Theory of Three-Photon Ionization of the Alkali Atoms

Three-photon ionization transition rates are calculated for the alkali atoms (except Li) using quantum defect Coulomb functions to evaluate the well-known perturbation formula. Several corrections of previous work are mentioned. The often-used approximation of keeping only a small number of intermediate states is investigated. The predicted three-photon ionization rate (in cgs units) of the alkalis for the ruby photon energy of 1.78 eV is of the order of ${10}^{-77\mathrm{}}$ × ${\mathrm{}\left(\mathrm{photon}\mathrm{flux}\right)\mathrm{}}^3$. Detailed results are presented as dispersion curves of the transition rate for photon energies between the three-photon and two-photon thresholds.