Interstellar H2 - The population of excited rotational states and the infrared response to ultraviolet radiation

Abstract

Molecular hydrogen in interstellar clouds absorbs ultraviolet radiation in lines of the Lyman and Werner systems. The subsequent fluorescence leads to dissociation or to the population of excited rotational-vibrational levels of the ground electronic state. The rotational-vibrational levels decay by means of quadrupole transitions which result in the emission of infrared photons and populate excited rotational levels of the lowest vibrational state. An additional input into the cascade is provided by the formation of molecules on grain surfaces. A concise tabular description of the rotation-vibration cascade is presented. Calculations of the infrared emission spectrum of H$sub 2$ are given for an illustrative cloud model which includes the processes of fluorescence and hot molecule formation. In favorable circumstances, some of the infrared lines may have detectable intensities. Due to distribution of lines, it may prove possible to detect interstellar H$sub 2$ using narrow-band filter photometry at a wavelength of 2.4 $mu$. (AIP)