292  CHIMIA 2011, 65, No. 5  NCCR MUST

Editorial

National Center of Competence in Research: Molecular Ultrafast Science and TechnologyOptical, atomic, and molecular sciences have always had a dramatic impact on fundamental science as well as on our society. They enabled the development of the laser, they triggered the development of quantum mechan-ics, they are at the heart of modern communication, they facilitate modern navigation systems, they help designing new drugs, or they play a key role in understanding molecules which determine the function of all living creatures. Research efforts in optical, atomic, and molecular sciences are still growing and are expected to have further im-pact on fundamental sciences as well as our daily life. Francis Crick, the co-discoverer of the double-helix DNA structure, once said “... If you want to understand function, study structure ...” To the present day, the best probes of equilibrium atomic structure in matter are nuclear magnetic resonance, electron spin resonance, high-energy electron scattering, X-ray scattering and X-ray absorption spectroscopy. Especially X-ray scattering has proven to be extremely powerful in unraveling the structure of large molecules, and, unlike any other field, research related to X-rays has been awarded with 19 Nobel prizes in chemistry, physics, and medicine. Time, however, is missing in Francis Crick’s statement, but  is  fundamentally  important  for chemical  reactions,  isomerization, or structural phase transitions where a sequence of events of structural and electronic modifications at the molecular level are essential for the understanding of the process. In other words we should probe time-dependent structures and redistribution of electronic charge. Thus, a full understanding of structural dynamic at the molecular level requires a thorough description of the electronic dynamics as the underlying driving force. We have now reached a point where not only do we have the time resolution to probe the motion of atoms within assemblies (molecules, solids, liquids, proteins) but also that of the electrons, which are the main actors in the formation and breaking of bonds.Over the past years, research along these lines has dramatically gained momentum in Switzerland with new re-search groups being established, with the implementation of new theoretical concepts, with the development of new sources of femto- and attosecond pulses in the soft and hard X-ray regimes, or with the implementation of various schemes of multidimensional spectroscopy. The Paul Scherrer  Institute  (PSI) successfully operates the FEMTO facility at the SLS providing hard X-ray pulses of approximately 100-fs duration and plans to further extend its focus in ultrafast science with a new hard X-ray free electron laser (SwissFEL), foreseen to be commissioned by 2017. The aim of MUST is to further promote these developments and to use them in tackling various fundamen-tal problems in molecular sciences by bringing together Swiss researchers active in the development and use of structure-sensitive ultrafast probes. The scientific questions addressed within MUST are focused on increasing our understanding of, and ultimately control of, matter at the level of atoms and electrons. Understanding matter on the atomic scale will address important challenges facing humanity including development of alternative sources of energy and improving health. To address these challenges we need to use and develop new tools that extend into higher spatial, temporal, and energy resolution to visualize and control both atomic and electronic motion in atomic compounds.Visualization of atomic motion in atomic compounds: The time scale for atomic motion is in the range of femtosec-onds to picoseconds and visualization relies on probing techniques which are even faster. Capturing in ‘real-time’ the motion of atoms, or groups of atoms, in molecules, liquids, solids, and proteins became possible with the ad-vent of femtosecond lasers delivering ultrashort pulses in the ultra-violet, visible, or infrared part of the spectrum. Recently, a major breakthrough has been the development of multi-dimensional spectroscopy in the infrared and, even more recently, in the visible and UV. These optical multi-dimensional spectroscopy techniques are analogues of NMR spectroscopy, with the major advantage that sub-ps temporal resolution is reached (NMR is limited to microseconds  at  best).  Furthermore,  couplings  between  electric  dipoles  are  orders  of  magnitudes  larger  than those between magnetic dipoles in NMR, which implies a huge increase in sensitivity. In addition to the progress in transferring NMR-like techniques to the ultrafast domain, X-ray and electron diffraction and X-ray absorption spectroscopy are reaching the relevant time scales. Major breakthroughs in probing ultrafast structural dynamics of chemical and biological systems as well as nano-systems and materials become accessible through ultrashort X-ray and electron pulses. Within MUST, we want to push the development of ultrafast multi-dimensional spectros-copy, of ultrafast X-ray and electron diffraction, and of X-ray absorption spectroscopy as highly sophisticated tools to follow the temporal evolution of structural rearrangements within matter of various sizes and in different environ-ments. Experimental efforts will be accompanied by theoretical work, and we expect that theoretical simulations will help designing appropriate experiments.Visualization of electronic motion: The time scale of electron motion on a molecular scale is that of attoseconds. Attosecond pulses and novel streaking methods using carrier envelope offset phase-stabilized laser pulses of few optical cycles are available  in university-based table-top laboratories and are typically  in the VUV to soft X-ray range. They are starting to allow for measurements reaching attosecond resolution for probing electron dynamics in atomic and molecular systems, such as the study of Auger processes, tunneling in atoms, or molecular tomography. Control of atomic motion in atomic compounds: Control of molecular processes through catalysis on surfaces is a well established method and is omnipresent in our daily life. Steady progress in surface- and nano-science will create a better understanding of such processes and help to devise new ones. Coherent control through optical pulse shaping techniques has been a major area of femtosecond spectroscopy since its birth and in a recent report from the US Department of Energy (Physics Today, July 2008 issue) it was identified as one of the most promising and attractive challenges for future research. MUST will increase its already strong efforts in optical pulse shap-ing and control will also be an integral part of many of the experimental investigations envisaged, as it can help in disentangling complicated measurements.Already today we witness the benefits of ultrafast science and technology for society and they would not have been possible without the efforts in fundamental research. For example femtosecond lasers are now commonly used in ophthalmology or in confocal microscopy of biological systems; they enable higher accuracy for frequency metrol-

NCCR MUST  CHIMIA 2011, 65, No. 5  293

ogy; they facilitate table-top hard X-ray sources that are being implemented in phase-contrast micro-imaging of biological samples or in high contrast radiography and mammography; they are also used in the processing and machining of solid materials and surfaces as well as in laser-assisted fabrication of new materials. In the future the extension of NMR-like techniques to the infrared and visible spectral domain promises a tremendous leap forward in applications similar to those of NMR, e.g. structural determination of molecular systems. Ultrafast spectroscopic measurements will help to better understand the photo-generation of hydrogen, the catalyzed reduction of oxygen, and the charge transfer reactions at the water-supercritical carbon dioxide interface, crucial steps in the upcoming energy problem. Ultrafast spectroscopy will help to understand the dynamics of light-induced electron transfer pro-cesses in supra-molecular systems and at interfaces; both being fundamental to the development of dye-sensitized solar cells and resulting photovoltaic systems. Through MUST we envision advance of knowledge in order to further improve the quality of daily life. New enabling technological innovations will help to further maintain competitiveness and to create new jobs. A major goal of MUST is to provide training and education of qualified personnel and to promote the scientific education of the general public.

Thomas Feurer and Ursula KellerUniversity of Bern and Eidgenössische Technische Hochschule Zürich (ETHZ)March 2011

Thomas Feurer was born in Kempten, Germany, in September 1963. He received his Diploma in Physics from the University of Wuerzburg, Germany in 1990. He then moved to the Rice University in Houston, Texas, where he worked on optically induced percolative phase transitions. He earned his PhD degree in Physics in 1994 at the University of Wuerzburg. In 1994 he went to the University of Jena and worked on ultrafast linear and nonlinear optics, femtosecond spectroscopy and coherent control of quantum systems, high-power short-pulse laser-matter interaction at relativistic intensities, generation of femtosecond hard X-rays and femtosecond time-resolved X-ray diffraction. He received the Habilitation in 2001 and moved to the M.I.T. in Cambridge, USA. His research interests were: High-frequency acoustic spectroscopy, ultrafast optics and pulse-shaping, nonlinear spectroscopy of liquids and solids, coherent control of collective excitations in solids, generation of phase-matched high harmonics, EUV nonlinear femtosecond spectroscopy. In 2002 he was appointed Research Associate at the M.I.T. and in 2004 he became full professor at the University of Bern, Switzerland. His current research interests are in ultrafast lasers and fiber optics, ultrafast coherent control and nonlinear spectroscopy. He has published more than 80 journal papers and holds several patents. In 1997, he received the Carl Zeiss Research Award, in 1999 the Werner-von-Siemens Medal, and  in 2001 he was awarded with a Max-Kade Fellowship. Thomas Feurer  is a member of  the Optical  Society of America (OSA) and the American Physical Society (APS).

Ursula Keller joined ETH as professor of physics in 1993. She received the PhD in Applied Physics from Stanford University in 1989 and the Physics diploma from ETH in 1984. She was a Member of Technical Staff (MTS) at AT&T Bell Laboratories in New Jersey from 1989 to 1993. Her research interests are exploring and pushing the frontiers in ultrafast science and technology: ultrafast solid-state and semiconductor lasers, frequency comb generation and stabilization, attosecond pulse generation and science using high harmonic generation. She has published more than 310 peer-reviewed journal papers and 11 book chapters and she holds or applied for 17 patents. She received the OSA Fraunhofer/Burley Prize in 2008, the Philip Morris Research Award in 2005, the first-placed award of the Berthold Leibinger Innovation Prize in 2004, and the Carl Zeiss Research Award in 1998. She is an OSA Fellow and an elected foreign member of the Royal Swedish Academy of Sciences and the German Academy Leopoldina. 

