Ultrafast electron dynamics and orbital-dependent thermalization in photoexcited metals

Abstract

We use a time-dependent density functional theory (TDDFT) to analyze nonequilibrium dynamics of laserexcited electrons in transition metals (Ni, Cr, Cu) and shed light on the ultrafast thermalization process from a microscopical point of view. As a first result, after instant increase of the electron temperature up to 50 000 K, we observe that the dynamics of electron density of states is faster than a laser subcycle, on the attosecond timescale. This is related to an ultrafast rearrangement of excited electrons in space to accommodate strong changes in ion screening. Secondly, we show that the electron thermalization dynamics strongly depends on the electronic structure of a given metal. Excited by a 7-fs laser pulse, d-block transition metals exhibit two subsystems of electrons, each one achieving its own temperature. Due to a higher localization, electrons from d block stay cold, while excited delocalized sp electrons rapidly reach a high temperature. Electrons of each band of energy are mutually thermalized within the time of the laser pulse. Much more time is however needed to reach equilibrium of the whole electronic system. These results redraw the validity limits of current two-temperature models during the laser irradiation time, potentially impacting further material reaction.

Direct treatment of interaction between laser-field and electrons for simulating laser processing of metals

“… 34 ]. Furthermore, first-principles electron dynamics in bulk Cu suggests different electron temperatures for 3 d orbital and 4 s , 4 p orbitlals 35 . As deduced from these facts, it is concluded that conversion of the data obtained from present ab initio results to parameters for the “two temperature model” is not straightforward.…”

Investigation of ultrashort laser excitation of aluminum and tungsten by reflectivity measurements

“…In these excitations conditions, the optical response remains dominated by the contribution of the bound d-band electrons in agreement with the experimental results (Rmeas  R0). In [15], it is shown that the excitation of a transition metal by an ultra-short pulse leads to the creation of two subsystems of electrons (associated respectively with the d and s/p bands) thermalized to two different temperatures. The equilibrium of the whole electronic system is reached after the end of the pulse, making the TTM inadequate in that case.…”

Phonon-induced long-lasting nonequilibrium in the electron system of a laser-excited solid

“…Kinetic models, like the Boltzmann equation 18,[29][30][31][32][33] or Monte-Carlo methods 25,34 allow to trace the evolution of the electronic nonequilibrium. The electronic thermalization 18,[29][30][31][35][36][37] and the influence of the different stages of electronic nonequilibrium distributions on further energy dissipation processes 15,18,[27][28][29]36,38 are in the focus of interest and their details are heavily discussed topics. Recently, also the influence of the nonequilibrium in the phonon system has attracted increasing attention [39][40][41] .…”