Excitation and Relaxation of Nonthermal Electron Energy Distributions in Metals with Application to Gold

TL;DR

This paper develops a semiempirical theory for the excitation and relaxation of nonthermal electrons in metals, incorporating electron-phonon, electron-electron, and electron-photon interactions, and applies it to gold to interpret ultrafast experiments.

Contribution

It introduces a comprehensive model for nonthermal electron dynamics in metals, extending existing theories to include highly excited states and phonon-assisted photon absorption.

Findings

Electron-electron scattering time for electrons 1 eV above Fermi level is 25-55 fs.

The theory accurately interprets ultrafast thermionic, photoemission, and Raman experiments on gold.

Explicit dependence on electronic density of states improves understanding of electron relaxation processes.

Abstract

A semiempirical theory for the excitation and subsequent relaxation of nonthermal electrons is described. The theory, which is applicable to ultrafast-laser excited metals, is based on the Boltzmann transport equation for the carrier distribution function $f(\epsilon,t)$ and includes electron-phonon, electron-electron, and electron-photon scattering integrals in forms that explicitly depend on the electronic density of states. Electron-phonon coupling is treated by extending the theory of Allen [Phys. Rev. Lett. 59, 1460 (1987)] to include highly-excited nonthermal electron distributions, and is used to determine the energy transfer rate between a nonthermal electron subsystem and a thermal phonon subsystem. Electron-electron scattering is treated with a simple energy-conserving electron-electron scattering integral. The electron-photon integral assumes photon absorption is phonon assisted. We apply the theory to analyze prior ultrafast thermionic emission, two-color photoemission, and electronic inelastic light (Raman) scattering experiments on Au. These analyses show that getting the details of $f(\epsilon,t)$ is necessary for proper interpretation of each experiment. Together, the photoemission and Raman-scattering analyses indicate an electron excited 1 eV above the Fermi level has an electron-electron scattering time in the range of 25 to 55 fs.