Having acquired some physical understanding of the most important molecular transitions, we next examine how these lines are employed in practice. The infrared emission from H2 requires more energetic conditions than are present in quiescent clouds. Hence, these lines are not useful as general diagnostics. Note, however, that the 2.12 pm line has proved effective in tracing shocks and stellar jets. The potentially important rotational transitions of H2O are obscured by atmospheric absorption; some of these lines have now been detected by satellite. Of the molecules we have considered previously, the most heavily used are CO, NH3, and OH. As we consider each species in turn, we focus on just a few of its common applications. We defer treatment of the maser emission from H2O until Chapter 14.
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