One-dimensional Thomas-Fermi model of atoms, molecules, and small clusters exposed to an intense laser field

Abstract

We have developed a one-dimensional time-dependent Thomas-Fermi model of atoms, molecules, and small clusters exposed to an intense subpicosecond laser field. In this model, the dynamics of an electronic cloud is governed by the hydrodynamic equations of motion, whereas the nuclei move in accordance with the Newton equations. The quantum mechanics enters in this approach only through the constitutive relation between the pressure and the density, which is derived from the application of Fermi-Dirac statistics to a noninteracting $T=0$ temperature electron gas. The time-independent version of the model, formulated in terms of the integral equation for the electric potential, is also discussed. We present numerical results for diatomic molecules and small clusters irradiated by the strong laser pulses. In the case of molecules, we observe the multielectron ionization accompanied by dissociation. The kinetic energy defect, understood as a reduction of the energy of resulting ions in comparison with a simple Coulomb explosion picture, is explained in terms of a screening effect of escaping fragments by the ejected electrons. For small clusters we find that the explosion of the cluster has a stepwise character; the consecutive layers of atoms are stripped off one by one. We observe the highly energetic (as compared with diatomic systems) atomic fragments even for relatively low laser intensities and a few atoms clusters. Also, the model predicts the nonuniform energy distribution among the same charge state ions and supports the idea of hot electrons generated in the cluster via a mechanism of inverse bremsstrahlung.