Two-Photon Ionization of Atomic Hydrogen. II

Abstract

This paper contains a generalization of results described in previous work. A general formula is given for the total cross section for two‐photon ionization of a hydrogenic state. An implicit method for evaluating the second‐order radial matrix elements that occur in this expression is described in detail. Numerical results were obtained for the states with principal quantum numbers n = 1 through 5. It is concluded that, when both one‐photon and two‐photon ionization are energetically possible, the effect of the latter may be expressed as an intensity‐dependent correction to the Gaunt factor which may therefore be written as G = g₁ + ζIλ³g₂. Here g₁ is the usual Gaunt factor, I is the intensity of the light in W cm⁻², λ is the wavelength in cm, g₂ is a dimensionless factor of order unity, and ζ is a constant given by ζ = 0.1504 W⁻¹ cm⁻¹. Graphs of g₂ are given as a function of electron energy. These results do not include the effect of three‐photon processes, which can also contribute to the first‐order intensity‐dependent correction to the Gaunt factor as a result of interference between the first‐ and third‐order amplitudes.