It is with great pleasure that the Editorial Board of CHIMIA warmly thanks Prof. Dr. Eric Vauthey and his fellow guest editors Prof. Thomas Feurer and Prof. Ursula Keller for their efforts in the planning and suc-cessful realisation of this interesting and topical issue on ‘NCCR MUST: A New Swiss Research Priority in Molecular Ultrafast Science and Technology’

290  CHIMIA 2011, 65, No. 5  Contents Inhalt sommaIre

NCCR MUST: A New Swiss Research Priority in Molecular Ultrafast Science and Technology

Editorial 292 T. Feurer U. Keller

294 AttoscienceatETHZurich:ShiningNewLightonOldQuestionsinQuantumMechanics U. Keller

299 High-HarmonicSpectroscopy:FromFemtosecondtoAttosecondMolecularDynamics H. J. Wörner

303 UltrafastX-rayAbsorptionStudiesoftheStructuralDynamicsofMolecularand BiologicalSystemsinSolution C. J. Milne, R. M. Van der Veen, V.-T. Pham, F. A. Lima, H. Rittmann-Frank, M. Reinhard, F. van Mourik, S. Karlsson, T. J. Penfold, M. Chergui*

308 UltrafastStructuralDynamicsinCondensedMatter P. Beaud*, S. L. Johnson, E. Vorobeva, C. J. Milne, A. Caviezel, S. O. Mariager, R. A. De Souza, U. Staub, G. Ingold

313 UltrafastTime-ResolvedVibrationalSpectroscopyatUniversityofZurich P. Hamm*

316 NearFieldEnhancementforTHzSwitchingandTHzNonlinearSpectroscopyApplications H. Merbold, F. Brunner, A. Cannizzo, T. Feurer*

320 TowardsHigh-powerSingle-cycleTHzLaserforInitiatingHigh-field-sensitivePhenomena C. Ruchert, F. Ardana, A. Trisorio, C. Vicario, C. P. Hauri*

323 CanEnergeticTerahertzPulsesInitiateSurfaceCatalyticReactionsonthePicosecondTimeScale? B. D. Patterson*, J. Sa, A. Ichsanow, C. P. Hauri, C. Vicario, C. Ruchert, I. Czekaj, R. Gehrig, H. C. Sigg, J. A. van Bokhoven, B. Pedrini, R. Abela

326 ComputationalSpectroscopyandReactionDynamics P.-A. Cazade, S. Lutz, M. W. Lee, M. Meuwly*

330 MixedQuantumMechanical/MolecularMechanical(QM/MM)SimulationsofAdiabaticand NonadiabaticUltrafastPhenomena B. F. E. Curchod, P. Campomanes, A. Laktionov, M. Neri, T. J. Penfold, S. Vanni, I. Tavernelli, U. Rothlisberger*

334 AcceleratingCalculationsofUltrafastTime-ResolvedElectronicSpectrawith EfficientQuantumDynamicsMethods M. Wehrle, M. Šulc, J. Vaníček*

339 Dispersedfs-FWMforInvestigationsofLowFrequencyVibrationsof TransientSpeciesinCombustion G. Knopp*, P. P. Radi, T. Gerber

342 Time-ResolvedPhotoelectronSpectroscopytoProbeUltrafastChargeTransfer andElectronDynamicsinSolidSurfaceSystemsandatMetal-MoleculeInterfaces L. Castiglioni*, M. Greif, D. Leuenberger, S. Roth, J. Osterwalder, M. Hengsberger

346 DiscriminatingBiomoleculeswithCoherentControlStrategies A. Rondi, D. Kiselev, S. Machado, J. Extermann, S. Weber, L. Bonacina, J.-P. Wolf*, J. Roslund, M. Roth, H. Rabitz

Contents Inhalt sommaIre  CHIMIA 2011, 65, No. 5  291

350 PhotoinducedElectronTransferReactions:FromtheElucidationofOldProblems inBulkSolutionsTowardstheExplorationofInterfaces M. Fedoseeva, J. Grilj, O. Kel, M. Koch, R. Letrun, V. Markovic, I. Petkova, S. Richert, A. Rosspeintner, P. S. Sherin, D. Villamaina, B. Lang, E. Vauthey*

353 PhotoinducedInterfacialElectronTransferandLateralChargeTransport inMolecularDonor–AcceptorPhotovoltaicSystems A. Punzi, J. C. Brauer, A. Marchioro, E. Ghadiri, J. de Jonghe, J.-E. Moser*

356 ArtificialPhotosynthesisatSoftInterfaces D. Schaming, I. Hatay, F. Cortez, A. Olaya, M. A. Méndez, P. Y. Ge, H. Deng, P. Voyame, Z. Nazemi, H. Girault*

Columns

360 SwiSS Science concentrateS Preparedby N. Bruns, V. Köhler, R. Kramer, P. Mauleón, F. Monnard, T. R. Ward

361 HigHligHtS of analytical cHemiStry in Switzerland Lighting-upCancerousCellsandTissueswithLanthanideLuminescence J.-C. G. Bünzli*, C. D. B. Vandevyver, A.-S. Chauvin, M. Gijs, H.-A. Lehr

362 interview «Lachimie,çaresteunescienceexpérimentale!» EntretienavecleprofesseurJacquesWeber Interviewer L. Weber

366 international year of cHemiStry 2011 EMS–DerführendeSpezialistfürHochleistungspolyamide S. Wick, C. Kruse*

370 FH-HES UsingMembrane-SupportedLiquid–LiquidExtractionfortheMeasurementofExtractionKinetics W. Riedl*, D. Mollet, G. Grundler

swissChEmiCalsoCiEty

373 AusschreibungderSCG-Preise2012/AppelàcandidaturespourlesPrixSSC2012

375 GeneralversammlungderSCG/AssembléegénéraledelaSSC

376 Awarmwelcometoournewmembers!

377 DivisionofAnalyticalChemistry

information

378 Honors

378 Conferences/Workshops/Symposia

378 Lectures

EuChEms

380 EuCHeMSNewsletter

ChimiarEport/CompanynEws

384 Markt,Apparate,ChemikalienundDienstleistungen

294  CHIMIA 2011, 65, No. 5  NCCR MUSTdoi:10.2533/chimia.2011.294   Chimia 65 (2011) 294–298  © Schweizerische Chemische Gesellschaft

*Correspondence: Prof. Dr. U. KellerETH ZürichDepartment of PhysicsWolfgang-Pauli-Str. 16CH-8093 ZürichTel.: +41 44 633 21 46Fax: +41 44 633 10 59E-mail: keller@phys.ethz.ch

Attoscience at ETH Zurich: Shining New Light on Old Questions in Quantum Mechanics

Ursula Keller*

Abstract: Ten years ago, we began a new research effort in attoscience at ETH Zurich, building on our ultrafast laser expertise in the few femtosecond regime. I present some of the technical highlights and explain how we continue within the NCCR MUST.

Keywords: Attosecond · Time-resolved spectroscopy · Tunneling time · Ultrafast science · Ultrafast solid-state lasers 

Attoscience describes a new research field in time-resolved spectroscopy and in sub-femtosecond lasers. Attosecond time reso-lution, where 1 attosecond (as) corresponds to 10–18 seconds, is required to resolve the dynamics of charge and energy transport on an atomic and molecular scale. These features are at the cutting edge of our NCCR MUST project, where the scientific driver is to better understand how matter functions at the electronic, atomic and mo-lecular level; how matter changes its struc-ture during a reaction; and how quanta of energy are transported on a microscopic spatial and ultrafast time scale. Attosci-ence is embedded in the MUST vision that we can contribute to important challenges such as alternative energy sources and im-proving health. We take a broader view to address these challenges through basic research, which we believe is fundamen-tally critical for breakthrough progress in these areas. My group started to work in attosecond science in 2001, mostly funded by a previous NCCR project in Quantum Photonics.

‘New Light’

Continued progress in laser sources enables new discoveries and expands our knowledge horizon. For attoscience, the key enabling technology was the discovery of high harmonic generation (HHG),[1,2] intense ultrafast near infrared pulses from Ti:sapphire laser systems,[3] and carrier envelope offset (CEO) phase stabilization[4–6] with which single at-tosecond pulse generation first became possible.[7] New demands on novel light sources also became apparent as we pro-gressed in attoscience. One key limita-tion is the low pulse repetition rate from Ti:sapphire amplifier systems, which typ-ically operate in the 1-kHz regime, and generate attosecond pulses in the 1-nJ re-gime for harmonics up to about 100 eV.[8] This means that we have at least a five or-ders of magnitude reduction in the signal-to-noise ratio for attosecond pump probe measurements, compared to femtosecond sources which deliver similar pulse ener-gies but at rates of about 100 MHz. How-ever, just increasing the pulse energy does not usually solve this problem, as space charge limitations set an upper limit to the usable pulse energy. This has motivated us to continue our laser development in intense pulse generation but at megahertz repetition rates.[9] High pulse repetition rate f

rep of course then corresponds to an

increase in average power Pav

, since Pav

= E

pfrep

, where Ep is the pulse energy. For

example, a pulse energy of 100 mJ at a pulse repetition rate of 5 MHz results in an average power of 500 W. Such high aver-age power is not practical for Ti:sapphire laser systems, and other solutions are cur-rently being explored. Excellent thermal management is one key issue for average power scaling. Heat needs to be extracted from the laser gain medium, which is op-

timized by a high surface-to-volume ratio of the gain medium as obtained in thin disk, fiber, or slab lasers, for example. Not surprisingly, all current world-lead-ing results have been obtained with such lasers. In my group, we have focused on power scaling of SESAM modelocked thin disk lasers, which we first started in 2000,[10] and pushed in performance to above 10 mJ in 2008[11] and currently to a record high average power of 140 W with an optical-to-optical pump efficien-cy of 40%.[12] In contrast to fiber[13] and slab lasers,[14] our results are generated directly from a laser oscillator – without any further external pulse amplification. Avoiding external amplifiers means that we avoid added noise from the amplifier and further system complexity. Our high power laser oscillators do not have any significantly higher complexity or more components than a standard low-power SESAM modelocked oscillator which is in any case required at the start of any amplifier system. Our goal is to explore the power scaling limitations of SESAM modelocked thin disk lasers in the 1-kW average output power regime, which we believe is reachable. At these power lev-els, an efficient, saturated power ampli-fier can always be added for further power scaling. A SESAM modelocked thin disk laser is ideally suited for power scaling because all optical elements inside the la-ser oscillator are used in reflection, laser mode areas can be increased as required, and SESAM damage can be avoided.[15] In collaboration with Prof. Günter Huber’s group in Hamburg, we are also exploring different laser materials to obtain shorter pulses,[16] as we currently typically obtain around 800-fs pulses at a center wave-length of around 1 mm, which then need to be compressed externally for attosecond pulse generation.[17,18]

