The excitation of interstellar C2

The probabilities of quadrupole transitions within the ground state, and of intercombination transitions between levels of the lowest triplet and singlet states, are computed in light of a theory developed to describe the excitation of the diatomic carbon molecule in interstellar clouds. Although the local temperature, density and radiation field strength parameters are not usually determinable from the C2 data alone, excited rotational level populations can be used as a diagnostic probe for them. The rotational populations calculated are presented for a range of physical conditions, and are used to interpret existing interstellar absorption line observations of C2. An unusual radio frequency spectrum of C2 is predicted.