Part IIIntroduction to

Two-Photon-Photoemission (2PPE) Spectroscopy

Martin Wolf

Sfb 658 Colloquium – 11 May 2006

...connection to molecular electronics:

X.Y. Zhu, Surf. Sci. Rep. 56, 1 (2004)

Motivation: Electron transfer across interfaces

hot e-

E

EF

e-transfer

substrate adsorbate

HOMO

LUMO

key step for interfacial and surface dynamics(femtochemistry, vibrational energy relaxation…)

e-

e-

~1 nm

Spectroscopic probe for electron transfer dynamics at interfaces

Outline Time-resolved two-photo-photoemission (2PPE)

Spectroscopy of charge transfer dynamics

Coherent control of surface currents

Electron motion controlled by the phase of light

momentum kII‌

ener

gy

left right

Ultrafast charge transfer

Time-resolved photoemission and ‘atomic clock‘ method

Decay of delocalized state versus localized core hole excitation in C6F6/Cu(111)

Electron dynamics in image potential states

Model system to study elementary processes

Electron thermatization and cooling in metals

Image potential states at metal surfaces

Courtesy: U. Höfer, Uni Marburg

z

Two-photon photoemission (2PPE) spectroscopy

Angle-resolved 2PPE spectra from Cu(100)

Berthold et al, PRL 88, (2002) 056805

n = 1

n = 22kinF hEE-E ν−Φ+=

EF

Evac

Ekin

hν1

hν2

hν

e-

Φ

ϕ

/mE2sink kin|| hϕ=

TOF

e-

energy- and time-resolution:

pulseduration:65 fs

momentum resolution:

~50 fs

How to distinguish occupied and unoccuppied states in 2PPE ?

Measure ΔEkin versus ħω

ΔEkin

ħωb

slope = 1

For surface states:(no k┴ dispersion)

Experimental setup for 2PPE

Courtesy: U. Höfer, Uni Marburg

Time-resolved 2PPE spectrscopy

Electron dynamics in image potential states

population dynamics

Cu(100)

Direct measurement ofthe lifetime using 2PPE

Excellent agreementwith many-body theory

P. Echenique et. al

Analysis of decay rates: Density matrix formalism

3 level system

Hamiltonian:

Quantum beat spectroscopy Coherent superposition of Rydberg-like image potential states (n>4)

Höfer et al, Science 277, (1997) 1480

Quantum beats:

Classical analog: electron motion at metals

Courtesy: U. Höfer, Uni Marburg

For recent review of this field see: Surf. Sci. Rep. 52, 219 - 318 (2004)

2PPE experiments

E-EF = Ekin + Φ - hνprobe

probe: 6 eV, 90 fs

pump: 1.5 eV, 55 fs

Gd/W(110) preparationAspelmeier et al.,

J. Magn. Magn. Mat. 132, 22 (1994)

• low excitation density: <10-4 e-/atominteraction with ‚cold‘ electrons

single quasi particle lifetimes

• high excitation density: >10-4 e-/atominteraction with ‚hot‘ electrons

hot electron temperature

or Ru(001)

transient energy distribution N(E):

Electron thermalization in metals

Electron dynamics in metals following optical excitation

Time-resolved 2PPE from Ru(001)

0.0001

0.001

0.01

0.1

1

2PPE

inte

nsity

1.51.00.50.0

E - EF (eV)

0.0001

0.001

0.01

0.1

1

0.0001

0.001

0.01

0.1

1

580 μJ/cm2

230 μJ/cm2

40 μJ/cm2Δ t / fs

0100200300400500

Ru(001)

e-UV probe800 nm pump

Pump-probe scheme:

probing electron dynamics around EFv

Lisowski et al., Appl. Phys. A 78, 165 (2004)

e-e scattering

1) Electron thermalization:

2) Relaxation by e-ph scattering

3) Energy transport in the bulk

Cooling of photoexcited electron gas

Summary I

Introduction:Electron transfer across interfaces

e-

e-

Ultrafast charge transfer

Time-resolved photoemission and ‘atomic clock‘ method

Decay of delocalized state versus localized core hole excitation in C6F6/Cu(111)

Electron dynamics in image potential states

Model system to study elementary processes

Electron thermalization and cooling in metals

Time-resolved photoemission as direct probe of electron dynamics

● Optical (visible) excitation ● Probed by transient absorption

τCT (rise) = 25-50 fs

● Core level excitation● Probed by core hole clock method

τCT (LUMO+1) < 3 fs

⇒ Goal: Systematic comparison on ONE organic model system: C6F6/Cu(111)

Motivation: Solar Energy Conversion in Grätzel Cell

[Schnadt et al., Nature, 418, 620 (2002)]

Ru-N3 dye

[Hannappel et al., J. Phys. Chem. B, 101, 6799 (1997)][Ashbury et al., J. Phys. Chem. B, 103, 3110 (1999)]

„antenna“Dye sensitizerTiO2

Ultrafast charge transfer at interfaces

Time-resolved two-photon photoemission

Resonant Auger-Raman(core hole clock method)

● Excitation of delocalized statesinto LUMO-resonance

● localized, element-specific excitation

● comparison with rate of core hole decay0.1 fs < τCT < 50 fs

● decay by filling on core hole time scale● Decay via electron transferback to substrate

