High-order above-threshold ionization of argon: Plateau resonances and the Floquet quasienergy spectrum

The Floquet quasienergy spectrum of argon in a strong laser field of 800 nm wavelength is calculated for intensities up to 7x1013 W cm-2, and beyond for some states, using a discrete complex basis set. Many of the dressed excited states of interest shift nonponderomotively in complicated ways but keep an ionization width narrow enough to produce sharp enhancements of above-threshold ionization (ATI) through Stark-shift-induced resonances. The quasienergy map is compared to high-resolution ATI spectra for 120 fs Ti:sapphire pulses [Nandor et al., Phys. Rev. A 60, R1771 (1999)]. The plateau enhancements happen at intensities where the dressed ground state is in resonance or in the wing of resonances with dressed excited states. The resonant dressed states are identified. In many cases, the same state is responsible for an enhancement of ATI in the low as well as the high orders. No evidence is found for enhancements that are not concomitant with any curve crossing and could thereby be interpreted as channel-closing enhancement