Ultrafast Electron Dynamics of non-thermal population in metals

1. Ultrafast Electron Dynamics of non-thermal population in metals Claudio Giannetti INFMand Università Cattolica del Sacro Cuore Dipartimento di Matematica e Fisica, Via Musei 41, Brescia.

2. ToF hν=3.14eV Occupied states n=1 IPS Log Scale 106 sensitivity Non-equlibrium Distribution 2-Photon Photoemission with P-polarized light Iabs=13 μJ/cm2 Introduction CW Light LINEAR PHOTOEMISSION: hν> Φ → mapping of EQUILIBRIUM ELECTRON DISTRIBUTION Femtosecond Light Pulses NON-LINEAR PHOTOEMISSION:hν< Φ →Mapping of NON-EQUILIBRIUM ELECTRON DISTRIBUTION Ag(100)

3. Opened Problems NON-LINEAR PHOTOEMISSION on metals is a powerful tool to investigate 2 main physical questions: 1. PHOTON ABSORPTION MECHANISMS 2. NON-EQUILIBRIUM ELECTRON DYNAMICS

4. Free-electron dispersion E Δk|| ΔE k|| Photon Absorption • PHOTON ABSORPTION MECHANISMS • PROBLEMS: The intraband transition between s-s states within the same branch is FORBIDDEN for the conservation of the momentum. • Recently the excitation mechanism has been attributed to: • Laser quanta absorption in electron collisions with phonons. • [A.V. Lugovskoy and I. Bray, Phys. Rev. B60, 3279 (1999)] • Photon absorption in electron-ion collisions. • [B. Rethfeld et al., Phys. Rev. B65, 2143031 (2002)] THE ENERGY ABSORPTION IS DUE TO A THREE-BODY PROCESS AND NOT TO A DIPOLE TRANSITION

5. scattering scattering scattering (IP/IS)3=0.69 Rtheor=0.29 Rexp=0.22±0.1 → The models predict a collision term: DEPENDENCE ON POLARIZATION in agreement with the measured RATIO (C. Giannetti et al., in preparation.) Photon Absorption • PHOTON ABSORPTION MECHANISMS • RESULTS: Ebin (eV) E-Ef (eV) SCATTERING-MEDIATED ABSORPTION and PHOTOEMISSION Evac Snapshot of the non-equilibrium electron distribution during the laser pulse duration (150 fs) k||=0 EF Z. Li, and S. Gao, Phys. Rev. B50, 15394 (1994)

6. photoemission decay dynamics of non-equilibrium electron distribution in Au film: PUMP: hν=1.84eV, Iabs=120μJ/cm2 PROBE: hν=5.52eV hν hν scattering W.S. Fann et al., Phys. Rev B46, 13592 (1992). Introduction • NON-EQUILIBRIUM ELECTRON DYNAMICS • PROBLEMS: Time Resolved 2-Photon Photoemission (TR-2PPE) e– Evac e– probe empty states Φ e– delay time τ Efermi pump occupied states This result is not compatible with Fermi-Liquid Theory

7. Non-Equlibrium Electron Dynamics • NON-EQUILIBRIUM ELECTRON DYNAMICS • PROBLEMS: • A. At our moderate laser intensities (Iabs=13 μJ/cm2), • the electron relaxation time τ is consistent with • Fermi-Liquid theory? • B.Indirect population of empty states such as Image Potential States?

8. Non-Equlibrium Electron Dynamics • NON-EQUILIBRIUM ELECTRON DYNAMICS • RESULTS: Time-Resolved Photoemission Spectroscopy Photemitted charge autocorrelation of different energy regions The Relaxation Time of the high-energy region is smaller than the pulse timewidth: τ<150 fs This result is compatible with Fermi-Liquid Theory (C. Giannetti et al., in preparation.)

9. Indirect population of IPS Ag(100) Ev n=1 Φ empty states Scattering Assisted Population NO DIPOLETRANSITION EF occupied states Non-Equlibrium Electron Dynamics • NON-EQUILIBRIUM ELECTRON DYNAMICS • RESULTS: Expected dipole selection rules: J=0 in S-pol J≠0 in P-pol Dipole selection rules Violated in non-resonant case G. Ferrini et al., Phys. Rev. Lett. 92, 2568021 (2004).

10. Conclusions NON-EQUILIBRIUM PHOTOEMISSION SPECTROSCOPY on Ag(100) • Role of scattering in the photon absorption mechanism • NON-EQUILIBRIUM electron dynamics at moderate laser intensities is well described by Fermi-Liquid theory • Demonstration of indirect population of empty states such as IMAGE POTENTIAL STATES

11. Responsibles: F. Parmigiani, G. Ferrini. Co-workers: F. Banfi, G. Galimberti, S. Pagliara, E. Pedersoli.