  We present a theoretical method for calculating multiphoton ionization
amplitudes and cross sections of few-electron atoms. The present approach is
based on an extraction of partial wave amplitudes from a scattering wave
function, which is calculated by solving a system of driven Schroedinger
equations. The extraction relies on a description of partial waves in terms of
a small number of Coulomb waves with fixed wave numbers. The method can be used
for photon energies below and above the ionization threshold and to treat
resonance-enhanced multiphoton ionization. We use it to calculate two-, three-,
and four-photon ionization cross sections of hydrogen and helium atoms for a
wide range of photon energies and to determine the asymmetry parameters of
photoelectron angular distributions for two-, three-, and four-photon
ionization of the helium atom.