NCCR MUST  CHIMIA 2011, 65, No. 5  295

advantages compared to photoelectron de-tection as shown for example in Fig. 2. They are less affected by space charge effects, ex-hibit a much better signal-to-noise ratio and a higher dynamic range, and allow for much faster acquisition times of minutes instead of hours. We could demonstrate attosecond control of absorption of different harmon-ics where a moderately strong infrared pulse introduces rapid absorption modulations. The relative phase of these oscillations in the individual harmonics can be controlled by the infrared pulse intensity.[29] This con-trol can be explained by the interference of transiently bound electron wave packets as every pulse in the attosecond pulse train (APT) creates a transiently bound electron-ic wavepacket. Two consecutive attosecond pulses within the APT create two electrons wave packets: the first one returns to the ion after its laser driven trajectory and then interferes with the newly created second one. The phase delay of this trajectory is intensity dependent and therefore the inter-ference and recombination is also intensity dependent. These process is repeated with half-cycle periodicity. This experiment rep-resents the first all-optical observation of electron wavepacket interference on atto-second timescale.

Attoline

Since 2001 my group has invested a significant amount of time and resources in developing an ‘attoline’ (Fig. 1) for at-tosecond pulse generation and attosecond spectroscopy. In the first attoline genera-tion, we were able to explore very funda-mental processes in HHG. We explored attosecond pulse train (APT) assisted HHG,[21,22] which promises a significant efficiency enhancement. Typical HHG ef-ficiencies at present are in the 10–6 to 10–8 regime. For a clear experimental demon-stration however, we required higher en-ergy from our Ti:sapphire laser system and improved mechanical stability for a spatially separated attosecond delay line. Solving these issues were not required, however, for the first observation of the theoretically predicted quantum path in-terference (QPI) in HHG.[23–26] The simple three step model in HHG predicted QPI because we have two or more electron tra-jectories contributing to harmonic emis-sion within the plateau harmonics, i.e. the short and long trajectory, which results in an intensity-dependent phase difference introduced by the classical excursion time of the excited electrons in the strong laser

field before recombination and harmonic emission. Phase matching and angular ap-ertures allowed us to balance the different contributions from the short and long tra-jectories such that the interference contrast was sufficient for observation in HHG.

Even though we could obtain many interesting results with our first attoline, it became apparent that significant limita-tions in mechanical stability did not allow for stable attosecond pulse characteriza-tion and for a larger variety of attosecond measurements. Our next generation at-toline (Fig. 1) was mainly designed and constructed by two graduate students Mirko Holler[19] and Florian Schapper[20] starting at the beginning of 2007. The much improved mechanical stability al-lowed us to measure attosecond pulses for the first time in May 2008[19,20] using the RABBITT technique[27] (Fig. 2).

Attosecond Transient Absorption

Attosecond transient absorption is an all-optical method for time-resolved mea-surements in the attosecond domain and has been widely used in the femtosecond domain. An all-optical method has several

Fig. 1. Attoline at ETH Zurich.[19,20] From left to right: The first three identical chambers accommodate the high-harmonics generation target and an interferometer for pump-probe-like experiments. The next chamber contains a toroidal focusing mirror directing the VUV-XUV and infrared beams into the experimental target chamber. This chamber is equipped with a time-of-flight (TOF) spectrometer. On the most right-hand side, an XUV spectrometer is attached for photon diagnostics. Note that the high power infrared laser amplifier system is not shown here and comes from the left into the first chamber for HHG.

296  CHIMIA 2011, 65, No. 5  NCCR MUST

probability distribution in ionization. The final momentum of ion and electron can be measured in coincidence using a COL-TRIMS (COLd Target Recoil Ion Momen-tum Spectroscopy)[32] setup and which we constructed with the help of Prof. Reinhard Dörner’s group from Frankfurt University. We measured an instantaneous tunneling delay time in helium in the low intensity regime,[31] within a measurement accuracy of around 10 attoseconds.

We then used the attoclock technique to ask fundamental questions regarding how fast a strong laser field can remove two electrons from an atom and if there are any correlation effects between these two electrons.[33,34] This has been studied with argon for a strong laser field process

Attoclock

The attoclock[30,31] is a novel attosecond streaking technique and allows for unprec-edented time resolution in the attosecond regime. The attoclock principle is based on an intense, close-to-circular polarized laser field, where the rotating electric field vec-tor gives the time reference similar to the hands of a clock. The attoclock has been used to measure the tunneling delay time in strong laser field ionization, which is de-fined by the angular difference between the maximum of the electric laser field, which induces the highest tunneling ionization rate, and the direction of the laser field when the electron exits the tunnel and can be accelerated by the electric field vector

direction at that exact moment at the exit of the tunnel. This measurement is based on the definition of ‘time’ by ‘counting cycles’ where the rotating electric laser field vector defines the cycle and where we count fractions of a cycle. The short laser pulse limits the ionization event within one single optical cycle, which is about 2.7 fs at a center wavelength of 800 nm. A tem-poral resolution of only a few attoseconds can be achieved because the measurement is based on an angular ‘peak search’ within one optical cycle. The observables are the final momentum of the electron and ion in coincidence with much less than one ionization event per laser pulse. The final distribution is averaged over many ioniza-tion events, which give a certain angular

a) b)

c) d)

Fig. 2. Characterization of an attosecond pulse train (APT)[19,20] with the RABBITT technique[27] which determines the phase difference between two adjacent harmonics (top left). For the electron time-of-fl ight detection (top right) delay steps of 107 as (i.e. 15 nm steps controlled with a piezo) with an acquisition time per step of 60 s have been applied. For a full RABBITT trace (bottom left) stable conditions for more than 30 hours are required. The recorded phase difference (bottom right) then determines the attochirp of the APT. Here we measured 450 as for the average pulse duration of the attosecond pulses within the APT with a transform limit of 160 as. With an additional 500-nm thick Al fi lter for dispersion compensation close to transform limited pulses can be achieved.[28]

NCCR MUST  CHIMIA 2011, 65, No. 5  297

referred to as double ionization. In strong field double ionization the standard picture typically distinguishes between sequential double ionization (SDI), where the elec-trons are assumed to tunnel-ionize inde-pendently, and non-sequential double ion-ization (NSDI), where the electrons cannot be treated separately. Electron correlation in double ionization by linearly polarized laser pulses has been studied extensively, both theoretically and experimentally. The question about electron correlation in double ionization by circularly polarized laser pulses is currently a hot topic in many theoretical studies. Electron correlation in strong field double ionization is dominated by recollision of the first ionized electron with its parent ion. With laser pulses that are close to circularly polarized, recolli-sion may be greatly modified or avoided and the electrons are usually assumed to be field ionized without mutual interac-tion. An open question remained whether other mechanisms rather than recollision can occur in strong field double ioniza-tion. Elliptically (close to circularly) po-larized pulses are ideally suited to answer this question, since recollision of the first emitted electron with the parent ion is pre-vented. Moreover with close to circularly polarized pulses we can apply the attoclock technique and determine the ionization time of the two electrons. The magnitude of the electron momenta follows the enve-lope of the laser pulse and gives a coarse timing for the electron releases (i.e. ‘the hour hand of the attoclock’). The emission angle of the electrons subsequently gives the fine timing (i.e. ‘the minute hand of the attoclock’). We have found unexpected results: First, coincidence momentum data exhibit an intensity dependence that is not captured by the standard SDI model, as shown with an oscillatory intensity de-pendence for the ratio of parallel to anti-parallel electron emission.[34] Second, the release time of the second ionized electron occurs significantly earlier than predicted by an SDI model.[33]

Old Questions in Quantum Mechanics

Novel time-resolved attosecond streaking techniques such as energy streaking[35] and the attoclock (i.e. angular streaking)[31] are currently being applied in an attempt to answer very fundamen-tal questions in quantum mechanics: How fast can light remove a bound electron or even many electrons from an atom or a solid? How fast can energy and charge be transported within a molecule, or from a surface to an adsorbed molecule? How fast is tunneling? Tunneling is one of the most fundamental concepts in quantum

mechanics and is of fundamental impor-tance for energy and charge transport in technology and nature.

