Dissociation dynamics of anionic carbon monoxide in dark states

A resonant system consisting of an excess electron and a closed-shell atom or molecule, as a temporary negative ion, is usually in doublet-spin states that are analogous to bright states of photoexcitation of the neutral. However, anionic higher-spin states, noted as dark states, are scarcely accessed. Here, we report the dissociation dynamics of CO− in dark quartet resonant states that are formed by electron attachments to electronically excited CO (a3Π). Among the dissociations to O−(2P) + C(3P), O−(2P) + C(1D), and O−(2P) + C(1S), the latter two are spin-forbidden in the quartet-spin resonant states of CO−, while the first process is preferred in 4Σ− and 4Π states. The present finding sheds new light on anionic dark states.