vuv (photoemission) spectroscopy k.c. prince, sincrotrone trieste, trieste, italy

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WUTA, Frascati, 8th October 2008

VUV (photoemission) spectroscopy

K.C. Prince,Sincrotrone Trieste, Trieste, Italy

Gas phase.1. Doubly excited states of helium.2. Biomolecules.3. Dichroism4. Two colours

Surfaces and solids.1. Classical application: band mapping2. Resonant photoemission from thin film catalysts, CeO2/Cu3. Adenine/Cu(110)

The future: the Fermi free electron laser light source.

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Experimental

Gas Phase photoemission beamline, Elettra.VG 220i electron energy analyser.

Our undulator

Our beamline

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Doubly excited states of helium are two electron states in which neither electron is in a 1s orbital:

Naively – nln’l’ n, n’>1Lowest energy series: 2snl, 2pnl, i.e. the states below the second IP, N=2.

For Helium, they are in the energy range 60-80 eV.

Why study them? - The simplest three body problem in atomic physics, - the simplest system where correlation is important,-a benchmark system, etc.

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Doubly excited states were sought (and one state found [1,2]) as early as 1930. After this, there was a pause.

[1] P. G. Kruger, Phys. Rev. 36, 855 (1930).[2] R. P. Madden and K. Codling, Phys. Rev. Lett. 10, 516 (1963); Astrophys. J. 141, 364 (1965). [3] M. Domke et al, Phys. Rev. A 52, 1424 (1996).

The doubly excited states of helium: a brief history

Madden and Codling in the early ’60s then used synchrotron radiation to measure two of the 3 predicted series [2].Fano and Cooper provided the theory, and there was another pause (for experimentalists) while theoreticians thought hard.

1990s: third generation synchrotron light sources threw some light on the matter. Domke et al: observed all three 1Po series below N=2, plus many more below higher N.

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Recent results:- partial VUV fluorescence yield spectra gives a different view of the doubly excited states compared with ion yield [4, 5]- the ion yield does not give the true cross-section for these states [5, 6] - spin-orbit coupling is important, even for He [7]- with photons, you can excite not only 1P states, but also triplets 3P, 3D [8]- He offers a window on “quantum chaology” [9]- Lifetime measurements of the fluorescence tell us about correlation [10]- Stark effects are also interesting (more later) [11, 12]

[4] M. K. Odling-Smee et al, Phys. Rev. Lett. 84, 2598 (2000).[5] Jan-Erik Rubensson et al, Phys. Rev. Lett. 83, 947 (1999).[6] K.C. Prince et al, Phys. Rev. A. 68, 044701 (2003).[7] Thomas Ward Gorczyca et al, Phys. Rev. Lett. 85, 1202 (2000). [8] F. Penent et al, Phys. Rev. Lett. 86, 2758 (2001). [9] R. Püttner et al, Phys. Rev. Lett. 86, 3747 (2001).[10] J. Lambourne et al, Phys. Rev. Lett. 90, 153004 (2003)[11] J. R. Harries et al, Phys. Rev. Lett. 90, 133002 (2003).[12] X.M. Tong and C. D. Lin, Phys. Rev. Lett. 92, 223003 (2004)

The doubly excited states of helium: recent history

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1.6x10-9

1.4

1.2Ion

yiel

d (A

)

64.1564.1464.1364.1264.1164.10Photon energy (eV)

1500

1000

500

Intensity (counts)

Ion yield (upper curve, left scale) and fluorescent UV photon yield (lower curve, right scale) at the (2,-13), (2,14) resonances.

The difference between VUV fluorescence and ion yield.

The long lived state decays by fluorescence, while the shorter lived state decays by autoionization (ion yield). So that state is stronger in partial fluorescence yield.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

Cro

ss s

ectio

n (M

barn

)

64.1564.1464.1364.1264.11

Photon energy (eV)

7x10-10

6

5

4

Ion yield (ampere)

2,14

2,-13Cross section (left axis)

Ion yield (right axis)

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Some recent work: the Stark effect for doubly excited

states.The Stark effect: the response of a quantum system to an external electric field – energy shifts and splitting of magnetic sub-levels.

