Multiphoton resonant ionization of a hydrogen-like atom by a strong electromagnetic field

Multiphoton resonant ionization in the case of a doubly degenerate intermediate bound state is studied. In the Keldysh-Feisal-Reiss approach, expressions for the energy and angular photoelectron distributions and the quasi-classical formula for the total rate of resonant ionization are obtained. It is shown that the ionization rate may be both higher and lower than the ionization rate in the usual case depending on the relationship between parameters. A situation with a strongly suppressed probability of resonant ionization is possible. In the near-threshold region, the angular dependence of the probability of photoelectron escape is shown to be weaker in comparison with the case of ionization via a nondegenerate level.