurring in the context of matter in intense laser fields (for a review, see, e.g., [1]). The underlying physical mechanism is laser-induced recollision, in which an electron returns to its parent ion and, by sharing part of its kinetic energy with the core, releases a second electron [2]. Due to the success of classical-trajectory models in reproducing key features in NSDI electron-momentum distributions, this correlation has been viewed as classical for over two decades. This holds especially in the direct-ionization regime, for which the second electrongains enough energy to overcome the second ionization potential [3].

In the present work, we address the question of whether features related to quantum interference may be visible in below-threshold NSDI, for which the the first electron, upon return, does not have enough energy to directly ionize the second electron. We focus on the recollision-excitation with subsequent ionization (RESI) mechanism, in which the first electron excites the second electron upon recollision with its parent ion. Using semi-analytic methods based on the strong-field approximation, we identify two types of quantum interference, related to (i) events displaced in time and electron indistinguishability; (ii) the electron being excited to different intermediate states, and provide analytical conditions for the interference fringes encountered. These conditions agree well with our computations, and give hyperbolic fringes [5], and go well beyond previous investigations inwhich this interference been found [6].

We show that both interference types are of paramount importance, and may survive the integration over the momentum components parallel to the laser-field polarization and focal averaging. This has several consequences.First, this implies that both types of interference can be observed experimentally, so that classical RESI models must be viewed with care. Second, by manipulating the interference effects one may obtain a myriad of shapes for the electron momentum distributions, includingcorrelated and anti-correlated. This means hat correlated NSDI


35

Creation of breathing ions by coherentattosecond shake-up

M. Khokhlova1, M. Ruberti1, P. Decleva2 and V. Averbukh1

1Blackett Laboratory, Imperial College London, UK2Department of Chemistry, University of Trieste, Italy

Coherent population of a series of shake-up states in the course of attosecond inner-shell/subshell ionisation creates an ionic wavepacket whose time evolution is reflected in the non-stationary electron density. We call the resulting states “breathing ions” and study their formation using the ab initiotime-dependent B-spline algebraic diagrammatic construction (ADC) method.

Inner-shell ionisation by broad-band attosecond pulses is fundamentally distinct from the traditional one, induced by synchrotron radiation. The cardinal difference lies in the capability of the attosecond pulses to coherently populate series of bound and autoionising states leading to new time-dependent physical phenomena. Here we study theoretically the attosecond regime of the most basic of the inner-shell processes: the atomic shake-up ionisation [1,2]. Our first-principles analysis shows that contrary to the monochromatic (synchrotron) case, attosecond ionisation generally leads to formation of coherent shake-up wavepackets that exhibit non-trivial temporal dynamics imprinting the ionising pulse properties.

wave function in the excitation classes. However, while the CI expansion is based on the Hartree-Fock reference state, the ADC approximation uses the perturbation-theoretically corrected reference, leading to a more compact, size-

30

Science with X-ray Pulses – New frontiers in exploring the science of matterat the

microscopic scale

Jon Marangos Imperial College

The new sources of ultrafast X-rays: X-ray free electron lasers and high harmonic generation from intense optical lasers, will be introduced. These can both generate X-ray pulses as short as 10-15 second (1 thousandths of a millionth of a millionth of a second!) and so enable us to study the very fastest events in nature that involve the rearrangement of electrons in matter. I will explain how this is leading to new insights into chemical reaction dynamics and ultrafast electronics.


34

consistent representation. Here we for the first time use the time-dependent version of the B-spline ADC(2) method explicitly including both singly and doubly excited configurations. This allows us to describe the dynamics of the attosecond shake-up process ab initio. We predict quantitatively the degree of coherence between the main ionised state and an arbitrary shake-up state, as well as the characteristics (amplitudes and phases) of the shake-up many-electron wavepacket. We show that by changing the XUV pulse parameters such as the frequency, intensity, duration and chirp, the superposition of coherently populated shake-up states can be controlled (see the scheme). The ensuing dynamics of the coherently populated shake-up states results in the complex oscillations of the electron density or, alternatively, of the expectation value of the radius-vector, Ψ(t)rΨ(t), reflecting the“breathing”-like behaviour of the ionic system. We show that the dynamics of the breathing ions can be probed by an ionising VUV pulse.

[1] T. Aberg, Phys. Rev. 156, 35 (1967).[2] M. Uiberacker et al., Nature 446, 627 (2007).[3] M. Ruberti and V. Averbukh, in “Chemistry and Molecular Physics on the Attosecond Timescale: Theoretical Approaches”, edited by M. Vrakking and F. Lepine, RSC Theoretical and Computational Chemistry series (2018).

