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others have unpaired electrons and are chemically active in the laboratory, such as CCH. Even carbon chain molecules of moderate length (such as HC11N) have been found. There are even some simple ring molecules. Many of these molecules were discovered by observations at millimeter wavelengths on telescopes such as that shown in Fig. 4.28(a).

The discovery of so many interstellar molecules was obviously a surprise. How could the predictions that molecules could not form have been so wrong? One answer is that the clouds in which the molecules have been found are not the same clouds that were studied at 21 cm. They have higher densities and visual extinctions, and lower temperatures. The higher densities mean that chemical reactions take place faster (remember the formation rate goes roughly as the square of the overall density). The higher visual extinctions provide shielding from the ultraviolet radiation that dissociates the molecules.

We don't see 21 cm emission from these clouds because the atomic hydrogen has been converted to molecular hydrogen. As we have already seen, the molecular hydrogen has no radio or optical spectrum. Since hydrogen is the most abundant element, we classify interstellar clouds by the form in which the hydrogen is found. For example, clouds in which the hydrogen is mostly atomic are called HI clouds. Clouds in which the hydrogen is mostly ionized are called HII regions (to be discussed in the next chapter). Clouds in which the hydrogen is mostly molecular are called molecular clouds.

14.5.2 Interstellar chemistry

Since the discovery of so many interstellar molecules, considerable effort has gone toward a better understanding of interstellar chemistry. It appears that some of the chemical reactions take place on grain surfaces. The grain surface provides a place for two atoms to migrate around until they find each other. They also provide a sink for the binding energy of a molecule. The example of molecular hydrogen is shown in Fig. 14.16. If two H atoms formed in the gas phase, the particular properties of the H2 molecule would keep it from radiating away the excess energy before the molecule flew apart. On a grain

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