INFORMATION ON ROTATIONAL ENERGY TRANSFER FROM MICROWAVE LINE-BROADENING
Ohio State University
Collision broadening of microwave absorption lines provides a useful tool for evaluating the state of understanding of rotational energy transfer during molecular collisions and, to some extent, in choosing between alternate hypotheses. Since individual events are not observed but only the averaged effect of collisions with molecules in various rotational states and various relative velocities and distances of closest approach, it cannot be said that agreement between theory and limited experimental observation demonstrates the correctness of the model. Lack of agreement, however, can show inadequacies of understanding of the collision process and agreement in a large number of cases can lend confidence to a belief that the basic processes are understood. The theory sets up a time-development matrix to yield expressions which are averaged over all internal quantum states of the colliding molecule and the velocity distribution function to give half-width and frequency-shift parameters which can be compared with experimental observation. A multipole expansion of the intermolecular potential is used, and considerable information can be obtained about the relative importance of the different interactions. Selection rules governing the energy transfer are derived, as well as a qualitative understanding of conditions under which these selection rules may be violated. Much of the existing experimental data on microwave linewidths is of lower accuracy than might be wished to allow critical comparison with theory. Some improvements in technique have been made, and other work is in progress in an effort to examine the line shape more rigorously and to look for the effects of collision-induced frequency shifts.
Author Institution: Department of Chemistry, The University of Texas at Austin