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 (a(3)?). Among the dissociations to O-(P-2) + C(P-3), O-(P-2) + C(D-1), and O-(P-2) + C(S-1), the latter two are spin-forbidden in the quartet-spin resonant states of CO-, while the first process is preferred in S-4(-) and (4)? states. The present finding sheds new light on anionic dark states.