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 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 multitemperature 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 multitemperature heat transport equations.

<a href= https ror org 01jkd3546 >P N Lebedev Physical Institute< a> of the <a href= https ror org 05qrfxd25 >Russian Academy of Sciences< a> Leninskij Prospekt 53 119991 Moscow Russia

Institute of Physics of the <a href= https ror org 02yhj4v17 >Czech Academy of Sciences< a> Na Slovance 1999 2 182 21 Prague 8 Czech Republic

Institute of Plasma Physics of the <a href= https ror org 053avzc18 >Czech Academy of Sciences< a> Za Slovankou 1782 3 182 00 Prague 8 Czech Republic

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