An early triumph of quantum mechanics.

Fields up to 84 kV/cm; new propensity rule.

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3 ways of looking at the Stark effect

Classical electrostatics: charge moves from one side of atom to other; gives an approximate value of Stark shift.

+ -

Mixing of atomic orbitals picture: explains asymmetric charge distribution.

+ + =+ -

Stark operator mixes states of opposite parity.Nearby states of even parity are mixed.Photoabsorption sees only 1Po states; there are “dark” S, Pe, D, F, etc. states.(Triplet states allowed by spin-orbit coupling, recently observed.)

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Purpose built apparatus: parallel plate condenser, gap 5 mm. Cost approx. 50 cents.

+

Experimental

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Electric field F || P, photon polarization.

Total VUV photon yield.

2500

2000

1500

1000

500

0

Inte

nsity

(arb

. uni

ts)

65.4065.2065.00Photon energy (eV)

(2,06)(2,-15),(2,16)

(2,07)

(2,-16),(2,17)

F=0

F=0.5 kV/cm

F=3.5 kV/cm

2500

2000

1500

1000

500

0

Inte

nsity

(arb

. uni

ts)


(2,06)(2,-15), (2,16) (2,-16), (2,17)

F=0

F=0.5 kV/cm

F=3.5 kV/cm

(2,-1n) and (2,1n) states lose intensity.(2,0n) states gain intensity (maybe anartefact) a broad shoulder develops at higher energy for higher n.

Fields are very low!

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Static field F perpendicular to polarization P

20x103

15

10

5

0

Fluo

resc

ence

yie

ld (a

rb. u

nits

)


(2,16)+

1 1 1 1 2 2

F=0

3 kV/cm

4 kV/cm

5 kV/cm

6 kV/cm

7 kV/cm

A new series is observed, labelled 1.Another new broad series labelled 2 is observed (as in other geometry.)Big increases in intensity at high n.

Quantum defect (1)=-0.435±0.005.Matches 1Pe series.

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How do we explain all this?

First, there are selection rules. Parallel geometry, ΔM=0perpendicular geometry, ΔM=±1.

M= zero for ground and S states, so in the perpendicular geometry there is no mixing of S states.

Then, theory confirms the series “1” is 1Pe.The broad features “2” are due to a pair of 1De states.In the parallel geometry, mixing occurs with 1Se states.

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Quantitative modelling of energies,5 kV/cm, F perpendicular to P.

First order perturbation theory – A. Mihelič and M. Žitnik, Ljubljana.

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VUV spectroscopy is a powerful method for investigating the fluorescencedecay dynamics of He doubly excited states.

The wave functions in the excited state can be probed in detailand agreement with experiment is satisfactory.

Stark effects on these states can be seen at moderate fields (< 1 kV/cm).

A new series of the He doubly excited states observed, the 1Pe series.Indications of other series observed, 1Se and 1De.

Conclusions

V. Feyer, M. Coreno,CNR-IMIP, Montelibretti (Rome), Italy, and INSTM, Trieste, Italy,R. Richter,Sincrotrone Trieste, Trieste, Italy,M. de Simone, A. Kivimäki, INFM-TASC,Trieste, Italy,and INSTM, Trieste, Italy,A. Mihelič and M. Žitnik,J. Stefan Institute, 1000 Ljubljana, Slovenia K.C. Prince et al, Phys. Rev. Lett. 96 (2006) 093001.

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Biomolecules: interaction with UV radiation

Interest: damage by ionizing radiationastrobiology: synthesis and destruction of pre-biotic molecules

in space (particularly Lyman α and He I)conformation and dynamics of this class of moleculesmass spectrometry

There exists a large body of electron impact ionization: why use UV radiation?Because it is more specific – dipole selection rules apply.

