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 circula rly polarized field. We solve numerically the time-dependent Schroding er equation for the atomic wave function, which beforehand is expanded in a Coulomb-Sturmian basis. We analyze the ionization from various i nitial states differing by their azimuthal quantum number m, the princ ipal and the angular quantum numbers being equal to 2 and 1, respectiv ely. 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 stud y the electron energy spectra for the same cases and show that they ar e characterized by a multiple splitting of the above-threshold-ionizat ion (ATI) peaks. The behavior of the ionization yield as well as the s tructure 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 t he aspects of the underlying dynamics by means of a very simple essent ial-state model.