Ultraviolet Measurements of Interstellar C2
  We analyzed archival spectra acquired with the Hubble Space Telescope for a
study of interstellar C2. Absorption from the electronic transitions, D
^1Sigma^+_u -- X ^1Sigma^+_g (0,0) as well as F ^1Pi_u -- X ^1Sigma^+_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 ^3Pi_u states and F ^1Pi_u and an avoided crossing between the
^3Pi_u states. Tunneling through a potential barrier caused by the 3 and 4
^1Pi_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.
