Freeman resonances in high-order above-threshold ionization

Photoelectron energy spectra for ionization in a strong laser field are calculated within the Floquet framework, for a one-electron model of argon and for other model potentials. The results indicate that, at least for linear polarization and not too short pulse durations, the plateau enhancements dominating high-order above-threshold ionization in rare gases originate from Stark-shift-induced resonances with dressed excited states similar in spatial extension to low-excited field-free states.