The energy spectrum of a charged particle bound by a short-range potential and subjected to an intense ac electromagnetic field is computed by using complex coordinates within the Floquet formalism. As the field strength is raised, additional discrete states appear and the lifetime of the states increase. The adequacy of a high-frequency model of the strong-field spectra is assessed.

• Received 28 November 1988

DOI:https://doi.org/10.1103/PhysRevA.39.2252