Even in the cold environments of dark clouds, one finds a great variety of molecules. Most are created when an ion of one species polarizes a nearby, electrically neutral atom, increasing their mutual attraction. A significant exception is H2 itself, which forms on the surfaces of dust grains. It is puzzling that these grains do not promptly sweep up other molecules within a cloud and remove them from the gas phase. Apparently, there are processes that liberate molecules from grains, but they are poorly understood at present.
Rotational, vibrational, and electronic transitions of molecules require increasing energy for excitation. A given transition radiates strongly when the ambient density is critical, i. e., just high enough to offset radiative decay through collisonal pumping. In most molecular clouds, H2 is undetectable, so observers principally utilize the rotational lines of CO. Denser cloud material may be traced through a complex of microwave NH3 lines that arise from quantum mechanical tunneling of the nitrogen atom. The high temperatures and densities created by shocks can overpopulate rotational levels of H2O, leading to maser emission. Finally, rotation of OH subtly affects the motion of this molecule's unpaired electron. The resulting quartet of lines near 18 cm is a valuable diagnostic of interstellar magnetic fields.
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