IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Ultrafast Electron Dynamics of non-thermal population in metals

INFM and

Università Cattolica del Sacro CuoreDipartimento di Matematica e Fisica, Via Musei 41, Brescia.

Claudio Giannetti

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Introduction

ToF

ΦEhνE Bkin

sinθ/2mEk 2kin//

LINEAR PHOTOEMISSION: hν > Φ

→ mapping of EQUILIBRIUM ELECTRON DISTRIBUTION

Femtosecond Light Pulses

NON-LINEAR PHOTOEMISSION: hν < Φ

→ Mapping ofNON-EQUILIBRIUM

ELECTRON DISTRIBUTION

CW Light

2-Photon Photoemissionwith P-polarized light

hν=3.14eV

Log Scale106 sensitivity

Iabs=13 μJ/cm2

Occupied states

Non-equlibrium Distribution

n=1 IPS

Ag(100)

IVC-16, Venice June 28-July 2, 2004

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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

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

*

2||

2

2m

kE

Free-electrondispersion

E

k||Γ

Photon Absorption

PHOTON ABSORPTION MECHANISMSPROBLEMS:

ΔE

Δk||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. B 60, 3279 (1999)]

•Photon absorption in electron-ion collisions. •[B. Rethfeld et al., Phys. Rev. B 65, 2143031 (2002)]

THE ENERGY ABSORPTION IS DUE TO A THREE-BODY PROCESS AND NOT TO A DIPOLE TRANSITION

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Photon Absorption

PHOTON ABSORPTION MECHANISMSRESULTS: Ebin (eV) E-Ef (eV)

Γ X

SCATTERING-MEDIATEDABSORPTION and PHOTOEMISSION

scatteringscattering

scatteringk||=0

Evac

EF

Z. Li, and S. Gao, Phys. Rev. B 50, 15394 (1994)

Snapshot of the non-equilibrium electron distribution during the laser pulse

duration (150 fs)

DEPENDENCE ON POLARIZATION

→ The models predict a collision term:

2

n

2n

absorb mω

ΔkEeJ

t

f(k)

in agreement with the measured RATIO

(C. Giannetti et al., in preparation.)

(IP/IS)3=0.69

Rtheor=0.29

Rexp=0.22±0.1

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Introduction

Evac

Efermi

occupied states

empty states

scatteringhν

hν

photoemission

decay dynamics of non-equilibrium electron distribution in Au film:

PUMP: hν=1.84eV, Iabs=120μJ/cm2

PROBE: hν=5.52eV

W.S. Fann et al., Phys. Rev B 46, 13592 (1992).

Time Resolved 2-Photon Photoemission (TR-2PPE)

pump

probe

delay timeτ

e–

e–

e–

Φ

NON-EQUILIBRIUM ELECTRON DYNAMICSPROBLEMS:

This result is not compatible with Fermi-Liquid Theory

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Non-Equlibrium Electron Dynamics

NON-EQUILIBRIUM ELECTRON DYNAMICSPROBLEMS:

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?

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Non-Equlibrium Electron Dynamics

NON-EQUILIBRIUM ELECTRON DYNAMICSRESULTS:

Time-Resolved Photoemission SpectroscopyPhotemitted charge autocorrelation of different energy regions

The Relaxation Time of the high-energy region is smaller than the pulse timewidth:

τ<150 fs

(C. Giannetti et al., in preparation.)

This result is compatible with Fermi-Liquid Theory

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Non-Equlibrium Electron Dynamics

NON-EQUILIBRIUM ELECTRON DYNAMICSRESULTS:

G. Ferrini et al., Phys. Rev. Lett. 92, 2568021 (2004).

n

P

S

Ag(100)

Dipole selection rules

Expected dipole selection rules: J=0 in S-polJ≠0 in P-pol

Violated in non-resonant case

EF

Ev

occupied states

emptystates

Φn=1

Indirect population of IPS

Scattering Assisted Population

NO DIPOLETRANSITION

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

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

IVC-16, Venice June 28-July 2, 2004

INFM D.M.F.

Responsibles:

F. Parmigiani, G. Ferrini.

Co-workers:

F. Banfi, G. Galimberti, S. Pagliara, E. Pedersoli.