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 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 ∼ 1 nm , respectively. It is found that, firstly, the electron-electron collisions make a minor contribution to a light absorbtion 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 0 < t < 4 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.Key words: femtosecond laser ablation, pump-probe, optics of hot Al and Au PACS: 52.38.Mf, 52.25.Os, 02.70.Ns
Supersonic heating and meltingFigures 1,2 show diagrams of processes in pump femtosecond laser pulse (fsLP) action on metal. The three time slices "ei", m 1 m 2 , and c 1 c 2 in Fig. 1 correspond to the following non-equilibrium processes: (e-i) the electron-ion thermal relaxation, (m) the melting of an overheated crystal lattice, and (c) the cavitation decay of a metastable state. Duration of fsLP τ L ∼ 40 − 100 fs is shorter than characteristic times of these three processes. They have very various time scales from subpicoseconds to nanoseconds. The electron overheating (T e ≫ T i ) starts from ei 1 when a fsLP arrives [1,2,3,4,5,6,7,8,9] and disappears at ei 2 when temperatures T e , T i equilibrate (t eq = t ei2 = 3 − 6 ps for Al at our intensities). The time is reckoned from the maximum of pump fsLP in Fig. 1. Since arriving of the pump to a target the conductivity electrons become much hotter than the ions. Two-temperature (2T) matter with hot electrons transits to a peculiar state with thermodynamic and optical characteristics different from one-temperature (1T) case. In 2T there are appearance of excesses of electron energy and pressure above equilibrium 1T ones. Also there are changes in elastic moduli and band structure. In semiconductor lattice the binding forces become weaker with increase of T e , while in metals situation is opposite. Large changes in optics of Au at high T e result from * +7- 495-7029317, Russian Federation, 142432, Chernogolovka Email address: nailinogamov@googlemail.com excitation of d-electrons. On account of the ion heat capacity C i (thermal "inertia" of a lattice) the beginning of melting t m1 ∼ C i T m /αT e is de...