Two-photon ionisation of atomic hydrogen in the presence of one-photon ionisation

A numerical method is proposed to calculate N-photon ionisation cross sections, when ionisation by N-1 photons is energetically possible. Some results obtained for N = 2 are presented. Cross sections obtained for two-photon ionisation of ns states of hydrogen by circularly polarised light are in excellent agreement with previous theoretical ones. As an example the method is also applied to lithium atoms. (author)