Two-temperature relaxation and melting after absorption of femtosecond laser pulse

The theory and experiments concerned with the electron–ion thermal relaxation and melting of overheated crystal lattice constitute the subject of this paper. The physical model includes two-temperature (2T) equation of state, many-body interatomic potential, the electron–ion energy exchange, electron thermal conductivity, and optical properties of solid, liquid, and two phase solid–liquid mixture. Two-temperature hydrodynamics and molecular dynamics codes are used. An experimental setup with pump–probe technique is used to follow evolution of an irradiated target with a short time step 100 fs between the probe femtosecond laser pulses. Accuracy of measurements of reflection coefficient and phase of reflected probe light are 1% and image1 nm, respectively. It is found that, firstly, the electron–electron collisions make a minor contribution to a light absorption in solid Al at moderate intensities; secondly, the phase shift of a reflected probe results from heating of ion subsystem and kinetics of melting of Al crystal during image ps, where t is time delay between the pump and probe pulses measured from the maximum of the pump; thirdly, the optical response of Au to a pump shows a marked contrast to that of Al on account of excitation of d-electrons.