For example two different attosecond measurement techniques addressed two different but related fundamental process-es: strong laser field ionization, where the strong laser field bends the binding poten-tial to emit an electron by tunneling (tunnel ionization)[31] and the photoelectric effect, where a single light quantum (a photon) is absorbed to emit an electron (photoemis-sion).[36] In the first case, an instantaneous tunneling delay time in helium was mea-sured within the experimental accuracy of a few tens of attoseconds, and in the latter case, a relative delay in photoemission of about 20 as between electrons originating from different bound states in neon was measured – from a semi-classical point of view a somewhat puzzling outcome! Strong laser field ionization involves the absorp-tion of many photons, and furthermore the electron emitted into the continuum (i.e. an unbound state) initially experiences a spatial separation from the ion of more than ten atomic units. For photoemission, in contrast, only one photon is absorbed without any initial lateral displacement. Why does photoemission, in contrast to tunnel ionization, have a measurable emis-sion delay, we may wonder? As we know, quantum mechanics may give unexpected results as viewed from a classical perspec-tive. Future results should hopefully help to resolve this potential controversy and give us a better physical picture to under-stand attosecond quantum mechanics.

More recently, we were able to add further insight.[37] After our first atto-clock experiments published in December 2008, we continued to perform laser tun-nel ionization experiments on both atomic helium and argon with attosecond time resolution over a larger intensity range, covering both the multiphoton ionization and the tunnel ionization regime. This work required very careful experimental efforts to prevent any artifacts and the the-oretical support from the group of Prof. Lars Madsen at Aarhus University to re-solve our initially unexpected results. By comparing the helium and argon results, we could show that a tunneling model cor-rectly describes the data, assuming instan-taneous tunneling delay time within the measurement accuracy of around a few 10 attoseconds. However tunneling occurs through a potential barrier that is modi-fied by all of the remaining electrons and from a state that is shifted significantly by the external field. The resulting modi-fied force terms influence the dynamics of the tunneled electron, changes important physical parameters and hence need to be accounted for in attosecond measurement techniques described above. The multi-

electron effects and the corrections to the tunnel potential identified in this work for the first time are universal in all atomic and molecular systems. These results af-fect attosecond streaking measurements because additional force terms need to be considered that have not been considered before. Leaving them away would result in a wrong delay time!

Our future work within MUST will greatly benefit from this new insight and will make sure that such attosecond streak-ing measurements are construed very care-fully, since these multi-electron effects will become even more severe for mol-ecules and surfaces, which will be a focus of our NCCR MUST projects. For a single atom the multi-electron dynamics were so fast that the static response was sufficient to explain our experimental results. It will be interesting to explore more complex and potentially slower multi-electron dynam-ics in molecules and surfaces. We are still at the beginning of a long and interesting journey with potentially many unexpected outcomes.

AcknowledgementsMany excellent graduate students and

postdocs have made essential contributions since I have joined the physics faculty at ETH Zurich in March 1993. I also would like to acknowledge the financial support from ETH Zurich and the Swiss National Science Foundation (SNSF) within the NCCR Quantum Photonics that allowed me to start a new research focus in attoscience in 2001 and the great introduction to high field laser physics I received in Prof. Anne L’Huillier’s group at the Lund University in Sweden during my sabbatical in 2001.

Received: March 6, 2011

[1] M. Ferray, A. L’Huillier, X. F. Li, L. A. Lompré, G. Mainfray, C. Manus, J. Phys. B: At. Mol. Opt. Phys. 1988, 21, L31.

[2] A. McPherson, G. Gibson, H. Jara, U. Johann, T. S. Luk, I. A. McIntyre, K. Boyer, C. K. Rhodes, J. Opt. Soc. Am. B 1987, 4, 595.

[3] P. F. Moulton, J. Opt. Soc. Am. B 1986, 3, 125.[4] H. R. Telle, G. Steinmeyer, A. E. Dunlop, J.

Stenger, D. H. Sutter, U. Keller, Appl. Phys. B 1999, 69, 327.

[5] A. Apolonski, A. Poppe, G. Tempea, C. Spielmann, T. Udem, R. Holzwarth, T. W. Hänsch, F. Krausz, Phys. Rev. Lett. 2000, 85, 740.

[6] D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, S. T. Cundiff, Science 2000, 288, 635.

[7] M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, T. Westerwalbesloh, U. Kleineberg, U. Heinzmann, F. Krausz, Nature 2002, 419, 803.

[8] U. Keller, IEEE Photonics Journal 2010, 2, April.

[9] T. Südmeyer, S. V. Marchese, S. Hashimoto, C. R. E. Baer, G. Gingras, B. Witzel, U. Keller, Nature Photonics 2008, 2, 599.

[10] J. Aus der Au, G. J. Spühler, T. Südmeyer, R. Paschotta, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, U. Keller, Opt. Lett. 2000, 25, 859.

http://www.ingentaconnect.com/content/external-references?article=0028-0836(2002)419L.803[aid=7089484]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2002)419L.803[aid=7089484]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2002)419L.803[aid=7089484]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2000)288L.635[aid=9573299]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2000)288L.635[aid=9573299]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2000)288L.635[aid=9573299]http://www.ingentaconnect.com/content/external-references?article=0740-3224(1986)3L.125[aid=561439]http://www.ingentaconnect.com/content/external-references?article=0740-3224(1986)3L.125[aid=561439]http://www.ingentaconnect.com/content/external-references?article=0740-3224(1986)3L.125[aid=561439]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2000)25L.859[aid=9573297]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2000)25L.859[aid=9573297]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2000)25L.859[aid=9573297]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2000)25L.859[aid=9573297]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2000)85L.740[aid=9573300]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2000)85L.740[aid=9573300]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2000)85L.740[aid=9573300]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2000)85L.740[aid=9573300]

298  CHIMIA 2011, 65, No. 5  NCCR MUST

[11] S. V. Marchese, C. R. E. Baer, A. G. Engqvist, S. Hashimoto, D. J. H. C. Maas, M. Golling, T. Südmeyer, U. Keller, Opt. Express 2008, 16, 6397.

[12] C. R. E. Baer, C. Kränkel, C. J. Saraceno, O. H. Heckl, M. Golling, R. Peters, K. Petermann, T. Südmeyer, G. Huber, U. Keller, Opt. Lett. 2010, 35, 2302.

[13] T. Eidam, S. Hanf, E. Seise, T. V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, A. Tünnermann, Opt. Lett. 2010, 35, 94.

[14] P. Russbueldt, T. Mans, J. Weitenberg, H. D. Hoffmann, R. Poprawe, Opt. Lett. 2010, 34, 4169.

[15] C. J. Saraceno, C. Schriber, M. Mangold, M. Hoffmann, O. H. Heckl, C. R. E. Baer, M. Golling, T. Südmeyer, U. Keller, IEEE J. Sel. Top. Quantum Electron. 2011, DOI: 10/1109/JSTQE.2010.2092753.

[16] T. Südmeyer, C. Kränkel, C. R. E. Baer, O. H. Heckl, C. J. Saraceno, M. Golling, R. Peters, K. Petermann, G. Huber, U. Keller, Appl. Phys. B 2009, 97, 281.

[17] T. Südmeyer, F. Brunner, E. Innerhofer, R. Paschotta, K. Furusawa, J. C. Baggett, T. M. Monro, D. J. Richardson, U. Keller, Opt. Lett. 2003, 28, 1951.

[18] C. J. Saraceno, O. H. Heckl, C. R. E. Baer, T. Südmeyer, U. Keller, Opt. Express 2011, 19, 1395.

[19] M. Holler, Ph.D. thesis, ETH Zurich Nr. 18782, 2009.

[20] F. Schapper, Ph.D. thesis, ETH Zurich Nr. 18783, 2009.

[21] K. J. Schafer, M. B. Gaarde, A. Heinrich, J. Biegert, U. Keller, Phys. Rev. Lett. 2004, 92, 023003.

[22] M. B. Gaarde, K. J. Schafer, A. Heinrich, J. Biegert, U. Keller, Phys. Rev. A 2005, 72, 013411.

[23] A. Zaïr, M. Holler, A. Guandalini, F. Schapper, J. Biegert, L. Gallmann, U. Keller, A. S. Wyatt, A. Monmayrant, I. A. Walmsley, E. Cormier, T. Auguste, J. P. Caumes, P. Salières, Phys. Rev. Lett. 2008, 100, 143902.

[24] T. Auguste, P. Salières, A. S. Wyatt, A. Monmayrant, I. A. Walmsley, E. Cormier, A. Zaïr, M. Holler, A. Guandalini, F. Schapper, J. Biegert, L. Gallmann, U. Keller, Phys. Rev. A 2009, 80, 033817.

[25] M. Holler, A. Zaïr, F. Schapper, T. Auguste, E. Cormier, A. Wyatt, A. Monmayrant, I. A. Walmsley, L. Gallmann, P. Salières, U. Keller, Opt. Express 2009, 17, 5716.

[26] F. Schapper, M. Holler, T. Auguste, A. Zair, M. Weger, J. P. Caumes, P. Salieres, L. Gallmann, U. Keller, Opt. Express 2010, 18, 2987.

[27] P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, P. Balcou, H. G. Muller, P. Agostini, Science 2001, 292, 1689.

[28] R. Lopez-Martens, K. Varju, P. Johnsson, J. Mauritsson, Y. Mairesse, P. Salieres, M. B. Gaarde, K. J. Schafer, A. Persson, S. Svanberg, C.-G. Wahlstrom, A. L’Huillier, Phys. Rev. Lett. 2005, 94, 033001.

[29] M. Holler, F. Schapper, L. Gallmann, U. Keller, Phys. Rev. Lett. 2011, 106, 123601.

[30] P. Eckle, M. Smolarski, P. Schlup, J. Biegert, A. Staudte, M. Schöffler, H. G. Muller, R. Dörner, U. Keller, Nat. Phys. 2008, 4, 565.

[31] P. Eckle, A. Pfeiffer, C. Cirelli, A. Staudte, R. Dörner, H. G. Muller, M. Büttiker, U. Keller, Science 2008, 322, 1525.

