The absorption and emission of radiation by the collision complex

The absorption and emission of radiation by interacting atoms has been used for many years to study atomic collisions. Generally called line broadening, it is actually a study of the diatomic collision complex. It is also a form of molecular spectroscopy, but it involves free rather than bound states, so it requires the study of intensity information rather than the wavelengths of bound-bound lines. This field is now approaching a very exciting prospect of studying the collision complex in reactive collisions. It has long been recognized that the same ideas and principles normally applied to two interacting atoms can also be applied to an atom-molecule or triatomic interaction. However, the complexity is very much greater. Thus, whereas line broadening by two colliding atoms has been studied for many decades in many laboratories, triatomics are just beginning to be studied in only a few laboratories. Yet this type of measurement holds great promise as a powerful diagnostic of chemical reactions and other atom-molecule collision processes. The author reviews the description of absorption and emission of radiation by diatomics, then generalizes to triatomics, discusses the new complications which result, and ends with an example under study in the laboratory. (Auth.)