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 ph otoionization of atoms by a short pulse of long wavelength laser radiation is examined in detail. Depression of the single ionization threshold to val ues of the electric field strength below the classical over-the-barrier thr eshold, a common occurrence in FMD, is shown to arise from the preexcitatio n of bound electrons into a continuum of unphysical low-lying excited state s. A connection is made to analogous calculations performed with the quantu m 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 p otential (V-H). Replacement of V-H by Q in the MD equations of motion resul ts in a large reduction in the number of excitations to unphysical bound st ates, while producing no essential change in the photoionization probabilit y. (C) 2001 Optical Society of America.