Photoinduced ultrafast local volume changes in intermediate-valence solids

Abstract

We present a theoretical model for the description of the ultrafast structural response of intermediate-valence solids to femtosecond laser excitation. The approach includes the calculation of the free energy of the hot electrons produced by the laser pulse and the determination of the changes in the thermodynamic and elastic properties of the solid as a consequence of the excitation. In particular and based on the promotional Ramirez-Falicov model, we consider the femtosecond laser heating of $\ensuremath{\alpha}$ cerium and the subsequent ultrafast lattice expansion dynamics. For this purpose, we determine the thermodynamic properties of cerium at very high electronic temperatures (simulating the laser excitation). The possibility for a nonequilibrium photoinduced inverse volume collapse transition is discussed. We consider both the laser-excited and the unexcited parts of the system in order to account for inertial confinement. The thermodynamic properties are obtained as function of time and used to calculate the shock velocity variation and the time scale for expansion of the heated spot into the surrounding (unheated) part of the sample. A transition on a subpicosecond time scale is predicted.