We have detected OH in the direction of about 50% of the continuum sources investigated. The OH abundance is one order of magnitude less than usually found in dust clouds. Most of the OH features have HI counterparts. This suggests that the OH radiation arises from the HI interstellar cold clouds. Our observations allowed in some cases the determination of the excitation temperatures in all four lines. A pumping model involving far-infrared radiation and collisions with neutral and charged particles has been proposed. It explains the observed excitation temperatures.
Nguyen-Q-Rieu,; Winnberg, A;  Guibert, J;  Lepine, J R.D.;  Johansson, L E.B.;  Goss, W M 
- Max-Planck-Institut fuer Radioastronomie, Bonn (F.R. Germany)
- Observatoire de Paris, Section de Meudon, 92 (France)
- Universidade Mackenzie, Sao Paulo (Brazil). Centro de Radio-Astronomia et Astrofisica
- Rymdobservatoriet, Onsala (Sweden)
- Commonwealth Scientific and Industrial Research Organization, Epping (Australia). Div. of Radiophysics