Stimulation of Interstellar OH by Phonons

Abstract

THE attempts which have so far been made to explain the observed features of the interstellar OH molecule cannot be regarded as entirely successful¹. The most interesting observations are the OH sources in emission, where it is clear that some form of maser action is involved. Any theoretical model to explain these data must provide a mechanism for producing stimulated emission while also accounting for the formation and stability of the OH molecule under the conditions prevailing in the emitting region. An optical pumping device involving absorption and re-emission at λ=3080 Å in the X²II→A²Σ⁺ bands was suggested by Perkins et al.², but this process has recently been criticized by Solomon³. It has been argued³ that the OH would be dissociated faster than it would be pumped by ultraviolet absorption. Solomon has in fact suggested that OH may form in the upper level of the Λ doublet by inverse pre-dissociation reactions taking place at a few thousand degrees Kelvin; but the possibility of other formation and stimulation mechanisms cannot be ruled out at the present moment.