Two-quantum processes in the coupled Hartree-Fock approximation

A method, based upon time-dependent Hartree-Fock theory, is developed for calculating the efficiencies of two-photon processes The lifetime for two-photon decay of He(2 1 S) is calculated with the result 2 0 × 10 -2 sec. Estimates for the cross section for the two-photon ionization of the ground state of helium are given for photon energies near the two-photon threshold. For incident photons at a wavelength of 1000 Å the cross section is 5 × 10 -31 I cm 1 w -1 , where I (w cm -2 ) is the intensity of the incident photons.