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Figure 5.16 Rotational levels of H2O. Two maser transitions, 616 ^ 523 and 414 ^ 321, are indicated on the right.
stars. At lower densities, rotational levels can still be excited by infrared continuum photons from heated dust grains.
Whatever their source of energy, the excited molecules tend to cascade downward to the rotational backbone, consisting of the states 101, 212, 303, etc. Inspection of Figure 5.16 reveals only two allowed transitions among the ortho -states, 414 —* 321 and 616 —► 523, by which molecules can leave the backbone without dropping directly to the next lower backbone level. The levels 321 and 523 thus accumulate molecules, but remain underpopulated relative to their adjacent backbone states. In fact, both transitions in question are observed as masers, the second being the 22.2 GHz line.
Like CO and NH3, H2O forms through gas-phase reactions. Within shocked cloud regions, the major sequence is
As we noted in §5.1, such neutral-neutral reactions are inoperative within cold, dark clouds.
Here, ion-molecule reactions can still proceed. The formation of H2O involves the chain
- O — H2 + OH+ H2 — OH+ + H H2 — H3O+ + H e" — H2O + H .
The last reaction is an example of the dissociative recombination process introduced in equation (5.2). Such reactions proceed very quickly given an adequate supply of free electrons.
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