Electron and phonon distribution relaxation in metal films under a femtosecond laser pulse

The simultaneous time evolution of electron and phonon distributions in a gold film submitted to a ferntosecond laser pulse has been theoretically investigated. A system of two coupled time-dependent Boltzmann equations describing electron and phonon dynamics has been solved by considering electron-electron (e-e) and electron-phonon (e-p) collisions. Phonon-phonon (p-p) collisions have been neglected. The collision probability of each process is derived by quantum-mechanical first order perturbation theory. The injected laser energy (source term) is directly absorbed by the electron gas through electron-photon collisions and a perturbation term is added to the Boltzmann equation for electrons. However, phonons do not directly absorb energy from the laser but they are affected by the irradiation due to their collisional interaction with electrons. The results emphasize the transient non-equilibrium behavior of the electron distribution function and details of electron and phonon relaxation dynamics.