Rate equation for intense core-level photoexcitation and relaxation in metals

A dynamical equation for intense core-valence photoexcitation and collisional relaxation in a metal is derived by combining cluster-model electronic-structure calculations and the equation of motion for density matrices. By invoking the Markov approximation, it can be reduced to a generalized rate equation, where the rate coefficients for collisional and radiative transitions are derived from the cluster Hamiltonian in a self-contained fashion; they incorporate energy-level shifts due to electron-hole interactions as well as modification of dipole transition matrix elements due to induced polarization. Numerical examples are shown for K-edge excitation of lithium metal by a laser pulse with the photon energy of 60 eV and the intensity of 10(14) W cm(-2). It is found that creation of multiple core holes gives rise to a shift of the absorption edge towards the high-energy side, leading to a reduction of the absorption coefficient within 10 fs timescale. Such an ultrafast switching mechanism may have relevance to recently demonstrated nonlinear transmission of soft x-ray and vacuum ultraviolet free-electron laser pulses through metallic targets.