[32] R. Dörner, V. Mergel, O. Jagutzki, L. Spielberger, J. Ullrich, R. Moshammer, H. Schmidt-Bocking, Phys. Rep. 2000, 330, 95.

[33] A. N. Pfeiffer, C. Cirelli, M. Smolarski, R. Dörner, U. Keller, Nat. Phys. 2011, 7, 428; K. Ueda, K. L. Ishikawa, Nat. Phys. 2011, 7, 371.

[34] A. N. Pfeiffer, C. Cirelli, M. Smolarski, R. Dorner, U. Keller, ‘Breakdown of the inde-pen dent electron approximation in sequen tial double ionization’, submitted.

[35] R. Kienberger, E. Goulielmakis, M. Uiberacker, A. Baltuska, V. Yakovlev, U. Heinzmann, M. Drescher, F. Krausz, Nature 2004, 427, 817.

[36] M. Schultze, M. Fiess, N. Karpowicz, J. Gagnon, M. Korbman, M. Hofstetter, S. Neppl, A. L. Cavalieri, Y. Komninos, T. Mercouris, C. A. Nicolaides, R. Pazourek, S. Nagele, J. Feist, J. Burgdorfer, A. M. Azzeer, R. Ernstorfer, R. Kienberger, U. Kleineberg, E. Goulielmakis, F. Krausz, V. S. Yakovlev, Science 2010, 328, 1658.

[37] A. N. Pfeiffer, C. Cirelli, M. Smolarski, D. Dimitrovski, M. Abu-Samha, L. B. Madsen, U. Keller, ‘Attoclock reveals geometry of laser-induced tunnel ionization’, http://arxiv.org/abs/1103.4803v2.

http://www.ingentaconnect.com/content/external-references?article=0028-0836(2004)427L.817[aid=9573303]http://www.ingentaconnect.com/content/external-references?article=0370-1573(2000)330L.95[aid=9573306]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2008)322L.1525[aid=9572968]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2010)328L.1658[aid=9573273]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2010)328L.1658[aid=9573273]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2011)106L.123601[aid=9573308]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2005)94L.033001[aid=9573309]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2005)94L.033001[aid=9573309]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2001)292L.1689[aid=9573310]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2008)100L.143902[aid=9573314]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2008)100L.143902[aid=9573314]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2004)92L.023003[aid=9573316]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2004)92L.023003[aid=9573316]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2003)28L.1951[aid=9573318]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2003)28L.1951[aid=9573318]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2010)35L.94[aid=9573321]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2010)35L.2302[aid=9573322]http://www.ingentaconnect.com/content/external-references?article=0146-9592(2010)35L.2302[aid=9573322]http://arxiv.org/

NCCR MUST� CHIMIA�2011,�65,�No.�5� 299doi:10.2533/chimia.2011.299�� Chimia�65�(2011)�299–302� ©�Schweizerische�Chemische�Gesellschaft

*Correspondence:�Prof.�Dr.�H.�J.�WörnerETH�ZürichLaboratory�for�Physical�ChemistryWolfgang-Pauli-Strasse�10CH-8093�ZürichTel.:�+41�44�633�4412Fax:�+41�44�632�1538E-mail:�woerner@phys.chem.ethz.ch

High-Harmonic Spectroscopy: From Femtosecond to Attosecond Molecular Dynamics

Hans�Jakob�Wörner*

Abstract: The�principles�of�high-harmonic�spectroscopy�(HHS)�are�illustrated�using�two�recent�examples�from�the�literature.�The�method�can�be�applied�as�a�probe�to�measure�the�evolution�of�the�electronic�structure�of�a�molecule�during�a�chemical�reaction�on�the�femtosecond�time�scale.�Alternatively,�it�can�be�used�as�a�compre-hensive�method�unifying�pump�and�probe�steps�into�a�single�laser�cycle�to�measure�electronic�dynamics�on�the�attosecond�time�scale.

Keywords: High-harmonic�spectroscopy

Introduction

Time-resolved measurements of molecular and chemical dynamics have long been lim-ited to several femtoseconds (1 fs = 10–15 s). This fundamental limit arose from the op-tical period of the carrier wave. In the past decade, new techniques have emerged that offer attosecond temporal resolution (1 as = 10–18 s).[1,2] They extend time-resolved chemistry from the atomic to the electronic time scale and provide the opportunity to study unexplored phenomena in the time domain.

The valence shell of molecules is char-acterized by typical energy intervals of a few electron-volts (eV), implying that valence-shell dynamics will be observ-able with light or electron pulses around and below 1 fs. The last few years have seen first successes in the observation of electronic dynamics on this time scale.[3–7] These techniques will enable a new experi-mental access to electronic structure and electron correlation, which is one of the most challenging problems in chemistry and physics.

All of these new techniques with at-tosecond temporal resolution rely on the

interaction of a molecule with an intense femtosecond laser pulse. Most of them share a common fundamental mechanism, often summarized in a three-step model.[8] Every time that the laser electric field reaches a maximum, a small portion of the valence electron wave function is moved into the continuum. This process can be understood as tunneling of the electron wave packet through the combined mo-lecular and laser electrostatic potential. The unbound electron wave packet is then exposed to the laser field, and can recollide with the parent molecule close to a zero-crossing of the electric field. This phe-nomenon, ‘laser-induced recollision’, en-ables new measurement techniques: pho-torecombination leads to attosecond pulse production[9] whereas elastic and inelastic electron rescattering offer new approaches to diffraction imaging.[2,10,11]

In typical experiments, the electron has acquired up to 100 eV of kinetic energy during its transit in the continuum and can release it in the form of an attosec-ond pulse. Since the emission takes place around each zero-crossing of the field, the described mechanism generates a train of attosecond pulses separated by half the pe-riod of the generating laser field. In the fre-quency domain, this emission corresponds to high-order harmonics of the fundamen-tal photon energy ω (Ω = ω, 3ω, ..., 101ω ... etc.).

In addition to being a source of ultra-short laser pulses, high-harmonic emission also encodes information about the elec-tronic structure of the molecule. This prop-erty arises from the de-Broglie wavelength of the recolliding electron which amounts to ~1–2 Å under typical conditions. Since the returning electron wave diffracts in the electrostatic molecular potential before it recombines, the high-harmonic spectrum

reveals structural signatures such as two-center interference[12] and features of elec-tronic structure.[13–16]

In addition, a high-harmonic spectrum also encodes dynamical information. Un-der precisely controlled experimental con-ditions, each photon energy in the spectrum is emitted by a unique electron trajectory, i.e. a unique time interval between ioniza-tion and recombination (0–1.7 fs for an 800 nm driving field). This has been used to track the motion of protons on the at-tosecond time scale[17,18] and to study elec-tron rearrangement following temporally confined ionization.[4–6]

The most unusual aspect of these new methods is certainly the possibility to measure attosecond dynamics using multi-femtosecond pulses.[2] This property origi-nates in the extreme nonlinearity of the laser-matter interaction at high intensities. The nonlinearity allows the laser period to be subdivided. Rather than following the envelope of the pulse profile, the ionized electron follows individual oscillations of the electric field. The problem remains tractable thanks to quantum mechanical diffusion: the second, third and subsequent returns of the electron wave packet have quickly decreasing amplitudes. Since the three-step model relies on simplifying as-sumptions, like the single-active electron and strong-field approximations, it should only be used as a guide to the intuition and improved by accurate calculations (quan-tum scattering etc.) when detailed informa-tion is being retrieved.

Probing Chemical Reactions

Photochemical reaction pathways are controlled by the valence electrons of the reacting molecule. An elucidation of the

300� CHIMIA�2011,�65,�No.�5� NCCR MUST

the coherent addition |E1,1

+ E2,2

|2 of the emissions from the ground and excited states. E

1,1 can be used as a static refer-

ence to measure dynamical changes in E

2,2. This detection scheme is highly sensi-

tive to weak emission and simultaneously probes its amplitude and phase[23,24] which are conveniently separated using the tran-sient grating method,[26,27] illustrated in Fig. 1a. Two synchronized replica of the pump pulse are crossed under a small angle in a supersonic expansion of the investi-gated molecules. The periodic modulation of the excitation fraction leads to a spatial modulation of the amplitude and phase of harmonic emission in the generating me-dium and results in diffraction along the direction of the modulation.[23]

Writing the emission from the ground and excited states in terms of the excitation fraction r = |c

2|2 and high-harmonic ampli-

tudes and phases, we obtain

EHHG

(Ω) = (1 − r) dg exp(if

g) + r d

e

exp(ife) (4)

where r = rmax

/2*(cos(kx)+1) is the exci-tation fraction modulated between 0 and r

max along the spatial dimension x. The

far-field profile that is observed on the detector is the Fourier transform of this quantity. The undiffracted high-harmonic signal is similar to a measurement carried out in collinear excitation with the same spatially averaged excitation fraction (see ref. [23] and [24]). The diffracted high-harmonic intensity is proportional to r2/4|d

e

exp(ife) − d

g exp(if

g)|2, and thus represents

a background-free measurement of the ex-cited state dynamics through 1:1 interfer-ence with the ground state emission. Since the excitation fraction is determined from the known absorption cross section and the measured pump laser intensity, the relative amplitudes and phases d

e(Δt)/d

g and f

e(Δt)

− fg can be extracted. The method described above has been

associated mechanisms ideally requires the time-resolved observation of the rear-rangement of the electronic structure as the reaction proceeds. Promising methods for this purpose thus need to be sensitive to the valence electron structure. Moreover, exci-tation fractions must remain low in femto-second single-photon excitation in order to avoid multiphoton processes upon excita-tion. Therefore, the methods should also be sensitive enough to detect the excited spe-cies. High-harmonic spectroscopy fulfills both of these requirements.

