The lowest of the strongly infrared-active vibrations of the fulleranes and astronomical emission band at a wavelength of 21 m

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Abstract
The lowest of the strongly infrared-active vibrations of the fulleranes of the family C60Hm is investigated by calculating its wavelength on the basis of two models. In one the molecule is represented by an elastic shell and in the other it is represented by a standard force field. The wavelength is found to be a strong function of m, the number of hydrogen atoms, increasing monotonically from about 19 µm for m = 0 to 23 µm for m = 60, and the transition is usually split into a number of components spread over a wavelength range of order 1 µm. It follows that the radiation in this transition from a mixed population of fulleranes is likely to consist of a broad band made up of many lines blended together, extending throughout much of the range from 19 to 23 µm. A comparison is therefore made with the emission band centred near 21 µm discovered by Kwok, Volk & Hrivnak in the shells around post-asymptotic giant branch stars, which has been measured to have similar properties. This band is only known in carbon-rich objects, many of which show the unidentified infrared features, in broad agreement with the idea that the features and the 21- µm band might both be carried by the fulleranes. It is suggested that in these objects both the 21- µm band and the unidentified infrared emission features may be collisionally excited.

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