Calculations of resonance enhanced multiphoton ionization of argon in a KrF laser field

We perform Floquet calculations of multiphoton ionization of argon in a KrF laser field for intensities up to 10(14) W cm(-2). As far as we are aware these results are the first ab initio Floquet calculations for a many-elect ron system at these intensities that are amenable to direct comparison with experiment. We discuss the accuracy and limitations of our calculations, i n particular with respect to the description of the argon atom. We also sho w that resonance features of the calculations may be understood in terms of two-state plus continuum models which depend on the relative strengths of the dipole interaction between the two stares and between each stare and th e continuum. We predict a substantial step-like rise in the underlying ioni zation rate as the laser intensity passes 10(14) W cm(-2) The calculations are performed using a new version of the R-matrix Floquet method.