Evidence for the Presence of C${\mathrm{H}}_2$ Molecules in Comets

The structure of the $\lambda 4050$ group in comets appears to be incompatible with the assumption of a diatomic emitter. Rather, the structure is in conformity with that expected for a ⊥ band of a nearly symmetric top molecule if the moment of inertia about the top axis is approximately 0.35×${10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$. Such a small value is possible only for a slightly bent X${\mathrm{H}}_2$ molecule with X = C, N, or O. For C${\mathrm{H}}_2$ and N${\mathrm{H}}_2^{+}$ a ⊥ band is to be expected in the region 4500-4000A. Of these two possibilities C${\mathrm{H}}_2$ is the most likely. Since the CH radicals observed in the comets must necessarily be formed from saturated hydrocarbons by successive photodecompositions one should indeed expect to find the spectra of intermediate molecules that lie in the accessible region.