Ionization of the Thomas-Fermi atom in intense laser fields: the static limit revisited

The problem of the Thomas-Fermi atom in strong static homogeneous electric fields is revisited. The static field situation is a reasonable approximation to the otherwise fully time-dependent problem of an atom in a laser pulse, provided that the laser frequency is not too high. Exact results for the ionization degree versus the applied electric field, obtained numerically, are compared with those from an approximate model which is much easier to solve. Very good agreement is found. Although the approximate model was published ten years ago, an unexplained discrepancy with both experimental results and the exact numerical solution of about one order of magnitude in the ionization degree had remained. In this paper it is shown that this discrepancy does not exist. A comparison with the simpler `over the barrier ionization' model and available experimental data for rare gases is made. A very appealing feature of the Thomas-Fermi model is its universality thanks to the scaling behaviour with respect to the nuclear charge Z .