Observations of in Dense Molecular Clouds

has been detected using infrared absorption spectroscopy along the lines of sight to six infrared 3 sources in dense molecular clouds : AFGL 2136, W33A, Mon R2 IRS 3, AFGL 961E, AFGL 2591, and AFGL 490. Upper limits to the column densities of are reported for an additional nine sources. The 3 column densities of CO toward Mon R2 IRS 3 and AFGL 961E have been determined from observations of the Ðrst-overtone lines of CO. For the six sources toward which was detected, a simple 3 model of chemistry has been used together with column densities of derived from infrared CO 3 2 measurements to estimate column lengths, mean number densities, and temperatures of molecular clouds. The derived column lengths are on the order of a parsec, the number densities are 104È105 cm~3, and the temperatures are D25È50 K. Subject headings : infrared : ISM: lines and bands È ISM: clouds È ISM: molecules È molecular processes