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During the past several years, the radio astronomical detection of a large number of complex organic and inorganic molecules has led to an entirely new concept of the chemical composition, kinematics and excitation of the interstellar medium. The microwave molecular transitions detected thus far fall into two natural categories: 1. „normal“ (i.e. nearly LTE) emission and absorption lines, and 2. anomalous or „masered“ emission lines. The first category includes molecules found in both high and low excitation regions whose intensity, linewidth and other characteristics may be described in terms of a physically realistic excitation temperature. Nearly all of the known microwave lines fall into this category. Typically, molecular lines of this type are found in HII regions, dark nebulae and, occasionally, extended circum-stellar shells. Only two molecules populate the second category: OH and H20. Emission from OH lambda doublet transitions in several rotational states and from the 618—523 rotational transition of H20 exhibits many strange characteristics: tremendous intensity, narrow line-width, non LTE distribution of hyperfine intensities, linear and circular polarization, and rapid variability. OH and H20 emission originates from extremely localized regions, often as small as a few A.U. in diameter, in or near some HII regions and some cool, late type stars. The behavior of OH and H20 emission lines is best explained by assuming that the microwave radiation is amplified by some narrow band non-linear process in the sources.