31

Many-electron eects on attosecond time delays

E. LindrothDepartment of Physics, Stockholm University AlbaNova University Center

SE-106 91 Stockholm, Sweden

I will discuss attosecond delays in laser-assisted photoionization with an emphasis on the theoretical treatment and the role of many-body eects, [1, 2, 3, 4]. With the recent possibility to combine high temporal and spectral resolution [5], attosecond experiments start to be sensitive to a range of many-body effects such as resonances and shake-up processes, highlighting the importance of electron correlation. In time-delay measurements attosecond XUV pulses are used to photoionize target atoms at well-defined times, followed by a probing process in real time by a phase-locked, infrared laser field. In this way, the laser field serves as a “clock” to monitor the ionization event. It is well established, see e.g. Ref. [6], that the observable delays do not correspond directly to the delay associated with single-photon ionization. Instead, a signicant part of the observed delay originates from a measurement induced process, which obscures the single-photon ionization dynamics. This eect has mostly been studied when the electron leaves a Coulomb field. Here the new situation when the there is no long range potential, as in a negative ion, will be discussed, as well as the effects of resonances and shake-up channels. Another interesting aspect is the angular dependence. Finally preliminary results for calculations on heavier systems where a relativistic treatment is called on will be shown.

[1] J. M. Dahlstr�om, T. Carette, and E. Lindroth. Diagrammatic approach to attosecond delays in photoionization. Phys. Rev. A, 86:061402, 2012.[2] T. Carette, J. M. Dahlstr�om, L. Argenti, and E. Lindroth. Multicongurational hartree-fock close-coupling ansatz: Application to the argon photoionization cross section and delays. Phys. Rev. A, 87:023420, 2013.[3] J M Dahlstr�om and E Lindroth. Study of attosecond delays using perturbation diagrams and exterior complex scaling. Journal of Physics B: Atomic, Molecular and Optical Physics, 47:124012, 2014.[4] Eva Lindroth and Jan Marcus Dahlstr�om. Attosecond delays in laser-assisted photodetachment from closed-shell negative ions. Phys. Rev. A, 96:013420, Jul 2017.[5] M. Isinger, R. J. Squibb, D. Busto, S. Zhong, A. Harth, D. Kroon, S. Nandi, C. L. Arnold, M. Miranda, J. M. Dahlstr�om, E. Lindroth, R. Feifel, M. Gisselbrecht, and A. L'Huillier. Photoionization in the time and frequency domain. Science, 358(6365):893{896, 2017.[6] J.M. Dahlstr�om, D. Guenot, K. Kl�under, M. Gisselbrecht, J. Mauritsson, A. L'Huillier, A. Maquet, and R. Ta�eb. Theory of attosecond delays in laser-assisted photoionization. Chemical Physics, 414(0):53 { 64, 2013.

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Abst rac ts

High harmonics source for probingultrafast demagnetization

François LégaréINRS, Centre EMT, 1650 boulevard Lionel-Boulet

Varennes (Québec) J3X 1S2, [email protected]

We generate harmonics in neon, reaching the cobalt M-edge absorption resonance. A pump-probe scheme then allows us to retrieve demagnetization curves by observing resonant X-ray magnetic scattering on a multilayer cobalt/platinum sample.

Using this setup, we are investigating the underlying physics of ultrafast demagnetization by changing the pump wavelength and its pulse duration. The discovery of the ultrafast optical demagnetization by Beaurepaire et al. [1], in 1996, has opened the way to faster data manipulation by optical means. This has been a motivation for extensive investigation of the ultrafast optical demagnetization phenomena, yet the mechanisms behind it are still being debated. In the work presented here, we make use of the HHG source of the AdvancedLaser Light Source laboratory (ALLS) to probe ultrafast optical demagnetization at the M-edge of cobalt by resonant magnetic X-ray scattering (RXMS). This scheme has been presented for the first time by Vodungbo et al. [2], and we have implemented this approach to investigate the fundamental temporal limit of ultrafast magnetization dynamics and its dependence on pump laser wavelength. The sample studied in this work is a [Co/Pt] multilayer film in which the magnetic domains present out-of-plane magnetization vectors.

FIGURE 1 - Schematics of the experimental set-up configuration. The sample is positioned with its domains aligned diagonally so that the peaks appear in the corners of the image on the camera. A magnetic force microscope image of the sample is presented in the inset, showing domains with a periodicity of ~145 nm. The scale bar represents 1 μm.

- Schematics of the experimental set-up configuration. The sample is positioned with its domains aligned

Abst rac ts

The inset of figure 1 shows a magnetic force microscope image of the sample's magnetic domains. When placed at the focus of a soft X-ray source tuned to the magnetic edge energy of the sample, light scattering occurs on the magnetic centers. The diffraction peaks have a width and angular distribution dependent on the size and orientation of the magnetic domains, and an intensity dependent on the magnetization vector's amplitude [3]. Through a pumpprobe measurements of the diffracted signal, ultrafast magnetization dynamics is tracked as a function of laser parameters, i.e. the pump pulse duration and its wavelength. Premilinary results are under analysed and will be presented at the meeting.

1. E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot, “Ultrafast spin dynamics in ferromagnetic nickel,” Phys. Rev. Lett. 76, 4250–4253 (1996).

2. B. Vodungbo et al., “Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network,” Nat. Commun. 3 (2012).

3. J. B. Kortright et al., “Soft-x-ray small-angle scattering as a sensitive probe of magnetic and charge heterogeneity,” Physical Review B - Condensed Matter and Materials Physics 64 (2001).

july 2-4, 2018 - · 2018. 6. 27. · taran driver imperial college london [email protected]...

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