Sfb 658 Colloquium 11 May Part II. Introduction to Two-Photon-Photoemission (2PPE) Spectroscopy. Martin Wolf

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1 Sfb 658 Colloquium 11 May 2006 Part II Introduction to Two-Photon-Photoemission (2PPE) Spectroscopy Martin Wolf

2 Motivation: Electron transfer across interfaces key step for interfacial and surface dynamics (femtochemistry, vibrational energy relaxation ) hot e - E LUMO...connection to molecular electronics: E F e- transfer substrate adsorbate HOMO e - X.Y. Zhu, Surf. Sci. Rep. 56, 1 (2004) e - ~1 nm Spectroscopic probe for electron transfer dynamics at interfaces

3 Outline Time-resolved two-photo-photoemission (2PPE) Spectroscopy of charge transfer dynamics Electron dynamics in image potential states Model system to study elementary processes Electron thermatization and cooling in metals Ultrafast charge transfer Time-resolved photoemission and atomic clock method Decay of delocalized state versus localized core hole excitation in C 6 F 6 /Cu(111) Coherent control of surface currents Electron motion controlled by the phase of light energy left momentum k II right

4 Image potential states at metal surfaces z Courtesy: U. Höfer, Uni Marburg

5 Two-photon photoemission (2PPE) spectroscopy energy- and time-resolution: E kin hν 2 e - TOF Angle-resolved How to distinguish 2PPE spectra occupiedfrom and Cu(100) unoccuppied states in 2PPE? n = 1 Measure ΔE kin versus ħω E vac pulse duration: 65 ~50 fs fs E F hν 1 Φ E - E F = E kin + Φ hν 2 n = 2 momentum resolution: ϕ hν e - ΔE kin For surface states: (no k dispersion) slope = 1 k = sin ϕ 2mEkin / h Berthold et al, PRL 88, (2002) ħω b

6 Experimental setup for 2PPE Courtesy: U. Höfer, Uni Marburg

7 Time-resolved 2PPE spectrscopy

8 Electron dynamics in image potential states population dynamics Cu(100) Direct measurement of the lifetime using 2PPE Excellent agreement with many-body theory P. Echenique et. al

9 Analysis of decay rates: Density matrix formalism Hamiltonian: 3 level system

10 Quantum beat spectroscopy Coherent superposition of Rydberg-like image potential states (n>4) Quantum beats: Höfer et al, Science 277, (1997) 1480

11 Classical analog: electron motion at metals Courtesy: U. Höfer, Uni Marburg For recent review of this field see: Surf. Sci. Rep. 52, (2004)

12 2PPE experiments or Ru(001) low excitation density: <10-4 e - /atom interaction with cold electrons Gd/W(110) single preparation quasi particle lifetimes Aspelmeier et al., high J. excitation Magn. Magn. Mat. density: 132, 22 >10 (1994) -4 e - /atom interaction with hot electrons hot electron temperature probe: 6 ev, 90 fs pump: 1.5 ev, 55 fs transient energy distribution N(E): E-E F = E kin + Φ - hν probe

13 Electron thermalization in metals Electron dynamics in metals following optical excitation

14 Time-resolved 2PPE from Ru(001) UV probe 800 nm pump μj/cm 2 Δ t / fs e - Pump-probe scheme: probing electron dynamics around E Fv 2PPE intensity μj/cm 2 1) Electron thermalization: e-e scattering μj/cm 2 Ru(001) 2) Relaxation by e-ph scattering 3) Energy transport in the bulk Cooling of photoexcited electron gas E - E F (ev) 1.5 Lisowski et al., Appl. Phys. A 78, 165 (2004)

15 Introduction: Electron transfer across interfaces Summary I e - e - 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 Ultrafast charge transfer Time-resolved photoemission and atomic clock method Decay of delocalized state versus localized core hole excitation in C 6 F 6 /Cu(111)

16 Ultrafast charge transfer at interfaces Motivation: Solar Energy Conversion in Grätzel Cell TiO 2 antenna Dye sensitizer Ru-N3 dye Optical (visible) excitation Probed by transient absorption τ CT (rise) = fs [Hannappel et al., J. Phys. Chem. B, 101, 6799 (1997)] [Ashbury et al., J. Phys. Chem. B, 103, 3110 (1999)] Core level excitation Probed by core hole clock method τ CT (LUMO+1) < 3 fs [Schnadt et al., Nature, 418, 620 (2002)] Goal: Systematic comparison on ONE organic model system: C 6 F 6 /Cu(111)

