Thermodynamics and transport properties in the transient regime

The nonequilibrium behaviour in the transient regime of metals excited by u ltrashort optical pulses is investigated by means of a second order expansi on of the Boltzmann equation. By definition, the transition range is locate d between the time necessary for the establishment of the electron temperat ure and the time where a description by the standard steady state equations is justified. Relaxation functions are derived for the electrical and ther mal currents, and the relaxation times related to them are determined. It i s shown that for the electrical transport the relaxation time corresponds t o Drude's momentum scattering time whereas the corresponding time for the h eat how is identified as the electron temperature relaxation time. Further, expressions for the electrical and thermal conductivity are obtained in th e case of a local thermal nonequilibrium between the electron and phonon su bsystems in first and second order, respectively. Consequences for the dete rmination of the temperature distributions inside metals are discussed. The solution of the Boltzmann equation is also used for the calculation of the time dependent energy distribution function of the electrons. The results are in good agreement with the experiment.