Electron-phonon energy relaxation in bismuth excited by ultrashort laser pulse: Temperature and fluence dependence

We present analysis of the experiments on excitation of bismuth by ultrafast laser pulses and compare with heating bismuth in equilibrium conditions. The analysis shows that the electron-phonon relaxation time is a strong function of the lattice temperature. We developed a kinetic theory, which predicts well the experimental results. We demonstrate that lattice heating and re-structuring with the temperature-dependent energy exchange rates occurs much faster than what follows from the two-temperature model with constant relaxation factor. The analytic formulae corrected by equilibrium and non-equilibrium data allowed the interpretation of various experiments without controversy. We demonstrate that all observed ultrafast transformation of bismuth are purely thermal in nature, thus excluding the conjectures about non-thermal melting.