ABSTRACT
This chapter discusses microscopic processes, especially those which directly or indirectly determine the temperature of the interstellar gas, for temperature is a dominating parameter. Since the formation and the presence of molecules are important for the properties of interstellar clouds, the chapter considers the reactions by which they arise through purely gas phase reactions. At the temperatures, densities, and radiation intensities in the interstellar medium, molecules do not form as readily as one might expect, and the product molecules do not persist indefinitely. Neutral exchange reactions between atoms and molecules or radicals may also occur in the interstellar medium if the reactions are exothermic. Species are grouped by the number of atoms they contain. All of them are potential probes of the wide range of physical conditions in the interstellar medium. Many of these molecular species may be found not only in interstellar clouds but also in circumstellar regions.
