|
The advent of femtosecond laser technique has stimulated investigations of electron relaxation processes in solids'8. At last investigators have obtained a unique tool for direct monitoring of ultrafast electron dynamics in real time scale. These investigations are of considerable importance both from scientific and practical points of view. Ultrafast electron processes are fundamental for the physics of surfaces. They determine a variety of surface processes such as adsorption, desorption, phase transitions, catalytic chemical reactions on a surface, etc., which have broad applications. The first studies of the electron relaxation in metals with the use of femtosecond laser pulses revealed that the electron distribution in metals is not equilibrium in the coarse of irradiation"2. It was shown that thermalization of the non-equilibrium electron gas is determined by the electron-electron relaxation and can be described by the Landau's Fermi-liquid theory. The latest detailed measurements ofthe energy dependence of the electron-electron relaxation times have supported this conclusion6'7. Some discrepancy between the experimental data and the Fermi-liquid model has been also reported for the cases when electrons from d-bands were involved in the process of relaxation4'8. It should be mentioned that thermalization of the electron gas takes place for the times comparable to the characteristic time of electron4attice energy transfer1 ; that is, the relaxation of the optically excited electron gas in a metal can not be treated as a two-step process9'1° where the electron-electron thermalization and electronlauice energy transfer are separated in time. One can expect that the electron distributions not thermal for any regime of optical excitation (to be correct, here the electron distribution averaged over the laser period is meant). So, for example, a study of the lattice temperature dependence of the electron-electron relaxation time in Au and Ag showed that the electron distribution is indeed nonthermal on the time scale of the electron-lattice energy transfer time . Inthis work we present a quantum mechanical kinetic theory for the electron gas in a metal excited by a short laser pulse. This work is ideologically based on our previous theory" ,where the electron distribution was determined from the solution of the Boltzmann equation with the additional phenomenologically introduced integral of electron-phonon collisions. Here we will consider both excitation due to laser-stimulated absorption of laser photons in electron-phonon collisionless and relaxation due to electron-electron and electron-phonon collisions not referring to any model assumptions. No other theory can now do that. The model calculations in' consider only the relaxation of the initially nonthermal distribution due to the electron-electron relaxation and do not explain how this initial distribution is created. The theory by Bejan and Raseev 'is based on the classical Boltzmann equation and phenomenologically accounts of only one-quantum absorption in a phenomenological way.
|
