Quantitative Theory of the Two-Photon Ionization of the Alkali Atoms

The theory of two-photon ionization of the alkali atoms is developed from the well-known perturbation formula for the second-order transition rate. Quantum defect Coulomb functions are used for both the negative and positive energy states. Spin-orbit splitting of the intermediate $p$ states is taken into account. The results are presented as dispersion curves of the transition rate for photon energies between the two-photon and one-photon thresholds. Cesium is of particular interest because the photon energy for the second harmonic of ruby laser light falls very near the $10p^2{P}_{\frac{3}{2}}$ level. For a photon energy of 3.57 eV, the transition rate (in cgs units) is found to be 2.6×${10}^{-49\mathrm{}}$×${\mathrm{}\left(\mathrm{photon}\mathrm{flux}\right)\mathrm{}}^2$.