We analyzed archival spectra acquired with the Hubble Space Telescope for a study of interstellar C2. Absorption from the electronic transitions, D 1Σ+u- X 1Σ+g(0,0) as well as F 1Πu- X 1Σ+g(0,0) and (1,0), was the focus of the study. Our profile syntheses revealed that the lines of the F - X bands were broadened as a result of a perturbation involving the upper levels. Further evidence for the perturbation came from anomalies in line strength and position for the F - X (0,0) band. The perturbation likely arises from a combination of triplet-singlet interactions involving spin-orbit mixing between 3Πu states and F 1Πu and an avoided crossing between the 3Πustates. Tunneling through a potential barrier caused by the 3 and 4 1Πu states and spin-orbit mixing with other close-lying triplet states of ungerade symmetry are less likely. Except for the broadening, lines in the F - X (1,0) band appear free from anomalies and can be used to study interstellar C2; new results for 10 sight lines are presented.
ISM: lines and bands - ISM: molecules - molecular data - ultraviolet: ISM