Dependence of photoabsorption spectra on long-range fields

Strongly energy-dependent photoabsorption cross sections near excitation thresholds have been parametrized in terms of separate short- and long-range field contributions. The largest portion of this energy dependence results from standard parameters characterizing the long-range field effects; these parameters are known analytically for the most common outer fields, and can be calculated for others independently of the complicated short-range dynamics. The approach is illustrated in the Appendix by using a semiempirical fit to the total ${\mathrm{H}}^{-}$ photodetachment cross section to predict partial cross sections.