Introduction

This book deals with the outermost shell of the collisionally dominated gaseous envelope of the Earth, the thermosphere and ionosphere. In the preceding chapters collision cross sections have appeared prominently in the physical descriptions and in the equations that govern the interaction of energetic photons, ions and electrons with the atmosphere. We learned, for example, that the optical depth (Equation (2.2.1)) is proportional to the column density of absorbing molecules, with the proportionality factor identified as the absorption cross section. Photoionization (Section 2.3) involves ionization cross sections and electron transport (Section 3.2) depends on elastic and inelastic collision processes and the associated collision cross sections. It is therefore appropriate to assess our understanding of cross sections and expand upon the part which they play in physical and chemical processes in the atmosphere.

It has been found convenient to classify collisions into several types: elastic, inelastic and reactive. When two particles collide and only kinetic energy and linear momentum are exchanged (and the total of each is conserved) then the collision is elastic. If one or both of the collision partners undergoes a change in internal energy then the collision is inelastic. Reactive collisions are those that involve the production of new species; such collisions are also inelastic.