Historical perspective

The study of interstellar chemistry started, appropriately, about 60 years ago. In 1926, Eddington discussed in his remarkable Bakerian Lecture the possibility of molecule formation and absorption in dark nebulae. At that time, only atomic species had been identified in interstellar space through their narrow absorption lines superposed on the spectra of background stars. In the next decade, several new interstellar features were detected which could indeed be ascribed to molecules: CH, CH+ and CN.

In spite of this early success, no other molecule was found in the interstellar gas for the next 25 years, until OH was detected in 1963 by its radio emission lines. In the next two decades, more than 70 different interstellar molecules were identified by centimeter and millimeter wavelength techniques. However, these radio emission line studies were mostly concerned with dense and dark clouds, whereas the early absorption line observations probed much more diffuse gas.

Although a wide variety of interstellar molecules has now been detected in dark clouds, still only a handful of molecules has been found in diffuse clouds. The molecules H2, HD, OH and CO were discovered in the 1970s by their absorption lines in the ultraviolet through rocket experiments and by the Copernicus satellite. Since the detection of C2 in 1977 by ground-based techniques, however, no new molecule has firmly been identified in diffuse clouds. The list of molecules sought but not detected is considerably longer and includes such interesting species as NH, HC1, NaH, MgH, H2O and C3 (see van Dishoeck and Black (1988a) for a recent summary).