Photodissociation of dicarbon: How nature breaks an unusual multiple bond

Significance It has long been observed that the coma of a comet is often green while its tail is not. While the explanation for this must be that the molecules responsible for the green emission, C 2 , are photodissociated, the mechanism was, until now, unknown. We have observed the photodissociation of C 2 in the laboratory for the first time and, in doing so, have determined its bond dissociation energy with unprecedented precision. Invoking the observed mechanism, the calculated lifetime of cometary C 2 is found to be consistent with astronomical observations.