The detection of the OH and other molecular lines in the radio spectrum of the interstellar medium

The detection of the ² -doublet lines of O¹⁶H¹at 1665.36 Mc and 1667.40 Mc has provided radio astronomy with a second spectral line for investigation and has already given rise to a number of significant results. In this paper the nature of -doublet levels are reviewed in an attempt to clarify the origin of the OH lines. The results of the OH observations are presented and the distinction between absorption and emission experiments is discussed in terms of the importance for the OH lines. The frequencies of the astrophysically important OH transitions in various molecular states and isotopic species are given and their detection possibilities are briefly discussed. Detection of the transitions of O¹⁸H¹appears possible with current techniques. Radio molecular lines of SH, SiH, CH, CN, and NH are also discussed.