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The chemical composition of the interstellar medium is determined by gasphase chemistry, assisted by grain surface reactions, and by shock chemistry.The aim of this study is to measure the abundance of the hydroxyl radical (OH)in diffuse spiral arm clouds as a contribution to our understanding of theunderlying network of chemical reactions. Owing to their high critical density,the ground states of light hydrides provide a tool to directly estimate columndensities by means of absorption spectroscopy against bright backgroundsources. We observed onboard the SOFIA observatory the 2Pi3/2, J = 5/2 3/2 2.5THz line of ground-state OH in the diffuse clouds of the Carina-Sagittariusspiral arm. OH column densities in the spiral arm clouds along the sightlinesto W49N, W51 and G34.26+0.15 were found to be of the order of 10^14 cm^-2,which corresponds to a fractional abundance of 10^-7 to 10^-8, which iscomparable to that of H_2O. The absorption spectra of both species have similarvelocity components, and the ratio of the derived H_2O to OH column densitiesranges from 0.3 to 1.0. In W49N we also detected the corresponding line of^18OH.

High-resolution absorption spectroscopy of the OH 2Pi 3/2 ground state line