Introduction

Our understanding of the structure of the interstellar medium has progressed enormously in the last 15 years largely as a result of the development of radio emission and ultraviolet absorption observations. From CO radio surveys we now know that about one-half of the interstellar gas is in the form of giant molecular clouds that are the formation sites of O and B stars, while measurements of highly excited gas, such as hot bubbles and expanding shells, indicate that a significant volume of the interstellar medium contains dilute, hot gas. The physical properties and dynamics of the interstellar gas are strongly influenced by the interaction of the massive O and B stars and supernovae. Supernova shells sweep up interstellar matter creating clouds and also impact them causing compression and fragmentation. The O and B stars produce large HII regions which expand and compress the interstellar gas. Thus on a theoretical basis we expect to find complex velocity fields, shock structures, and density inhomogeneities in the diffuse and translucent component of the interstellar clouds.

Today it is hard to imagine studying the diffuse interstellar clouds without relying on observations of molecular lines. In an excellent review article by Dalgarno and McCray (1972) on the heating, ionization and cooling processes of HI regions, these authors stated that ‘The processes leading to interstellar molecule formation are only partially understood …’. The central role of molecules as both important components and physical probes was just becoming evident.