Dissociative attachment of low-energy electrons to the electronically excited and bent ${}^1{B}_2$ state of ${\mathrm{CS}}_2$ is studied. The measurements are carried out in a crossed three-beam geometry in which a laser beam is used to prepare the excited molecules. The absolute cross sections for the formation of ${\mathrm{S}}^{-}$ and ${\mathrm{CS}}^{-}$ by dissociative attachment to the ${}^1{B}_2$ excited state are determined by a measurement of the relative number density in the excited state. Based on the earlier observation of the formation of ${\mathrm{S}}_2^{-}$ from the linear symmetric ground state of ${\mathrm{CS}}_2,$ an increase in the ${\mathrm{S}}_2^{-}$ cross section was expected from the bent excited state. However, ${\mathrm{S}}_2^{-}$ is found to be absent from this state. Possible mechanisms are proposed to explain this counterintuitive result.

• Received 19 May 2000

DOI:https://doi.org/10.1103/PhysRevA.64.012707