Observations of the molecular cloud boundary layer near the H II region S68 reveal an over-abundance of (C-12)O and (C-13)O relative to CO-18 consistent with a simple model of isotope-selective photodestruction of the rarer CO species. Self-shielding and the isotopic shift of the UV dissociative transitions increase the lifetime of the more abundant isotopes of carbon monoxide in a UV irradiated environment. As a result, large variations occur in the abundance ratios of CO isotopes in the surface layer of clouds and near internal UV sources. This effect is important for the determination of molecular abundances, cloud masses, isotope ratios, and molecular excitation conditions.
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Bally et al. (1982) studied this question.