4 Conclusion

The population distribution of the hyperfine levels in the OH $^2\Pi _{3/2}(J=7/2)$ state is determined. The measured distribution corresponds well to a statistical form. With this result, the nascent population of the hyperfine levels of OH out of photodissociation of $\rm H_2O$ is determind and can be used for astrophysical models (e.g. Thissen et al. 1999). The statistical population distribution and the much smaller dipole transition moment (see Table 1) rules out maser emission on the satellite lines in the OH $^2\Pi _{3/2}(J=7/2)$ state. In fact the satellite lines of this state are not observed in emission or in absorption (Matthews et al. 1986; Baudry & Diamond 1998).

In contrast to this the OH ground state shows laser emission on the satellite line. Secondary processes (e.g. IR relaxation from excited OH or IR pumping) has to be taken into account to explain these maser actions.

Acknowledgements

This work was supported by the Sonderforschungsbereich 216 der Deutschen Forschungsgemeinschaft. We also thank the department of Molecular and Laser Physics of the University Nijmegen for the free disposal of the microwave system.