In the spirit of the three-step model, the emitted high-harmonic field can be written as a product of three terms

EHHG

(Ω) = aion

(Ω,θ)aprop

(Ω)arec

(Ω,θ) (1)

where Ω is the emitted photon energy and θ characterizes the orientation of the mole-cule relative to the polarization of the laser field. The last step, photorecombination, is now well understood in terms of quantum scattering calculations developed in the context of photoionization.[15,16,19] Most remarkably, the strong laser field does not lead to a measurable distortion of the elec-tronic wave functions within the accuracy of current experiments, even in polarizable systems like xenon[20] or in molecules with appreciable extension along the direction of the laser field like CO

2.[4,5] The second

step, propagation of the electron wave-packet, is largely dominated by the laser field and is well described by classical equations as evidenced by, e.g. the chirp of attosecond pulses.[21,22] The first two steps together produce an electron wave packet that weakly depends on the molecular ori-entation, whereas recombination is highly sensitive to the molecular frame. Eqn. (1) describes the case where the molecule is initially in the electronic ground state and multiphoton ionization prepares the cation in a single electronic state. We now extend the formalism to describe a photochemical

reaction.[23,24] A weak femtosecond pump pulse prepares the molecule in a coherent superposition of the ground state and an excited electronic state through one-pho-ton absorption

Ψ(Δt) = c1ψ

1 + c

2(Δt)ψ

2(Δt) (2)

with ψ

i being the multielectron wave func-

tion of state i and Δt the time delay after excitation. The high-harmonic field emit-ted from this coherent superposition can be expressed as

EHHG

(Ω) = |c1|2E

1,1 + |c

2|2E

2,2 + c

1c

2*E

1,2 + c

2c

1*E

2,1 (3)

The first two contributions describe high-harmonic generation in a single elec-tronic state and are well described by Eqn. (1). E

1,2 and E

2,1 represent ‘cross terms’ that

are sensitive to the electronic coherence between states 1 and 2. In addition, these contributions are also proportional to the overlap of the nuclear wave functions in the two electronic states, which vanishes quickly in the case of photodissociation.[23]

The high-harmonic emission from a photoexcited molecule can thus be un-derstood from the coherent addition of the electromagnetic fields emitted by the ground and excited electronic states. In addition to Eqn. (1), the relative phase of the high-harmonic emission from the two states contributed by the propagation of the electron in the laser field plays a role. It is well approximated by ΔI

pτ where τ is the

electron transit time in the continuum.[25] This finding is intuitive since the contri-bution to the relative phase is essentially equal to the difference in phase accumu-lated by the bound state.

When a photochemical reaction is probed by HHS, the ground electronic state, ψ

1 in Eqn. (2), is essentially static,

whereas ψ2 undergoes the interesting dy-

namics. Hence, the measured intensity is

800 nmprobe pulse

400 nmpump pulses

a) b)

0 20 40 60 80 100

2.5

3

3.5

4

4.5

(Å)

pump−probe delay (fs)

wavepacket calculation

experimental data

Fig.�1.�High-harmonic�transient�grating�spectroscopy.�(a)�Experimental�setup:�Two�synchronized�excitation�pulses�(400 nm)�set�up�a�transient�grating�of�excitation�in�the�molecular�beam.�A�delayed�800�nm�pulse�generates�high�harmonics�from�the�excited�sample.�(b)�Internuclear�separation�of�Br2�as�a�function�of�the�time�delay�after�photoexcitation�to�the�C�1P1u�state.�Data�from�ref.�[23].

NCCR MUST� CHIMIA�2011,�65,�No.�5� 301

of the laser field, the transit time τ scales approximately linearly with the wave-length λ, enabling a controlled variation of the relative phase.

Since the minimum observed in the emission from aligned CO

2 molecules had

been attributed to either structural[14] or dynamical origins,[4] we have measured high-harmonic spectra of aligned CO

2 at

different wavelengths and intensities.[5] The observed spectra are shown in Fig. 2. For a generating wavelength of 800 nm, the minimum in aligned CO

2 is observed

at 42 eV, whereas at 1200 nm, it is ob-served at 57 eV. This clearly shows that the minimum is not of structural origin because its position would be independent of the laser parameters, as in the case of argon.[6] Does the observed position agree with a dynamical minimum? At 800 nm, the relative phase accumulated by the two orbitals amounts to ~3π but at 1200 nm, the minimum corresponds to ~4.5π. De-tailed calculations show that the differ-ence arises from the photorecombination dipole moment of HOMO that undergoes a sudden phase variation at the position of the structural minimum.[5] Thus, the mini-mum observed in CO

2 is mostly dynami-

cal but its position is affected by structural properties as well.

In the future, high-harmonic spectros-copy of attosecond dynamics may be op-erational and relevant in larger molecular systems in which the orbital relaxation upon ionization is significant. In such cases the orbitals of the neutral molecule differ from those of the ion and ionization will

applied to the photodissociation of Br2

molecules.[23,24] Single-photon excitation at 400 nm leads to the repulsive C 1P

1u

state that dissociates into Br atoms in the 2P

3/2 ground electronic state. The relative

high-harmonic amplitudes and phases for a range of harmonic orders (H13 to H21) have been determined from the experiment.[23] The amplitude measurements revealed a harmonic-order dependent minimum as a function of the pump-probe time delay. These minima occur when the de-Broglie wavelength of the recombining electron wave emitting the qth harmonic fulfills a destructive interference condition with the internuclear separation of the molecule R = 3/2λ

q. Using this relation and the measured

position of the amplitude minima, one can determine the internuclear separation as a function of time as shown in Fig. 1b. In addition to the amplitudes, one also obtains the relative harmonic phases. The phases undergo a fast variation with the pump-probe delay that reflects the variation of I

p

with the internuclear separation. The phase of the photorecombination dipole also con-tributes to the high-harmonic phase and characterizes the change in the electronic structure of the dissociating molecule.

Probing Electronic Dynamics

After discussing the application of HHS as a probe in femtosecond time-resolved experiments, we now turn to the attosecond time scale. High-harmonic generation can be used as a comprehensive method unify-ing the pump step (multiphoton ionization) and the probe step (photorecombination) into a single laser cycle. The experiment is thus repeated in each half-cycle of the laser field and the temporal information is read on the photon energy axis, exploiting the one-to-one relationship between transit time and emitted photon energy.[17]

Recent experiments show that strong-field ionization does not exclusively pre-pare the ground state of the cation[4–6,28] but that excited states can also be populated. The probability of accessing a certain ion-ic state is exponentially sensitive to I

p[29]

but also to the shape of orbitals,[30] which can enhance the contribution of a deeply bound orbital. When ionization accesses two states of the ion and the laser-induced dynamics in the ion is neglected, high-harmonic generation has two contributions E

HHG(Ω) = E

1,1 + E

2,2 (5)

which correspond to ionization from and recombination to one of the valence orbit-als of the molecule. Remarkably, a cross term of the form encountered in Eqn. (3) is absent from Eqn. (4) because recombi-nation cannot take place to an occupied

orbital. The photorecombination step thus measures the phase shift accumulated by the superposition of two ionic states be-tween ionization and recombination. How-ever, the process is insensitive to the coher-ence between the two states. This differs from other recently introduced methods, like attosecond transient absorption[7] and strong-field ionization[31] to probe elec-tronic dynamics.

Carbon dioxide is an instructive model system to understand multielectron effects in HHS. When CO

2 is aligned parallel to

the laser field generating the high harmon-ics, emission from HOMO of π

g symmetry

is suppressed by the presence of a nodal plane along the direction of tunneling and recombination. HOMO-2 of s

u symmetry

lies 4.3 eV below HOMO but its emission is not suppressed by the presence of nodal planes. HOMO-1 is of π

u symmetry and is

thus suppressed by both a nodal plane and a larger I

p. Thus, HHS can be described as

a sum of contributions from HOMO and HOMO-2, i.e.

EHHG(Ω) = αion, i(Ω,θ)αprop, i(Ω)αrec, i(Ω,θ)i=1

2

∑(6)

Minima in EHHG

(Ω) may thus be of structural origin and appear in a

rec,i(Ω,θ)

or of dynamical origin and result from de-structive interference in the coherent sum. These two situations can be distinguished by varying the relative phase well approxi-mated by ΔI

pτ[25] of the two propagation

terms in Eqn. (6). For a given intensity

30 40 50 60 70

0.001

0.1

10

alignedrandom

30 40 50 60 70

0.001

0.1

10

photon energy (eV)

harm

onic

inte

nsity

(arb

.u.)

1200 nm, 1.2.1014 W/cm2

800 nm, 2.1.1014 W/cm2

Fig.�2.�High-harmonic�spectroscopy�of�aligned�CO2�molecules.�The�central�wavelength�and�peak�intensities�of�the�laser�pulses�used�to�generate�the�high-harmonic�spectra�are�indicated�in�the�figures.�

302� CHIMIA�2011,�65,�No.�5� NCCR MUST

trigger an electronic wave packet whose motion encodes the effects of electron cor-relation.

