Excitatioin Temperature of the 18-CM Line of OH in H i Regions

OH radio absorption observations have led to estimates of the OH/H abundance ratios, but such estimates have been based on an assumed excitation temperature of OH. A value of ~~,1O0 K, an order of magnitude less than for H, has often been used. In this paper the physical mechanisms which compete to establish the OH excitation temperature in a typical H i cloud are examined and transition rates evaluated. It is shown that OH-ion collisions and 18-cm radiation are the most important processes. The excitation temperature is evaluated for an H i cloud and shown to be ,~3Ø0 K for a neutral hydrogen density of 10 atoms/cm3. Observations place an upper limit of 20_100 K on the excitation temperature, suggesting that Oil may be prevalent in H i clouds having low abundances of heavy ions