Low-frequency theory of multiphoton ionization

To describe multiphoton ionization in a strong field of low frequency ω, we develop an asymptotic expansion in powers of ω2 of the ac quasienergy; the leading term is the (cycle-averaged) dc quasienergy. We conjecture that the ac width and ac shift (calculated within this framework) satisfy a dispersion relation, similar to that satisifed by the dc width and dc shift. We have calculated the first few terms of the expansion for the ground-state hydrogen atom in a circularly polarized field, and we find good agreement with the exact values for the shift and width down to wavelengths of about 300 nm, at intensities of order 1014 W/cm2 or more