● direct time domain approach10 fs < τCT < 1 ps

2PPE & Resonant Auger-Raman Spectroscopy

electron density: high lowGallivan and Dougherty, Org. Lett. 1, 103 (1999)

Electronic structure of benzene and C6F6

C atoms: sp2 hybridizedσ-bonds ⇒ plane backbonepz-orbitals ⇒ delocalized π-system

H substituton by F:electronegative F ⇒ shift of e-density

towards F⇒ stabilization of orbitals

perfluor effect: π-orbitals are less stabilizedthan σ-orbitals (charge transfer along C-F)

2PPE spectroscopy of C6F6/Cu

2PPE spectra vs coverage

Gahl et al., Faraday Discussion 117, 191 (2001)

• Preparation of a specific coveragethrough annealing

• Unoccupied molecular resonanceobserved at ~3 eV above EF

• Preparation of a specific coveragethrough annealing

• Unoccupied molecular resonanceobserved at ~3 eV above EF; meff ≈2 me

• Evolution of meff: Formation of adsorbatebandstructure with increasing coverage

Population dynamics of C6F6/Cu(111)

2PPE cross-correlation traces:

Delayed population risein state A at k|| = 0,

single exp. decay for ‘B‘

C6F6/Cu(111)molecular

resonance A

ADAR

tNtNdt

tdNττ

)()(~)(−=

rate equationanalysis:

low excitation regime: ni=const.

τARτAD‘A‘ ‘A‘τAD

Population dynamics of C6F6/Cu(111)

2PPE cross-correlation traces:

Delayed population risein state A at k|| = 0,

single exp. decay for ‘B‘

C6F6/Cu(111)molecular

resonance A

ADAR

tNtNdt

tdNττ

)()(~)(−=

rate equationanalysis:

C6F6/Cu(111)

decay timesrise /

Results:

Core hole clock method

Narrowband excitation from C1s level into molecular resonance (LUMO)

hνRes ~288 eV

For a review see: Rev. Mod. Phys. 74, 703 (2002), Chem. Phys. 251, 141 (2000)

Collaboration with S. Vijayalakshmi, A. Föhlisch, W. Wurth, Universität Hamburg

Core hole spectroscopy of C6F6/Cu(111)

NEXAFS: Resonant Auger-Raman:

with Raman fraction f and τΓ = 7,7 fs for C1s hole

Results:

Analysis of chargetransfer times:

C6F6/Cu(111) LUMO resonance

Ultrafast charge transfer dynamics

Charge transfer times for C6F6/Cu(111) LUMO resonance

37 fs

1.5 - 2.5 fs

2PPE spectroscopy core hole clock method

Considerably shorter time scale obtained with core hole clock method

Coverage dependence is qualitatively different

nearly independentfrom coverage!

Discussion: ‘2PPE vs atomic clock’

2PPE probes charge transfer (population decay)to delocalizd states in the substrate

Core hole clock method probes dephasing of coreexcited state mainly by intramolecular delocalization

screeningof core hole shifts resonancetowards EF

Coherent control of photoionization

Example: Ionization of Rb atoms in the gas phase

relative phase Φa – 2Φb

elec

tron

yiel

d

Generation of coherent surface currents

Principle

Courtesy of J. Güdde, U. Höfer, University of Marburg

2PPE from n=1 image potential state Cu(001)

Courtesy of J. Güdde, U. Höfer, University of Marburg

left right

Φ =

Φ =

Summary

Ultrafast charge transfer

Comparison between 2PPE and ‘atomic clock‘ method

2PPE monitors charge transfer back to metal substrate

Ultrafast dephasing of core excited state: intra- versusinter-molecular delocalization

Time-resolved two-photo-photoemission (2PPE)

Spectroscopy of charge transfer dynamics

Electron dynamics in image potential states

Model system to study elementary processes

Coherent control of surface currents

Electron motion controlled by the phase of light

momentum kII‌

ener

gy

left right

thanks to

€ € € DFG: Sfb 658, SPP 1093, FUB, EU € € €

U. Bovensiepen, P. Kirchmann, M. Lisowski, P. Loukakos, FU Berlin

Collaborations: A. Föhlisch, V. Vijayalakshmi, W. Wurth, Univ. Hamburg

Image state dynamics: AG Ulrich Höfer, Univ. Marburg

H. Petek and S. Ogawa, Femtosecond time-resolved two-photon photoemission studiesof electron dynamics in metalsProgress in Surface Science 56, 239-311 (1998).

Literature:

P.M. Echenique, R. Berndt, E.V. Chulkov, Th. Fauster and U. Höfer, Decay of electronic excitations at metal surfacesSurface Science Reports 52, 219-318 (2004).

M. Bonn, H. Ueba and M. WolfTheory of sum-frequency generation spectroscopy of adsorbed moleculesby density matrix method - Broadband vibrational SFG and applicationsJ. Phys.: Condens. Matter 17, 201 (2005)

M. Weinelt,Time-resolved two-photon-photoemission from metal surfacesJ. Phys.: Condens. Matter 14, R1099 (2002).