Hubble Space TelescopeMeasurements of Vacuum Ultraviolet Lines of Interstellar CH

Three interstellar absorption lines near 1370 Å seen toward ζ Oph have been assigned by Watson to Rydberg transitions in the G - X (or 3 d - X ) band of CH. Our survey of a dozen diffuse interstellar lines of sight shows that the three absorption lines are consistent with the known column densities of CH, by deriving the following oscillator strengths: f (1368.74) = 0.019 ± 0.003, f (1369.13) = 0.030 ± 0.005, and f (1370.87) = 0.009 ± 0.001. We also determined intrinsic line widths that correspond to decay rates of (1.5 ± 0.6) × 10 11 , (3.8 ± 0.7) × 10 11 , and (1.1 ± 0.6) × 10 10 s -1 for λλ1368, 1369, and 1370, respectively. These rates are significantly higher than those associated with radiative decays and, thus, are readily attributable to predissociation of the Rydberg state. A fourth interstellar line near 1271 Å has been conjectured by Watson to be the strongest transition in the 4 d - X Rydberg band of CH. We detected this line along four sight lines and our spectrum syntheses show that with f (1271.02) = 0.007 ± 0.002, it is also consistent with the known column densities of CH. In addition, we conducted a search for the F - X band of CH near 1549 Å, and successfully discovered two of its absorption features along four sight lines. The astronomical oscillator strengths derived for these features are f (1549.05) = 0.021 ± 0.006 and f (1549.62) = 0.013 ± 0.003. Finally, the X Per sight line provided us with an astronomical detection of another CH band via two D - X features near 1694 Å. Comparisons with results of available theoretical calculations for the four CH bands are presented.