Abstract
Theoretical photoabsorption cross sections for alkali-metal atoms in a static electric field are derived for energies near the zero-field ionization threshold (), extending a previous development for hydrogen. Spectra result from zero-field dipole matrix elements and a density-of-states matrix . The factors represent a renormalization of the photoelectron's wave function necessitated by the long-range Coulomb plus Stark potential. The matrix contains all spectral information on quasidiscrete Stark levels and continuum resonances, expressed as an algebraic function of (1) quantum defects embodying core effects, (2) a frame transformation between spherical and parabolic coordinates in the pure Coulomb field outside the alkali-metal core, and (3) factors and calculated from asymptotic amplitudes and phases of hydrogen-Stark wave functions of the parabolic dissociation channels. Hydrogenic parameters are calculated semianalytically within the WKB approximation; and dipole matrix elements are known independently. Predicted cross sections for photoionization of the excited state of Na agree with experiment. The theory reproduces (a) asymmetric resonances observed at , parametrized as a Beutler-Fano profile for a simple case, and (b) the oscillations observed at , attenuated by factors from their predicted depth in H. Dependences on light polarization are sorted out for two-photon excitation.
- Received 26 January 1982
DOI:https://doi.org/10.1103/PhysRevA.26.2656
©1982 American Physical Society