An analysis of the nonlocal thermodynamic equilibrium resulting from collisions, direct pumping of infrared radiation, and UV fluorescence was made to identify the observational 'signatures' and estimate the efficiency of each process in production of vibrational line protons. The relative populations of the vibrational levels are characterized by a single excitation temperature for all modes of excitation; this is shown by the solution of the full rate equations governing the statistical equilibrium of the CO level populations. The high emissivities estimated for the CO IR band indicate that CO will dominate over H2 in the cooling of high-temperature molecular gas at densities above 10 to the 7th/cu cm; such conditions occur in a shock-heated gas in the cores of molecular clouds or in protostellar nebulae.
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Scoville et al. (1980) studied this question.