Theory of the Nonhydrogenic Stark Effect

Photoionization spectra of alkali atoms in a dc electric field are calculated semianalytically and agree with experiment. Cross sections depend on two distinct sets of parameters: (a) zero-field dipole matrix elements and quantum defects and (b) eigenfunctions of the external Coulomb-Stark potential. The field's effect on photoionization is condensed into a density-of-states matrix ${〈{\ensuremath{\Psi}}^{\ensuremath{'}}|\ensuremath{\Psi}〉}^{\ensuremath{-}1}$, whose use may be appropriate to long-range effects in all spectral phenomena.