Selection Rule forΣ+↔Σ−Transitions in Electron-Molecule Collisions

Abstract

The differential cross section for the electron-impact excitation of the $b{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{+}$ state from the ground state of ${\mathrm{O}}_2$ is found to decrease rapidly as the scattering angle approaches zero. This behavior is the diatomic-molecule analog of "parity-unfavored" transitions in electron-atom scattering. A group-theoretic argument is given which explains the observations and provides a selection rule for ${\Sigma }^{+}\leftrightarrow {\Sigma }^{-}$ transitions in diatomic molecules.