Introduction

Ultraviolet radiation is a crucial ingredient in any theory of interstellar chemistry. In the interplay of molecule formation and destruction processes, ultraviolet photons adopt a multiple role, destroying neutral species on the one hand, while creating chemically reactive ions and depositing thermal energy on the other. It has long been recognized (e.g. Stief et al. (1972)), that dust in a cloud's outer layers attenuates ambient Galactic ultraviolet starlight, thereby enhancing the survival of molecules against photodestruction. Unfortunately, however, the degree of attenuation is sensitive to the grain scattering properties, which are not well determined at ultraviolet wavelengths (Sandell and Mattila 1975, Leung 1975, Whitworth 1975, Bernes and Sandqvist 1977, Sandell 1978, Flannery, Roberge, and Rybicki 1980). Since even a small amount of ultraviolet radiation has profound consequences in dark regions, the chemical and ionization balance of such regions has remained uncertain.

The early studies of dust shielding may have been overly pessimistic about uncertainties, however, as noted by Chlewicki and Greenberg (1984a,b). This is due in part to the existence of strong constraints on grain properties that follow from secure observational data, and also to the discovery of the chemical consequences of the ultraviolet emission associated with gas-cosmic ray interactions (Prasad and Tarafdar (1983); see also Chapter 16). The interaction produces an ultraviolet field in clouds which, at great depths, destroys molecules more rapidly than cosmic rays or attenuated starlight. As a result, the role of starlight is restricted to a relatively narrow region near a cloud's surface, where the effects of uncertainties in grain properties are moderate.