Temperature Dependence of Electron-Phonon Interactions in Gold Films Rationalized by Time-Domain Ab lnitio Analysis

The nonequilibrium dynamics of excited electrons in metals is probed by ultrafast laser measurements. Using a real-time Kohn-Sham time-dependent density functional theory and nonadiabatic molecular dynamics, we report direct modeling of such experiments, rationalizing the observed temperature dependence. Focusing on thin gold films, we analyze the effect of temperature on film structure, electronic state densities, nonadiabatic electron-phonon coupling, elastic electron-phonon scattering times, and electron-phonon relaxation rates. The effective electron-phonon coupling constants calculated at different temperatures are in good agreement with the values deduced from experiments and an alternative theory. A temperature increase accelerates both inelastic and elastic electron-phonon scattering and allows a larger number of higher-frequency phonon modes to couple to the electronic subsystem. The inelastic electron-phonon coupling is largest between nearest states, indicating that carrier relaxation involves transitions over small energy increments. In contrast, the elastic electron-phonon scattering is strongest for pairs of electronic states that are distant in energy. The electron-phonon interactions exhibit mild energy dependence, with both nonadiabatic electron-phonon coupling and elastic electron-phonon scattering times decreasing with increasing electron excitation energy. The detailed ab initio analysis of the electron-phonon interactions emphasizes the nonequilibrium nature of the relaxation processes and provides important insights into the electron-phonon energy exchange in metal films in general.