The third astronomical maser species was discovered when Snyder and Buhl (1974) detected a group of new molecular emission features near 86 GHz in the Orion molecular cloud. The narrow linewidths and the compactness of the emission region suggested that this was a new maser, which Snyder and Buhl promptly identified as the J = 2 → 1 transition of the first vibrationally excited state (v = 1) of SiO. The almost immediate detection of two more rotational transitions, J = 1→ 0 and 3 → 2, in the same vibration state by Davis et al. (1974) and Thaddeus et al. (1974) confirmed this bold proposal. Subsequently, maser emission has been detected in rotational transitions in both v = 2 (Buhl et al. 1974) and v = 3 (Scalise and Lepine 1978). The rotational quantum numbers were also extended to higher values: J = 4 → 3 masers were discovered by Schwartz, Zuckerman and Bologna (1982); J = 5 → 4 by Clemens and Lane (1983); J = 6 → 5 by Jewell et al. (1987). SiO was the first molecule to present such a rich spectrum of maser lines. Figure 3.3 provides the energy level diagram, with detected maser transitions marked.
KeywordsOptical Depth Column Density Maser Emission Pump Rate Vibration State
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