Theory of two- and three-photon ionisation of metastable helium atoms

Calculations of discrete oscillator strengths, of AC Stark shifts and of single and multiphoton ionisation cross sections are presented for helium atoms. The wavefunctions of excited (bound and continuum) states are determined in the framework of the central field approximation; two optimal parametric potentials are determined for singlet and triplet systems. The reliability of the wavefunctions is shown by comparing oscillator strengths for various transitions and photoionisation cross sections for the 21S, 23S, 21P and 23P states with previous accurate theoretical results. Cross sections for two- and three-photon ionisation of 21S and 23S metastable states are calculated for linearly polarised light for a large range of photon frequencies. The AC Stark shifts of resonant transitions are evaluated. Cross sections and shifts are calculated by means of perturbation theory; the infinite summations over intermediate states are evaluated by means of perturbed-function solutions of single-particle equations. The results are compared with experiment as well as with previous calculations.