Introduction

A detailed knowledge of collisional excitation processes is important in various aspects of the study of interstellar clouds. Because local thermodynamic equilibrium rarely obtains in such environments, the diagnosis of physical conditions, such as temperature, particle density, and radiation density, requires a quantitative understanding of all microscopic processes (i.e. collisional excitation and deexcitation and radiative decay and absorption) which influence the excitation conditions. Quite often, only rotational excitation of simple molecules need be considered. For instance, dense, cold cloud gas is studied primarily by observing millimeter and submillimeter emission features arising from transitions between different rotational levels of molecules in their ground electronic and vibrational states. In diffuse clouds, simple diatomics such as H2, CN and C2 are observed through electronic absorption transitions involving different rotational levels of the ground vibrational and electronic state. Even some atomic species such as C and C+ are observed in various fine structure states. This information is important for the diagnosis of diffuse clouds. Observable emission from vibrationally excited molecules arises in hotter gas or in regions exposed to a strong ultraviolet radiation field, but the collisional excitation of vibrational states is not well understood for the appropriate temperature range. However, the observed emissions due to the decay of collisionally excited fine structure levels of atomic species, such as C and C+ can be used to investigate clouds.

Second, collisional excitation of atomic and molecular species is always followed by spontaneous radiative emission leading to a loss of energy from the medium which is an important cooling process of the interstellar gas.