High-Resolution Photodetachment Study ofSe−Ions

Abstract

Photodetachment of ${\mathrm{Se}}^{-}$ ions is studied in a crossed-beam experiment, in which a 2-keV negative-ion beam is intersected by a pulsed tunable dye laser ($\Delta \lambda \approx 1-2$ Å). The threshold behavior of the photodetachment cross section $\sigma$ is compatible with Wigner's law ${\sigma }_L\propto k^{2L+1}$ ($k$ is the magnitude of the momentum of the outgoing electron and $L$ is its orbital angular momentum) over only about 5 meV above threshold in this case ($L=0\mathrm{}$). Some differences are observed for the threshold behavior of transitions terminating in different final states of $\mathrm{Se}\left({}_{}{}^3{}_{}{}^{}P_{2,1,0}^{}{}_{}{}^{}\right)$. These results are discussed in the light of existing theory. The observation of four of the six possible fine-structure transition thresholds allows us to determine unambiguously the electron affinity (EA) of $\mathrm{Se} \left\{\mathrm{EA}\right[\mathrm{Se}=(16297\mathrm{}\pm 2) {\mathrm{cm}}^{-1\mathrm{}}\cong (2.0206\mathrm{}\pm 0.0003) \mathrm{eV}\left]\right\}$ and the spin-orbit splitting between the ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ states in ${\mathrm{Se}}^{-}\left[\right(2279\mathrm{}\pm 2\left) {\mathrm{cm}}^{-1\mathrm{}}\cong \right(282.6\mathrm{}\pm 0.3\left) \mathrm{MeV}\right]$. Comparison of experimental results for this splitting in ${\mathrm{O}}^{-}$, ${\mathrm{S}}^{-}$, and ${\mathrm{Se}}^{-}$ is made with predictions from isoelectronic extrapolation. The transition strengths for the various fine-structure transitions are determined and agree well with predictions by Rau and Fano. Finally, the absolute cross section for ${\mathrm{Se}}^{-}\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$ detachment is reported.