Conclusions and Outlook

The two examples discussed in this arti-cle highlight the potential of high-harmonic spectroscopy towards two different direc-tions. The method will be applied as a probe to measure the rearrangement of the valence electronic structure of molecules undergo-ing chemical reactions on the femtosecond time scale. In the future, this will be particu-larly interesting to probe changes in elec-tronic configurations occurring at conical intersections. High-harmonic spectroscopy will be sensitive to electronic symmetries and map them into the amplitude, phase and polarization of the emitted radiation. On the attosecond time scale, high-harmonic spec-troscopy may become an important tool to investigate purely electronic dynamics. In the majority of polyatomic molecules, the orbital relaxation upon ionization is significant. In such cases, high-harmonic spectroscopy will offer a new approach to probing electron correlation – a property that is hard to measure by other means. We anticipate fruitful collaborations on both of these topics within NCCR MUST.

AcknowledgementsThe work described in this article was

performed at the National Research Council of Canada in collaboration with Mr. J. B. Bertrand, Dr. Y. Mairesse, Prof. P. B. Corkum and Dr. D. M. Villeneuve. Our current work in Zürich is supported by the Swiss Science Foundation under grant PP00P2_128274.

Received: March 5, 2011

[1] M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, T. Westerwalbesloh, U. Kleineberg, U. Heinzmann, F. Krausz, Nature 2002, 419, 803.

[2] H. Niikura, F. Légaré, R. Hasbani, A. D. Bandrauk, M. Y. Ivanov, D. M. Villeneuve, P. B. Corkum, Nature 2002, 417, 917.

[3] P. Eckle, A. N. Pfeiffer, C. Cirelli, A. Staudte, R. Dörner, H. G. Muller, M. Büttiker, U. Keller, Science 2008, 322, 1525.

[4] O. Smirnova, Y. Mairesse, S. Patchkovskii, N. Dudovich, D. Villeneuve, P. Corkum, M. Y. Ivanov, Nature 2009, 460, 972.

[5] H. J. Wörner, J. B. Bertrand, P. Hockett, P. B. Corkum, D. M. Villeneuve, Phys. Rev. Lett. 2010, 104, 233904.

[6] S. Haessler, J. Caillat, W. Boutu, C. Giovanetti-Teixeira, T. Ruchon, T. Auguste, Z. Diveki, P. Breger, A. Maquet, B. Carré, R. Taïeb, P. Salières, Nat. Phys. 2010, 6, 200.

[7] E. Goulielmakis, Z.-H. Loh, A. Wirth, R. Santra, N. Rohringer, V. S. Yakovlev, S. Zherebtsov, T. Pfeifer, A. M. Azzeer, M. F. Kling, S. R. Leone, F. Krausz, Nature 2010, 466, 739.

[8] P. B. Corkum, Phys. Rev. Lett. 1993, 71, 1994. [9] E. Goulielmakis, M. Schultze, M. Hofstetter,

V. S. Yakovlev, J. Gagnon, M. Uiberacker, A. L. Aquila, E. M. Gullikson, D. T. Attwood, R. Kienberger, F. Krausz, U. Kleineberg, Science 2008, 320, 1614.

[10] H. Niikura, F. Légaré, R. Hasbani, M. Y. Ivanov, D. M. Villeneuve, P. B. Corkum, Nature 2003, 421, 826.

[11] M. Meckel, D. Comtois, D. Zeidler, A. Staudte, D. Pavičić, H. C. Bandulet, H. Pépin, J. C. Kieffer, R. Dörner, D. M. Villeneuve, P. B. Corkum, Science 2008, 320, 1478.

[12] M. Lein, N. Hay, R. Velotta, J. Marangos, P. Knight, Phys. Rev. Lett. 2002, 88, 183903.

[13] J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pépin, J. C. Kieffer, P. B. Corkum, D. M. Villeneuve, Nature 2004, 432, 867.

[14] T. Kanai, S. Minemoto, H. Sakai, Nature 2005, 435, 470.

[15] T. Morishita, A.-T. Le, Z. Chen, C. D. Lin, Phys. Rev. Lett. 2008, 100, 013903.

[16] H. J. Wörner, H. Niikura, J. B. Bertrand, P. B. Corkum, D. M. Villeneuve, Phys. Rev. Lett. 2009, 102, 103901.

[17] S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirilǎ, M. Lein, J. W. G. Tisch, J. P. Marangos, Science 2006, 312, 424.

[18] S. Baker, J. S. Robinson, M. Lein, C. C. Chirilǎ, R. Torres, H. C. Bandulet, D. Comtois, J. C. Kieffer, D. M. Villeneuve, J. W. G. Tisch, J. P. Marangos, Phys. Rev. Lett. 2008, 101, 053901.

[19] A.-T. Le, R. R. Lucchese, S. Tonzani, T. Morishita, C. D. Lin, Phys. Rev. A 2009, 80, 013401.

[20] A. D. Shiner et al., Nat. Phys. 2011, doi: 10.1038/nphys1940.

[21] Y. Mairesse, A. de Bohan, L. J. Frasinski, H. Merdji, L. C. Dinu, P. Monchicourt, P. Breger, M. Kovačev, R. Taïeb, B. Carré, H. G. Muller, P. Agostini, P. Salières, Science 2003, 302, 1540.

[22] G. Doumy, J. Wheeler, C. Roedig, R. Chirla, P. Agostini, L. F. DiMauro, Phys. Rev. Lett. 2009, 102, 093002.

[23] H. J. Wörner, J. B. Bertrand, D. V. Kartashov, P. B. Corkum, D. M. Villeneuve, Nature 2010, 466, 604.

[24] H. J. Wörner, J. B. Bertrand, P. B. Corkum, D. M. Villeneuve, Phys. Rev. Lett. 2010, 105, 103002.

[25] T. Kanai, E. J. Takahashi, Y. Nabekawa, K. Midorikawa, Phys. Rev. Lett. 2007, 98, 153904.

[26] H. J. Euchler, P. Günther, D. Pohl, ‘Laser-Induced Dynamic Gratings’, Springer Verlag, 1986.

[27] Y. Mairesse, D. Zeidler, N. Dudovich, M. Spanner, J. Levesque, D. M. Villeneuve, P. B. Corkum, Phys. Rev. Lett. 2008, 100, 143903.

[28] B. K. McFarland, J. P. Farrell, P. H. Bucksbaum, M. Gühr, Science 2008, 322, 1232.

[29] M. V. Ammosov, N. B. Delone, V. P. Krainov, Zh. Eksp. Teor. Fiz. 1986, 91, 2008.

[30] J. Muth-Böhm, A. Becker, F. H. M. Faisal, Phys. Rev. Lett. 2000, 85, 2280.

[31] H. J. Wörner, P. B. Corkum, J. Phys. B: At. Mol. Opt. Phys. 2011, 44, 041001.

http://www.ingentaconnect.com/content/external-references?article=0953-4075(2011)44L.041001[aid=9572944]http://www.ingentaconnect.com/content/external-references?article=0953-4075(2011)44L.041001[aid=9572944]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2000)85L.2280[aid=9572945]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2008)322L.1232[aid=9572947]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2008)100L.143903[aid=9572948]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2007)98L.153904[aid=9572949]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2010)105L.103002[aid=9572950]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2010)105L.103002[aid=9572950]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2010)466L.604[aid=9572951]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2010)466L.604[aid=9572951]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2009)102L.093002[aid=9572952]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2009)102L.093002[aid=9572952]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2008)101L.053901[aid=9572955]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2003)302L.1540[aid=9572953]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2006)312L.424[aid=9572956]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2006)312L.424[aid=9572956]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2009)102L.103901[aid=9572957]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2009)102L.103901[aid=9572957]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2008)100L.013903[aid=9572958]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2005)435L.470[aid=9572959]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2005)435L.470[aid=9572959]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2004)432L.867[aid=9037358]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2002)88L.183903[aid=9572960]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2008)320L.1478[aid=9572961]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2003)421L.826[aid=9572962]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2003)421L.826[aid=9572962]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2008)320L.1614[aid=9572963]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2008)320L.1614[aid=9572963]http://www.ingentaconnect.com/content/external-references?article=0031-9007(1993)71L.1994[aid=9037357]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2010)466L.739[aid=9572964]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2009)460L.972[aid=9572967]http://www.ingentaconnect.com/content/external-references?article=0036-8075(2008)322L.1525[aid=9572968]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2002)417L.917[aid=9572969]http://www.ingentaconnect.com/content/external-references?article=0028-0836(2002)419L.803[aid=7089484]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2010)104L.233904[aid=9572966]http://www.ingentaconnect.com/content/external-references?article=0031-9007(2010)104L.233904[aid=9572966]http://dx.doi.org/10.1038/nphys1940http://dx.doi.org/10.1038/nphys1940

NCCR MUST� CHIMIA�2011,�65,�No.�5� 303doi:10.2533/chimia.2011.303�� Chimia�65�(2011)�303–307� ©�Schweizerische�Chemische�Gesellschaft

*Correspondence:�Prof.�Dr.�M.�CherguiÉcole�Polytechnique�Fédérale�de�Lausanne�(EPFL)Laboratory�of�Ultrafast�SpectroscopyEPFL-SB-ISIC-LSUStation�6CH-1015�LausanneTel.:�+�41�21�693�0447Fax:�+�41�21�693�0365E-mail:�majed.chergui@epfl.ch

Ultrafast X-ray Absorption Studies of the Structural Dynamics of Molecular and Biological Systems in Solution

Christopher�J.�Milne,�Renske�M.�Van�der�Veen,�Van-Thai�Pham,�Frederico�A.�Lima,�Hannelore�Rittmann-Frank,�Marco�Reinhard,�Frank�van�Mourik,�Susanne�Karlsson,�Thomas�J.�Penfold,�and�Majed�Chergui*

Abstract: We�review�our�recent�studies�of�excited�state�structures�and�dynamics�of�chemical�and�biological�sys-tems�with�pico-�and�femtosecond�X-ray�absorption�spectroscopy�in�the�liquid�phase.

