Ultraviolet Measurements of Interstellar C₂

Abstract

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 ¹Π_u - 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 ³Π_u states and F ¹Π_u and an avoided crossing between the ³Π_u states. Tunneling through a potential barrier caused by the 3 and 4 ¹Π_u 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.