Formation of HS− and DS− by Dissociative Attachment in H2S, HDS, and D2S

Abstract

We have studied the formation of HS⁻ by dissociative attachment in a total ionization chamber, with a RPD electron gun. The onset of the ionization efficiency curve ($\text{1.45 ± 0.1}$ eV) is equal to the minimum energy necessary for the formation of HS⁻ ($\text{1.55 ± 0.03}$ eV). Nevertheless the curve does not exhibit the usual shape characteristic of a vertical onset process. The cross section at the maximum is $\mathrm{\sigma \; (}{\mathrm{HS}}^{-}/{\mathrm{H}}_2\mathrm{S}{\text{)=1.7 × 10}}^{\text{−18}}\hspace{0.17em}{\mathrm{cm}}^2.$ We measured the isotope effects separately, in a mass spectrometer, and we found that the cross section in D₂S is much smaller: $\mathrm{\sigma \; (}{\mathrm{HS}}^{-}/{\mathrm{H}}_2\mathrm{S}\mathrm{)\; /\sigma \; (}{\mathrm{DS}}^{-}/{\mathrm{D}}_2\mathrm{S}\text{)=25 ± 3}$ . We tried to reproduce the experimental results by a semiempirical calculation, using the expression of O'Malley for the cross section and parameterizing the final state potential curve of the negative ion. We have shown that the experimental results are consistent with the assumption of a very shallow bound potential in the Frank–Condon region. Quantitative calculations are impossible because the formula of O'Malley is incorrect for that sort of potential curve.