Keywords: X-ray�absorption�spectroscopy

1. Introduction

Chemical reactivity and biological func-tion are intimately linked to structure and structural dynamics on the molecular level. The advent of ultrafast spectroscopy in the mid-1980s has allowed the probing of chemical reactions in real-time.[1] How-ever, these studies are carried out using probe pulses in the optical domain, which only delivers information on the electronic structure. Therefore, over the past 10–15 years a huge effort was made aimed at us-ing electron or X-ray probe pulses that can deliver geometric structural information such as bond lengths and bond angles, via scattering, diffraction, or X-ray spectros-copy which additionally delivers informa-tion about the electronic structure.[2] Our primary efforts have mostly been focussed on developing and using time-domain X-ray absorption spectroscopy (XAS).[3,4] X-ray absorption spectra are characterized by absorption edges, which are due to excita-tion of the core electrons of a given atom to the ionization threshold. An absorption edge consists of two regions: the X-ray ab-sorption near edge structure (XANES) at

energy just below and just above (typically 50 eV) above the edge. For an atom embedded in a molecule, crystal or even a liquid, both XANES and EXAFS exhibit modulations, which are due to the multiple and single scattering of the X-ray excited photoelec-tron onto the nearest neighbours.

Our choice of XAS is motivated by the fact that: a) it is element specific; b) it delivers information about the local struc-ture (coordination numbers and bond dis-tances) around the atom of interest, c) just below the edge bound-bound transitions occur, which contain information about the electronic structure (such as oxida-tion state, occupancy of valence orbitals, charge transfer, etc.) of the system; d) last but not least, it can be implemented in any phase of matter, and in particular in liquids, which are the natural environment of biol-ogy and of most of chemistry.

Time-resolved XAS requires tunable X-radiation with sufficiently short pulses, which can be synchronized to an external light source (e.g. a pump laser). Storage rings, such as Swiss Light Source (SLS) at the Paul-Scherrer-Institut in Villigen, offer the required specificities in terms of flux, energy range and stability, but the synchrotron X-ray pulses have widths of 50–100 ps. Therefore, in order to push into the femtosecond time domain, which is the fundamental time scale in chemistry, materials science and biology, the slicing scheme was implemented in the hard X-ray range at the MicroXas beamline of the SLS. It provides 100 fs hard X-ray pulses albeit at the cost of a dramatic reduction in flux.[5] In all cases (ps or fs) the method-ology is based on recording the transient (difference spectra of the excited minus the unexcited sample) absorption on a pulse-

to-pulse basis.[6] In the following sections, we illustrate the methodology on a few ex-amples and how it helped answer specific scientific questions.

2. Intramolecular Charge Transfer in Metal Complexes

Transition metal complexes play an important role in chemistry and in many cases in biology. They are also of impor-tance for solar energy research and for the development of molecular devices. Their visible absorption spectrum is dominated by the metal-to-ligand-charge-transfer (MLCT) bands. For the MLCT states, there is an issue of structural changes as a result of charge transfer. We addressed this question in a picosecond XAS study of aqueous ruthenium(ii)-tris-2,2’-bipy-ridine, ([RuII(bpy)

3]2+).[7] The system was

excited in the 1MLCT band, which leads to population of the 3MLCT state on ultra-fast time scales.[8] The L

3 (2p

3/2-4d) and L

2

(2p1/2

-4d) edges of ruthenium were probed, delivering information on the electronic structure, i.e. the occupancy of the d(t

2g)

and d(eg) orbitals in the excited triplet state.

Information about the excited state struc-ture was obtained via the electronic struc-ture (based on the relationship between lig-and field splitting and coordinating struc-ture) and via the EXAFS at the L

3 edge.

Both showed that the structure change is a small (–0.037 ± 0.0135 Å) contraction of the Ru–N bonds.[7b] This relatively weak bond contraction is due to a balance be-tween strong attractive forces between metal and ligand after charge transfer, and steric effects resulting from the three bpy ligands being in a constrained geometry, already in the ground state. Our results were later confirmed by quantum chemi-cal calculations.[9]

304� CHIMIA�2011,�65,�No.�5� NCCR MUST

rium distances elongated by ~0.1 Å, ~0.2 Å, and ~0.3 Å, respectively, relative to the ground state (and MLCT) bond distance. Light excitation into the 1MLCT state leads to population of the lowest quintet state 5T

2 with unity quantum yield.[14] For the

[FeII(bpy)3]2+ complex, which is the small-

est of the family of Fe(ii)-complexes, the quintet state lifetime is the shortest, ~650 ps at room temperature. As a consequence, the quintet state structure could not be determined by quasi-static methods (X-ray diffraction or X-ray absorption spectros-copy). One of the questions that arises is whether the quintet lifetime is determined by the Fe–ligand bond distance, bearing in mind that nearly all SCO complexes exhibit the same bond elongation (typically ~0.2 Å) in the HS state.[17] We determined its HS state structure by recording the Fe K edge XAS of the system with 70 ps X-ray pulses.[18] Fig. 3 shows the XANES of the molecule in the ground state, the transient (difference) spectrum at 50 ps time delay and the XAS spectrum of the quintet state, as retrieved from the ground state and the difference spectra and from the photoly-sis yield determined in laser-only experi-ments (and convoluted to match the much longer X-ray probe width). The structural analysis of the excited state delivered an Fe–N bond elongation of 0.203 ± 0.008

Similar studies have also been car-ried out on halogenated rhenium-carbonyl polypyridine complexes by probing the electronic and geometric structure at both the L-edges of rhenium and the K-edge of the halogen ligand. These will be discussed in a forthcoming publication.

3. Bond Formation in Bimetallic Complexes

The triplet excited states of binuclear d8-d8 platinum, rhodium, and iridium com-plexes (bridged by various ligands) have attracted much attention owing to their re-markable photophysical and photochemi-cal properties, which are strongly deter-mined by their structure. We investigated the tetrakis-m-pyrophosphitodiplatinate(ii) [Pt

2(P

2O

5H

2)

4]4– ion, which is the most

studied binuclear metal complex due, among other reasons, to its photocatalytic activity. In the ground state the two metal atoms are not chemically bound, but exci-tation of the first singlet state results in a bond owing to the promotion of an electron from the antibonding ds* (d

z2-derived) to

the bonding ps (pz-derived) orbitals, which

leads to a contraction of the metal–metal bond. The system then undergoes intersys-tem crossing to the long lived (~1 ms) low-est triplet states.[10] Using time-resolved XAS, van der Veen et al. resolved its geo-metric[11] and electronic[12] structure via EXAFS and XANES, respectively. Fig. 1 shows the ground state Pt L

3-edge XAS

spectrum (black trace) as well as the tran-sient spectrum, integrated from 0 to 150 ns to improve the signal-to-noise ratio. The inset shows the XANES region for the ground-state complex and its transient spectrum, wherein dramatic changes ap-pear. In particular, a new absorption shows up at 11.574 keV, just below the absorption edge, which is due to the creation of a hole in the 5ds* orbital upon laser excitation that can then be accessed from the 2p

3/2

core orbital (L3 edge).[12] Clear changes are

visible in the EXAFS region (Fig. 1b), re-flecting structural modifications between the ground and excited triplet states.

From the transient EXAFS spectrum (Fig. 1b), the magnitude of the Pt–Pt bond contraction as well as, for the first time, the changes affecting the ligand bonds were extracted. In particular (see scheme in Fig. 1b), while the Pt–Pt bond contracts, the Pt–P bonds slightly elongate, in agreement with theoretical predictions.[13] This work underscores the ability to retrieve details of the excited state structure of a rather com-plex molecular system in solution. Howev-er, the X-ray determined structure deviates somewhat from that obtained from optical spectra (the system happens to be highly harmonic in both the ground and the ex-

cited state),[10] and we believe that this may stem from the neglect of the solvent con-tribution in modeling the EXAFS. These effects and eventually coordination of sol-vent molecules to the excited complex is an aspect which we are now investigating in detail (see below).

4. Spin Cross-over Complexes

One of the fascinating features of Fe(ii)-polypyridine complexes is their abil-ity to undergo dramatic spin changes under temperature, pressure or irradiation. They have therefore been named spin cross-over complexes (SCO complexes).[14] In the predominantly octahedral field of the ligands, all electrons are in the lower t

2g sub-shell forming the low spin

(LS) ground state, while transferring elec-trons to the e

g orbitals increases the spin

state. Because the eg

orbitals derive from the d

x2-y

2 and dz2 orbitals, they are antibond-

ing in six-fold coordinated complexes, which leads to a striking metal–ligand bond elongation in the high spin (HS) state. A generic diagram of the potential energy curves of Fe(ii) SCO complexes is shown in the inset to Fig. 2, as a function of the Fe–N bond length. The ligand field states 1,3T, 5T and 5E have their equilib-

Fig.�1.�a)�Static�Pt�L3�XAS�spectrum�of�[Pt2(P2O5H2)4]

4–�in�solution�(black�line,�left�axis)�and�the�transient�(excited�–�unexcited)�XAS�spectrum�(red�circles,�right�axis)�integrated�up�to�150�ns�after�excitation.�The�inset�zooms�into�the�XANES�region.�b)�Transient�EXAFS�data�and�best�fit�(solid�line)�with�the�following�results:�a�Pt–Pt�contraction�of�0.31(5)�Å,�a�Pt-ligand�bond�elongation�of�0.010(6)�Å.[11]�

NCCR MUST