Interpretation of Electric-Field-Induced Oscillations in the Cross Section of a One-Electron Atom above the Ionization Limit

Exact calculations of the photoionization cross sections of ground-state H in the presence of an external electric field are presented. The oscillations obtained above the zero-field ionization limit for $\pi$ polarization do not arise from specific properties of $m=0\mathrm{}$ states, but from cancellation effects in the oscillator strengths due to the symmetry properties of the wave functions and of the light. The dependence of the cross section upon the light polarization is thoroughly explained and agrees with recent experiments on Rb.