SCALING LAWS FOR INELASTIC-COLLISION PROCESSES IN DIATOMIC-MOLECULES

A variety of fitting and scaling laws have been developed for the purpose of modeling rotational energy transfer (RET) in diatomic molecules. These include exponential energy gap (EGL), statistical power gap (SPG), and dynamically based angular-momentum scaling laws (e.g., the energy-corrected sudden approximation, ECS). These scaling laws are tested against state-to-state energy-transfer data for diatomic halogens, and stimulated Raman Q-branch band shapes in nitrogen. For state-to-state RET in halogens, an ECS scaling law, modified to account for restrictions on angular-momentum transfer, is found to be superior to the EGL. When all available data on Raman band shapes in N2, particularly including the collision-induced Raman line shifts, are taken into account, the angular-momentum-based ECS-EP scaling law again provides the best representation of the data. We conclude that dynamically based scaling laws are to be preferred for modeling rotational energy transfer in diatomic molecules. Several unresolved questions and possible future directions for energy-transfer scaling laws and fitting procedures are discussed, including extension to polyatomic systems, possible contributions to the line width from elastic dephasing processes, and the development of global fitting procedures which will simultaneously account for line shape, line shift, and (when available) state-to-state RET measurements on molecular systems.