Threshold phenomena in the cross sections for atomic photoprocesses in a strong laser field

Using the effective-range theory in the complex-quasienergy method for describing a weakly bound electron in a laser field, it has been shown that the cross sections for above-threshold ionization and generation of the high harmonics of laser radiation increase significantly (resonant-like) near the threshold of one of the multiphoton ionization channels. The dependence of the enhancement effects on the orbital angular momentum of the bound electron shows that these effects are caused by the threshold phenomena in the cross sections for multichannel reactions that are modified by the strong laser field.