In the search for new molecules in the interstellar medium (ISM), it is becoming increasingly clear that anions are more significant than previously believed.  The l-C3H- anion has recently been put forth as the carrier of rotational lines observed in the Horsehead nebula photodissociation region originally attributed to l-C3H+.  The existence of the anion in such a region is feasible due to its possession of not just a dipole-bound excited state necessary for its formation but also from the stabilizing presence of a valence excited state above the ground state.  More than two-dozen other small anions have been found through high-level ab initio coupled cluster computations to possess at least a dipole-bound excited state.  Several, like l-C3H-, possess an additional excited valence state with CCSiN- exhibiting two valence excited states below the dipole-bound state.  A number of anions have already been detected in various regions in the ISM, and the corresponding neutrals for many of the anions shown to possess electronically excited states are also present in the ISM.  The electronically excited states of various anions ranging in size from tetra-atomics to polycyclic aromatic hydrocarbons are discussed as well as the computation of rovibrational reference data that will aid in their interstellar detection.  Some of the anions examined may even be applicable as carriers of the diffuse interstellar bands or other unattributed interstellar spectra.

Electronically excited states of interstellar anions

247th National Meeting (2014)

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