Time-dependent and time-independent methods applied to multiphoton ionization of helium

A comparison is made between results from the R -matrix Floquet approach and the direct numerical integration of the full-dimensional time-dependent Schrödinger equation for the multiphoton ionization of helium. Good agreement is obtained away from resonances in the intensity range 5 × 10 13 to 4 × 10 14 W cm -2 at a laser wavelength of 248.6 nm. This agreement confirms error estimates made independently of the comparison. We review and contrast the two approaches, and discuss details of the convergence tests by which error estimates are obtained.