IONIZATION OF AN EXCITED HYDROGEN-ATOM BY A HIGH-FREQUENCY CIRCULARLY-POLARIZED PULSED-FIELD

We study the ionization of atomic hydrogen by a high-frequency circularly polarized field. We solve numerically the time-dependent Schrodinger equation for the atomic wave function, which beforehand is expanded in a Coulomb-Sturmian basis. We analyze the ionization from various initial states differing by their azimuthal quantum number m, the principal and the angular quantum numbers being equal to 2 and 1, respectively. At high field intensities we observe important differences in the ionization yield: strong stabilization is observed for m = 0 and m = 1 while for m = -1 the atom is almost completely ionized. We also study the electron energy spectra for the same cases and show that they are characterized by a multiple splitting of the above-threshold-ionization (ATI) peaks. The behavior of the ionization yield as well as the structure observed in the ATI spectra are explained in terms of the ac-Stark shift and width of the atomic levels. We also consider some of the aspects of the underlying dynamics by means of a very simple essential-state model.