2022ApJ...933..192E

The high cosmic abundance of carbon monoxide (CO) and the ubiquitous nature of aluminum-coated dust grains sets the stage for the production of weakly bound triatomic molecules AlCO (X ²Π) and AlOC (X ²Π) in circumstellar envelopes of evolved stars. Following desorption of cold AlCO and AlOC from the dust grain surface, incoming stellar radiation in the 2-9 eV wavelength range (visible to vacuum ultraviolet) will drive various photochemical processes. Ionization to the singlet cation state will cause an immediate Al-X (X = C, O) bond dissociation to form Al⁺ (¹S) and CO (X ¹Σ⁺) coproducts, whereas ionization to the higher-lying triplet states will lead to stabilization of AlCO⁺ (X ³Π) and AlOC⁺(X ³Π) in deep potential wells. In competition with ionization is electronic excitation. Excitation to the spectroscopically bright 1 ²Π and 2 ²Σ⁺ states will lead to either highly Stokes-shifted fluorescence, or photodissociation to yield Al (²D) + CO (X ¹Σ⁺) products via nonadiabatic pathways, making AlCO and AlOC good candidates for electronic experimental studies. These many photoinduced pathways spanning orders of magnitude of the electromagnetic spectrum will lead to the depletion of AlCO and AlOC in astronomical environments, potentially explaining the lack of observational detection of these molecules. Furthermore, these results indicate new catalytic pathways to the freeing of aluminum atoms trapped in solid aluminum dust grains. Additionally, the results herein implicate an ion-neutral reaction as a possible important pathway in [Al, C, O] cation formation.