Observations of H-3(+) in dense molecular clouds

dH(3)(+) has been detected using infrared absorption spectroscopy along the lines of sight to six infrared sources in dense molecular clouds: AFGL 213 6, W33A, Mon R2 IRS 3, AFGL 961E, AFGL 2591, and AFGL 490. Upper limits to the column densities of H-3(+) are reported for an additional nine sources. The column densities of CO toward Mon R2 IRS 3 and AFGL 961E have been det ermined from observations of the first-overtone lines of CO. For the six so urces toward which H-3(+) was detected, a simple model of H-3(+) chemistry has been used together with column densities of H-2 derived from infrared C O measurements to estimate column lengths, mean number densities, and tempe ratures of molecular clouds. The derived column lengths are on the order of a parsec, the number densities are 10(4)-10(5) cm(-3), and the temperature s are similar to 25-50 K.