Near-threshold ionization of atomic hydrogen

We consider near-threshold ionization of atomic hydrogen by strong laser light. Nonperturbative features of the photoelectron spectrum and double optical resonance spectrum (in the V configuration) are obtained and discussed. We show that in contrast to the usual sign of the ac Stark shift the ionization threshold is dynamically lowered due to ``pulling'' by the atoms near threshold Rydberg levels. Dynamical widths of the spectra are also found. Our results are obtained analytically by means of a variant of the single-pole approximation and are qualitatively consistent with experimental results.