Introduction

Infrared line emission from vibrationally excited molecular hydrogen (H2) has been observed in many objects in the Galaxy including planetary and reflection nebulae and various molecular cloud complexes such as those in Orion and the vicinity of the Galactic center. Similar emission has also been detected in diverse extragalactic sources such as Seyfert and interacting galaxies and objects in the Small Magellanic Cloud. Most interstellar molecular hydrogen does not emit substantial vibrational emission since it exists in cold clouds at temperatures less than 100 K while the vibrational energy levels lie many thousands of degrees above the molecular ground state. Considerable infrared vibrational emission is produced, however, in regions which are heated to sufficiently high temperatures where the hydrogen molecules are thermally excited by collisional processes, or in regions where an efficient nonthermal molecular excitation mechanism is operating.

Many of the observed H2 emitting regions are physically associated with sources of intense ultraviolet radiation. Photons with wavelengths longward of the Lyman limit can escape the ionized clouds of hydrogen gas that usually surround the radiation sources and penetrate into neutral gas clouds called photodissociation regions. The thermal and chemical structures of these regions are critically influenced by the ultraviolet radiation. The molecular hydrogen that is present in these clouds is vibrationally excited by the discrete absorption of ultraviolet photons, and it may also be collisionally excited in warm gas heated by the ultraviolet radiation. The radiative decay of the excited molecular hydrogen produces an infrared spectrum that depends on a variety of physical parameters which may vary considerably from one interstellar photodissociation region to another.