Multi-temperature atomic ensemble: nonequilibrium evolution after ultrafast electronic excitation

Ultrafast laser radiation or beams of fast charged particles primarily excite the electronic system of a solid driving the target transiently out of thermal equilibrium. Apart from the nonequilibrium between the electrons and atoms, each subsystem may be far from equilibrium. From the first principles, we derive the definition of various atomic temperatures applicable to electronically excited ensembles. It is shown that the definition of the kinetic temperature of atoms in the momentum subspace is unaffected by the excitation of the electronic system. When the electronic temperature differs from the atomic one, an expression for the configurational atomic temperature is proposed, applicable to the electronic-temperature-dependent interatomic potentials (such as ab-initio molecular dynamics simulations). We study how the configurational temperature behaves during nonthermal phase transition, triggered by the evolution of the interatomic potential due to the electronic excitation. It is revealed that upon the ultrafast irradiation, the atomic system of a solid exists temporarily in a multi-temperature state: separate equilibria in the momentum and configurational subspaces. Complete equilibration between the various atomic temperatures takes place at longer timescales, forming the energy equipartition. Based on these results, we propose a formulation of multi-temperature heat transport equations.