17 2PPE & Resonant Auger-Raman Spectroscopy Time-resolved two-photon photoemission Resonant Auger-Raman (core hole clock method) Excitation of delocalized states into LUMO-resonance Decay via electron transfer back to substrate direct time domain approach 10 fs < τ CT < 1 ps localized, element-specific excitation decay by filling on core hole time scale comparison with rate of core hole decay 0.1 fs < τ CT < 50 fs

18 Electronic structure of benzene and C 6 F 6 electron density: high low Gallivan and Dougherty, Org. Lett. 1, 103 (1999) C atoms: sp 2 hybridized σ-bonds plane backbone p z -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 stabilized than σ-orbitals (charge transfer along C-F)

19 2PPE spectroscopy of C 6 F 6 /Cu 2PPE spectra vs coverage Preparation of a specific coverage through annealing Unoccupied molecular resonance observed at ~3 ev above E F ; m eff 2 m e Evolution of m eff : Formation of adsorbate bandstructure with increasing coverage Gahl et al., Faraday Discussion 117, 191 (2001)

20 Population dynamics of C 6 F 6 /Cu(111) 2PPE cross-correlation traces: C 6 F 6 /Cu(111) molecular resonance A rate equation analysis: dn( t) dt ~ N( t) = τ AR N( t) τ AD τ AD A τ AD A τ AR low excitation regime: n i =const. Delayed population rise in state A at k = 0, single exp. decay for B

21 Population dynamics of C 6 F 6 /Cu(111) 2PPE cross-correlation traces: C 6 F 6 /Cu(111) molecular resonance A rate equation analysis: dn( t) dt C 6 F 6 /Cu(111) ~ N( t) = τ AR N( t) τ AD Results: rise / decay times Delayed population rise in state A at k = 0, single exp. decay for B

22 Core hole clock method Collaboration with S. Vijayalakshmi, A. Föhlisch, W. Wurth, Universität Hamburg hν Res ~288 ev Narrowband excitation from C1s level into molecular resonance (LUMO) For a review see: Rev. Mod. Phys. 74, 703 (2002), Chem. Phys. 251, 141 (2000)

23 Core hole spectroscopy of C 6 F 6 /Cu(111) NEXAFS: Resonant Auger-Raman: C 6 F 6 /Cu(111) LUMO resonance Results: Analysis of charge transfer times: with Raman fraction f and τ Γ = 7,7 fs for C1s hole

24 Ultrafast charge transfer dynamics Charge transfer times for C 6 F 6 /Cu(111) LUMO resonance 2PPE spectroscopy core hole clock method 37 fs fs nearly independent from coverage! Considerably shorter time scale obtained with core hole clock method Coverage dependence is qualitatively different

25 Discussion: 2PPE vs atomic clock screening of core hole shifts resonance towards E F 2PPE probes charge transfer (population decay) to delocalizd states in the substrate Core hole clock method probes dephasing of core excited state mainly by intramolecular delocalization

26 Coherent control of photoionization Example: Ionization of Rb atoms in the gas phase electron yield relative phase Φ a 2Φ b

27 Generation of coherent surface currents Principle Courtesy of J. Güdde, U. Höfer, University of Marburg

28 2PPE from n=1 image potential state Cu(001) Φ= left right Φ= Courtesy of J. Güdde, U. Höfer, University of Marburg

29 Summary Time-resolved two-photo-photoemission (2PPE) Spectroscopy of charge transfer dynamics Electron dynamics in image potential states Model system to study elementary processes 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- versus inter-molecular delocalization Coherent control of surface currents Electron motion controlled by the phase of light energy left momentum k II right

30 thanks to U. Bovensiepen, P. Kirchmann, M. Lisowski, P. Loukakos, FU Berlin Collaborations: A. Föhlisch, V. Vijayalakshmi, W. Wurth, Univ. Hamburg DFG: Sfb 658, SPP 1093, FUB, EU Image state dynamics: AG Ulrich Höfer, Univ. Marburg

31 Literature: H. Petek and S. Ogawa, Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals Progress in Surface Science 56, (1998). M. Weinelt, Time-resolved two-photon-photoemission from metal surfaces J. Phys.: Condens. Matter 14, R1099 (2002). P.M. Echenique, R. Berndt, E.V. Chulkov, Th. Fauster and U. Höfer, Decay of electronic excitations at metal surfaces Surface Science Reports 52, (2004). M. Bonn, H. Ueba and M. Wolf Theory of sum-frequency generation spectroscopy of adsorbed molecules by density matrix method - Broadband vibrational SFG and applications J. Phys.: Condens. Matter 17, 201 (2005)