Carbon monoxide adsorption at forsterite surfaces as models of interstellar dust grains: An unexpected bathochromic (red) shift of the CO stretching frequency

Carbon monoxide (CO) is one of the most abundant species in the interstellar medium (ISM). In the colder regions of the ISM, it can directly adsorb onto exposed Mg cations of forsterite (Fo, Mg 2 SiO 4 ), one of the main constituents of the dust grains. Its energetic of adsorption can strongly influence the chemico-physical evolution of cold interstellar clouds; thus, a detailed description of this process is desirable. We recently simulated the CO adsorption on crystalline Fo surfaces by computer ab initio methods and, surprisingly, reported cases where the CO stretching frequency underwent a bathochromic (red) shift (i.e., it is lowered with respect to the CO gas phase frequency), usually not experimentally observed for CO adsorbed onto oxides with non-d cations, like the present case. Here, we elucidate in deep when and under which conditions this case may happen and concluded that this red shift may be related to peculiar surface sites occurring at the morphologically complex Fo surfaces. The reasons for the red shift are linked to both the quadrupolar nature of the CO molecule and the role of dispersion interactions with surfaces of complex morphology. The present work, albeit speculative, suggests that, at variance with CO adsorption on simple oxides like MgO, the CO spectrum may exhibit features at lower frequencies than the reference gas frequency when CO is adsorbed on complex oxides, even in the absence of transition metal ions.