Ion-pair dissociation (IPD) usually occurs in molecular superexcited states, exhibiting complicated dynamics. Electron-impact-induced IPD is scarcely investigated, due to challenges both in the experimental measurements and data interpretation. Here, we report an experimental study of the IPD dynamics of carbon monoxide (CO) by low-energy (25–45 eV) electron impact, using an anion velocity map imaging technique. Branching ratios between the anionic yields ${\mathrm{C}}^{-}$ and ${\mathrm{O}}^{-}$ indicate a predominance of ${\mathrm{O}}^{-}$ production, while a slight enhancement of the minor product ${\mathrm{C}}^{-}$ with an increase of electron-impact energy is observed. The electron-impact energy dependences of the ${\mathrm{O}}^{-}$ and ${\mathrm{C}}^{-}$ velocity images are insignificant while they do exist at relatively low-impact energies. Moreover, the ${\mathrm{O}}^{-}$ and ${\mathrm{C}}^{-}$ velocity images exhibit distinctly different forward-backward asymmetries at a common electron energy. The electron-impact IPDs observed here are classified into an indirect process via doubly excited Rydberg states of CO and a direct process from the repulsive ion-pair states, and the ${\mathrm{O}}^{-}$ and ${\mathrm{C}}^{-}$ angular distributions are further elucidated with partial-wave analyses of quantum scattering theory.

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• Electron & positron scattering
• Scattering of atoms, molecules, clusters & ions