ar X iv : a st ro - p h / 06 09 13 2 v 1 5 S ep 2 00 6 Chlorine in the Galactic ISM : Revised f - values with FUSE and STIS

Cl I is the atomic species most directly coupled to molecular hydrogen due to its chemistry. Its weakest lines are thereby probably the best tracer of optically thick H 2 components in diffuse clouds. We report on the empirical determination of the oscillator strengths for four Cl I absorption lines predicted to be weak and often detected toward moderately reddened sight lines observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). We compared our oscillator strength estimates with the oscillator strength calculations listed in Morton (2003). We find that our empirical oscillator strength values for the Cl I 1004, 1079, 1090 and 1094Å lines differ from the theoretical predictions by factors of ∼3.1, 1.2, 2.4 and 0.42, respectively. We briefly discuss the value of Cl I as tracer of molecular gas for our star sample.