Benchmark multiphoton ionization rates for He at 390 nm

We compare single-electron multiphoton ionization rates of He obtained by the R-matrix Floquet approach with those obtained by the numerical integration of the time-dependent Schrodinger equation. The calculations are performed at a laser wavelength of 390 nm, a wavelength accessible to Ti:sapphire lasers via frequency doubling. For intensities between 1 x 10(14) W cm(-2) and 2.5 x 10(14) W cm(-2), we find general agreement between the two approaches within 10%. Over this range of intensities we further find our single-ionization rates typically two orders of magnitude greater than those predicted by the ADK model. The ionization rates are strongly enhanced by resonances. This resonance structure is strongly distorted by the laser field: the 1s3d D-1 state lies below the 1s3s S-1 state at the present intensities.