Impacts of Hot Electron Diffusion, Electron-Phonon Coupling, and Surface Atoms on Metal Surface Dynamics Revealed by Reflection Ultrafast Electron Diffraction

TL;DR

This study uses reflection ultrafast electron diffraction to investigate how hot electron diffusion, electron-phonon coupling, and surface atoms influence the structural dynamics of metal surfaces after femtosecond laser excitation, revealing metal-specific behaviors.

Contribution

It demonstrates the effectiveness of reflection UED in analyzing surface dynamics and uncovers metal-dependent differences in atomic motions and electron-phonon interactions.

Findings

Hot electron diffusion reduces surface excitation.

Gold exhibits larger vibrational amplitudes than predicted.

Reflection UED reliably captures transient surface structural dynamics.

Abstract

Metals exhibit nonequilibrium electron and lattice subsystems at transient times following femtosecond laser excitation. In the past four decades, various optical spectroscopy and time-resolved diffraction methods have been used to study electron-phonon coupling and the effects of underlying dynamical processes. Here, we take advantage of the surface specificity of reflection ultrafast electron diffraction (UED) to examine the structural dynamics of photoexcited metal surfaces, which are apparently slower in recovery than predicted by thermal diffusion from the profile of absorbed energy. Fast diffusion of hot electrons is found to critically reduce surface excitation and affect the temporal dependence of the increased atomic motions on not only the ultrashort but sub-nanosecond times. Whereas the two-temperature model with the accepted physical constants of platinum can reproduce the observed surface lattice dynamics, gold is found to exhibit appreciably larger-than-expected dynamic vibrational amplitudes of surface atoms while keeping the commonly used electron-phonon coupling constant. Such surface behavioral difference at transient times can be understood in the context of the different strengths of binding to surface atoms for the two metals. In addition, with the quantitative agreements between diffraction and theoretical results, we provide convincing evidence that surface structural dynamics can be reliably obtained by reflection UED even in the presence of laser-induced transient electric fields.