Nonequilibrium Band Occupation and Optical Response of Gold After Ultrafast XUV Excitation

Abstract

Free electron lasers offer unique properties to study matter in states far from equilibrium as they combine short pulses with a large range of photon energies. In particular, the possibility to excite core states drives new relaxation pathways that, in turn, also change the properties of the optically and chemically active electrons. Here, we present a theoretical model for the dynamics of the nonequilibrium occupation of the different energy bands in solid gold driven by exciting deep core states. The resulting optical response is in excellent agreement with recent measurements and, combined with our model, provides a quantitative benchmark for the description of electron-phonon coupling in strongly driven gold. Focusing on sub-picosecond time scales, we find essential differences between the dynamics induced by XUV and visible light.

Optical Properties of Gold After Intense Short-Pulse Excitations

“…Moreover, the sp and d-bands are assumed to be in equilibrium, i.e., they share a common temperature and chemical potential. This condition sets the lower temporal limit when our approach is valid since the occupation of these bands may stay out of equilibrium for several hundreds of femtoseconds [56,57]. The assumption of equilibrated upper bands is fulfilled in many existing experiments [29][30][31]33] since such a quasiequilibrium state is reached at probe time [56].…”

“…This condition sets the lower temporal limit when our approach is valid since the occupation of these bands may stay out of equilibrium for several hundreds of femtoseconds [56,57]. The assumption of equilibrated upper bands is fulfilled in many existing experiments [29][30][31]33] since such a quasiequilibrium state is reached at probe time [56]. For such cases, we can describe the material properties with quasi-equilibrium electron states that are uniquely defined by the electron temperature.…”

“…Here, we consider optical excitation of gold, where two active bands contribute, namely, the 5d valence and the 6sp conduction electrons. The additional excitation of core electrons is possible with X-rays or XUV pulses [39,56] but should not be addressed here, mainly, due to the ultrashort lifetimes of such core holes.…”