Abstract
RECENT radio astronomical investigations of the 18-cm ground-state ⋀-doublet transition of the OH radical have suggested that maser amplification of the radiation is taking place in the interstellar medium. So far, no entirely satisfactory theoretical explanation of the observations has been published. A number of workers (for example, Cook1 and Litvak et al.2) have suggested that optical pumping of the OH molecule is responsible for the apparent population inversion. Another possibility is that the OH molecule is formed in an excited state in the vicinity of HII regions. In either case, an appreciable number of OH molecules may be found in excited levels. If the populations of the excited rotational levels of the ground electronic and vibrational state were sufficiently large, it would be expected that it would be possible to observe ⋀-doublet transitions originating in these levels.
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References
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ZUCKERMAN, B., PALMER, P. & PENFIELD, H. A Search for the ⋀Doublet Transition in the 2Π1/2, J=1/2 State of OH. Nature 213, 1217–1218 (1967). https://doi.org/10.1038/2131217a0
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DOI: https://doi.org/10.1038/2131217a0
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