Relaxation of Femtosecond Non-equilibrium Electrons in a Metallic Sample

TL;DR

This paper presents a kinetic model for femtosecond electron energy relaxation in metals, highlighting the slowing down near the Fermi energy due to Fermionic blocking, which differs from the traditional Two Temperature model.

Contribution

The paper introduces a novel kinetic model that accounts for Fermionic blocking effects in electron-phonon relaxation, extending understanding beyond the Two Temperature model in femtosecond regimes.

Findings

Peaking of hot electron distribution at Fermi energy observed.

Slowing down of relaxation due to Fermionic blocking near Fermi energy.

Model predicts time-resolved evolution of hot electrons.

Abstract

A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons prepared in a metallic sample that has been subjected to homogeneous photo-excitation by a femtosecond laser pulse. The model assumes that the delta pulse photoexcitation creates two interpenetrating electronic subsystems, initially comprising a dilute energy-wise higher-lying non-degenerate hot electron subsystem, and a relatively dense, lower-lying electron subsystem which is degenerate. In the femtosecond time regime the relaxation process is taken to be dominated by the electron-(multi) phonon interaction, resulting in a quasi-continuous electron energy loss to the phonon bath. The kinetic model is given for this time regime, beacuse in this time regime the usual Two Temperature model is not applicable. The Two Temperature model assumes that the hot electrons and phonons are in their respective equilibrium states (Fermi and Bose) but at a different temperatures. One uses the Bloch-Boltzmann-Peierls transport formula to calculate the energy transfer rate. In the present Kinetic model a novel physical feature of slowing down (due to Fermionic blocking of interaction phase space) of electron-phonon relaxation mechanism near the Fermi energy of degenerate electronic subsystem is considered. This leads to a peaking of the calculated hot electron distribution at the Fermi energy. This feature, as well as the entire evolution of the hot electron distribution, may be time-resolved by a femto-second pump-probe study.