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5

4

3

2

1

0

806040200

21.2 eV

16.67 eV

11.62 eV

10.0 eV

9.56 eVparentionx10

Photofragmentation with laboratory sources of UV

glycine5

4

3

2

1

0

120100806040200

21.2 eV

16.67 eV

11.62 eV

10.0 eV

9.56 eV

proline

4

3

2

1

0

Inte

nsity

(arb

. uni

ts)

140120100806040200m/z

21.2 eV

16.67 eV

11.62 eV

10.0 eV

leucine

Aliphatic amino acids and Pro show strong fragmentation, even close to threshold.Mostly loss of HCOO.(Our work and Lago et al, Chem. Phys. 307(2004) 9).

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O. Plekan et al, Chem. Phys. 334 (2007) 53–63.

6

5

4

3

2

1

0

160140120100806040200

21.2 eV

16.67 eV

11.62 eV

10.0 eV

9.56 eV

parent ionC4H10NCOOH+C4H10N+

Methionine is different:- loss of COOH is not the main channel-at low energies, the parent ion is the dominant peakWhy?

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5000

4000

3000

2000

1000

0

Inte

nsity

(arb

. uni

ts)

20.0 17.5 15.0 12.5 10.0 7.5

Binding energy (eV)

glycine

proline

methionine

nN

nO

OO

nS

Ar IHe I Ne I Kr I Xe I

UV photoionization removes an electron from a valence orbital.

The Highest Occupied Molecular Orbital of aliphatic amino acids has nitrogen lone pair character.

The HOMO of methionine has S lone pair character. The S atom accommodates the charge without fragmenting.

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Amino acids with aromatic groups.

Tyrosine

5

4

3

2

1

0

Inte

nsity

(arb

. uni

ts)

250200150100500m/z

21.2 eV

16.67 eV

11.62 eV

10.0 eV

8.43 eV

c)

Phenylalanine Tryptophan

For all three amino acids, there is a significant parent ion signal at low photon energy.Fragmentation pattern differs: aromatic ring breaks off from rest of molecule.HOMOs have π character.

Tryptophan mass spectrum

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For amino acids we measured all ions produced by ionization of several orbitals.If we measure ions and photoelectrons in coincidence, we observe the fragments due to the ionization of a specific orbital.

E.g. Methanol, CD3OH.

Next step – coincidences.500

400

300

200

100

0

coun

ts

26 24 22 20 18 16 14 12 10electron binding energy in eV

any ion D+

CD3+ OD+

DCO+ COD+

D2COH+

D3COH+

2a"

5a'1a"4a'

3a'2a'

Work done at Spring-8, in collaboration with R. Richter, K. Ueda, G. Pruemper.

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Dilute species, biomolecules

What is the conformational (folding) structure of a free bio molecule?

Circular dichroism (CD) in the near UV is a standard tool for secondary structure determination/control of large molecules. [I(left)-I(right)]/I~0.001-0.0001.

+ Sample in solvent, structure is “true” structure.- Sample in solvent, wavelength range limited to 180-250 nm circa.- low info content (few peaks).

CD spectrum and secondary structure of proteins.

“Conventional” CD is best for: monitoring conformational changes due to a perturbation, quality control etc. Less good for absolute structure.

Being extended to 120 nm (Daresbury-> Australian synchrotron, Brookhaven etc.)

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Recently natural CD of small chiral molecules investigated. Optimal conditions: CD signal=[I(left)-I(right)]/I~0.04.

Dichroism

Can we use VUV spectroscopy (and later Fermi) to obtain structural (folding) information?

Higher energy-> no windows-> molecules in vacuum.

Free molecules-> range extended above the IP-> potentially larger info content

-0.02

0.00

0.02

0.04

0.06

b)R(+)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fig. 1

Inte

nsity

(arb

. uni

ts)

a)

CD

(arb

. uni

ts)

CD

(arb

. uni

ts)

Inte

nsity

(arb

. uni

ts)

R(+)

4 5 6 7 8 9 10 11 12 13

-0.06

-0.04

-0.02

0.00

0.02

Kinetic Energy (eV)

d)

S(-)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

c)S(-)

S. Turchini, N. Zema, G. Contini, G. Alberti, M. Alagia, S. Stranges, G. Fronzoni, M. Stener, P. Decleva, and T. Prosperi, Phys. Rev. A 70, 014502 (2004)

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500x103

400

300

200

100

0

-100

Inten

sity (

arb.

units

)

109876 Kinetic energy

Signal, left Signal, right difference, x10

Proline, valence band spectrum

Dichroism from 0 to 4%.

