Alkali negative ions. III. Multichannel photodetachment study ofCs−andK−

Abstract

A crossed ion-laser beam apparatus has been used to obtain ${\mathrm{Cs}}^{-}$ and ${\mathrm{K}}^{-}$ cross sections for photodetachment into the ground (${}_{}{}^2{}_{}{}^{}S$) and first excited state (${}_{}{}^2{}_{}{}^{}P$) of the neutral atom in the photon energy range near the threshold for production of ${}_{}{}^2{}_{}{}^{}P$ neutrals. A total cross section is determined from the cross section for production of neutral atoms and the partial cross sections are obtained from the cross section for production of photoelectrons transmitted through a low-pass kinetic-energy filter. The filter characteristics are determined empirically and the discrimination between ground-and excited-state detachment channels is approximately 1000. It is apparent from the data that the detachment channels are strongly coupled in the threshold region, a result attributed to long-range forces between the detached electron and the neutral atom. The data are compared with a semiempirical multichannel photodetachment model developed by Lee; the model is quite successful in its prediction of the energy-dependent cross section for production of Cs ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2},\frac{3}{2}}^{}{}_{}{}^{}$ from knowledge of the previously measured ${\mathrm{Cs}}^{-}$ total photodetachment cross section. We have produced an approximate fit to the ${\mathrm{K}}^{-}$ data with this model. The following electron affinities have been determined: for Cs, 0.4715(3) eV, and for K, 0.50147(10).