Photoionization of atoms described by Fermi Molecular Dynamics: toward a firmer theoretical basis

The application of Fermi Molecular Dynamics (FMD) to the modeling of the photoionization of atoms by a short pulse of long wavelength laser radiation is examined in detail. Depression of the single ionization threshold to values of the electric field strength below the classical over-the-barrier threshold, a common occurrence in FMD, is shown to arise from the preexcitation of bound electrons into a continuum of unphysical low-lying excited states. A connection is made to analogous calculations performed with the quantum Hamilton-Jacobi equation, in which the time-dependent quantum potential ($Q$) plays a role similar to that played in FMD by the so-called Heisenberg potential (V H). Replacement of (V H) by Q in the FMD equations of motion results in a large reduction in the number of excitations to unphysical bound states, while producing no essential change in the photoionization probability.