100x10-3

80

60

40

20

0

D pa

rame

ter

1614121086Kinetic energy (eV)

A_state

-40x10-3

-20

0

20

40

D pa

rame

ter


B_state

-40x10-3

-20

0

20

40

D pa

rame

ter


C_state

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Inte

nsity

(arb

. uni

ts)

11.0 10.0 9.0 8.0Binding energy (eV)

0

1

2

3

4

5

6

0.44

0.42

0.40

0.38

I(8.9

)/I(9

.5)

440435430425420415410405

Temperature (K)

3.80

3.75

3.70

3.65

ln (I

(8.9

)/I(9

.5))

2.45x10-32.402.352.302.25

Temperature (1/K)

Valence band photoemission.He I.The two highest MOs are due to two pairs of conformers.

By measuring spectra as a function of temperature, can extract free energy difference, 6-9 kJ/mol.

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Surfaces and solids

Photoemission from O/Ag(110).

One of the most important applications of VUV photoemission spectroscopy has been band mapping. It will continue to be a standard method.

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Z.-X. Shen and co-workers are prolific users of low energy photons and Angle Resolved UPS.Huge output of results on oxide materials.

ARUPS-> direct access to valence band structure.

What science will be done?The valence band: gaps in high Tc superconductors, transport properties, bonding, etc.Oxides and related materials have large unit cells-> high momentum resolution required-> high angular resolution at low electron energy.

K.M. Shen et al, PRL

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Microscopy: Pb/Au/Si(111)

SPELEEM photoemission microscope, (now closed, upgraded to Nanospectroscopy). Sample: Au/Si(111) + 5 ML Pb. The Au induces layer by layer growth.Area: about 1 micron diameter.Time: one photon energy, one kinetic energy, all angles: about 60 s.One photon energy, all valence kinetic energies, all angles: tens of min.

Valence band.

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Inverse model catalyst: CeO2/Cu

Many catalysts consist of metal particles supported on oxides.Oxide maintains dispersion, is a reservoir for oxygen, participant in SMSI (strong metal support interaction), catalyst, etc.

Difficult to prepare single crystal oxides, easy to prepare metal crystals.->Prepare oxides on metals.

HR-TEM image of (a) Cu loaded ceria powder(b) elemental mapping of the area; white - cerium, red – copper.

F. Šutara, V. Matolín et al, Thin solid Films, in press.

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We can grow defect-free, well-ordered epitaxial CeO2 layers on Cu(111)

LEED of CeO2/Cu(111), E = 98 eV, (a) discontinuous, (b) 2.5 ML, (c) 5 ML.Arrows mark substrate spots.

Can we check for point defects (oxygen vacancies) with resonant photoemission?

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Inten

sity,

arb

. unit

s

8 6 4 2 0BE(eV)

115eV 122eV 124.5eV

Ep=

O 2p

Ce 5d, 6s, 4f

Ce3+Ce0

Ce0

Ce4+

Resonant spectra show primarily Ce4+.

Conclusion: film is epitaxial (LEED) and has low point defect density.F. Šutara et al, Thin Solid Films, 516 (2008) 6120.

Configurations CeO2, Ce4+, (4d105p6) 4f0, resonates at hv=124.5 eVCe2O3, Ce3+, (4d105p6) 4f1, resonates at hv=122 eVCe metal, (4d105p6) 4f15d16s2, resonates at hv=122 eV

Resonant process:Ce 4d104fn 4d94fn+1 4d10fn

interferes with direct valence photoemission.

Resonant photoemission

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A DNA base on a metal surface: a prototypical bio/metal interface.Has been studied by STM, vibrational spectroscopy, theory.

