Excited hydrogenic atom in a strong low-frequency electromagnetic field

Abstract

The multiphoton ionization of a bound electron state which is twofold degenerate with respect to its orbital angular momentum is considered in a quasiclassical approximation. It is shown that the ionization probability increases strongly in an intense electromagnetic field, in which nonresonant mixing of the levels forming the degenerate state is significant, in comparison to the case described by the Keldysh formula. It is also shown that such degeneracy leads to a sharp increase in the intensity of the radiation scattered by the bound electron, and the high-frequency cutoff of the emission spectrum is shifted to higher frequencies.