Ultrafast electron dynamics in laser-driven warm dense gold

Abstract

We present a theoretical model for ultrafast charge emission from 20 nm gold nanofilms driven by femtosecond laser excitation at absorbed fluences of 0.346 − 1.0 J/cm2. A two-temperature model coupled with molecular dynamics using electron temperature-dependent interatomic potentials provides transient thermal profiles. These profiles feed a self-consistent finite-disk emission model that uses Richardson–Dushman thermionic emission, Schottky barrier lowering, and collective space charge feedback. The model reveals a clear front–rear emission asymmetry. The illuminated surface emits ∼106 electrons, while the rear surface is limited to ∼105 electrons due to delayed energy transport across the film. The emission dynamics are controlled by the transient electric field established by the separation of emitted electrons and the positively charged surface, which regulates both the total yield and the expansion velocity of the charge cloud. The front surface rapidly enters the space charge limited regime, whereas the rear surface remains temperature-limited due to its lower electron temperature. The predicted electron yield is reduced by up to seven orders of magnitude. The subpicosecond ballistic escape fraction and return-current timescale are calibrated against experimentally observed electron kinetic energies. This model bridges atomistic temperature dynamics and macroscopic charge-cloud observables, providing predictive access to the subpicosecond sheath-formation regime that current single-shot deflectometry cannot resolve.