Q. Chen and N. V. Richardson, Nature Mat. 2 (2003) 324Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219D. J. Frankel, Q. Chen and N.V. Richardson, J. Chem. Phys. 124, 204704 (2006).

Adenine, C5N5H5

DNA bases II: adsorption of adenine, C5H5N5, on Cu(110)

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Experimental conclusions from the literature: the molecule is lying flat (Yamada et al) the molecule is tilted slightly (Chen et al) or the molecule is strongly tilted (McNutt et al)

Theory agrees: bonding through amino group.Molecule tilted up from surface, 18-26º.

Preuss et al, PRL 94, 236102 (2005)

Vibrational studies:Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219A. McNutt et al, Surf. Sci, 531 (2003) 131.T. Yamada et al, Surf. Sci. 561 (2004) 233–247.

Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219

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Bonding is very different at high and low coverage. N is involved in bonding.In particular, the two amino N atoms have unsaturated (imino)At low coverage: N 1s (398.7 eV) characteristic of π bonded N.

N 1s photoemission

Gas phaseDepositedDesorb excessOrdering

0.6 ML > 1 ML

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NEXAFS of adenine: Near Edge X-ray Absorption Fine Structure Spectroscopy

0.3 ML, annealed 430 K 1 ML, annealed 430 K

Fractional monolayer: molecule lying almost parallel to the surface.Saturated monolayer: molecule(s) tilted

1.0x106

0.8

0.6

0.4

Inten

sity (

arb.

units

)

420415410405400395Photon energy (eV)

Grazing incidence

Normal incidence

150x103

100

50

0

Inten

sity (

arb.

units

)

420415410405400395Photon energy (eV)

Normal incidence

Grazing incidence

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Collaborators.

The Gas Phase team at ElettraO. Plekan, V. Feyer, R. Richter, M. Coreno, M. de Simone,

Materials Science (Czech) BeamlineT. Skala, V. Chab, F. Sutara, V. Matolin

Oksana

VitaliyRobert

Monica

Marcello

Tomas

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Fermi specs.

A seeded free electron laser: -> a conventional laser bunches the electrons -> the bunches pass through an undulator -> the electrons in each bunch emit coherently -> intensity is proportional to square of number of electrons, and square of number of periods in undulator.

Fermi FEL 1: 12-30 eV; FEL 2: 30-126 eV.Pulses of 50 fs-1 ps50 Hz GW power levels, 1014 photons/pulseStart operation at the beginning of 2009

Free Electron Lasers.

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If you can control the timing of light, you can learn new things…

Eardweard Muybridge, 1878: do all 4 horse’s hooves leave the ground at once?A bet by Leland Stanford, wealthy ex-governor of California: early research at Palo Alto. Sub-second resolution.

Harold E. Edgerton, 1964. Microsecond strobe.

Or applied research, H. E. Edgerton, 1934.

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Two photon double ionization

Physics is different for two photon (h>39.5 eV)and many photon (h~ few eV) double ionization.->control of relative field strengths.

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FEL light is so intense that it ionizes all molecules in its path

-> ultradilute samples.Clusters and “flying proteins”.

Nanospray set-up for spectroscopy,with “unfree” lasers, T.R. Rizzo et al, PPCM, EPFL.

Our schematic setup.

http://lcpm.epfl.ch/Bioproject/vacuumchambercut.jpg

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Setting up with Synchrotron Radiation: two photon spectroscopy of neon

Synchrotron light: 30-65 ps bunch length, 2 ns interval.Neon atoms excited with synchrotron light. Then excited by a laser in a second step to resonant ionizing states.Two photon transitions: Δl=0,2.Proof of principle of lifetime measurements on nanosecond time scale.

A. Moise et al, Nucl. Instrum. Methods A 588 (2008) 502.

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Where is Fermi being built?

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ELETTRA

FERMI@Elettra FEL

ELETTRAStorage Ring FEL

1010 Increase

P ~ Ne

P Ne2

Brightness

vuv (photoemission) spectroscopy k.c. prince, sincrotrone trieste, trieste, italy

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