Robert C. Amme
Dept. of Phys., Univ. of Denver, Denver, CO 80208
Ultrasonics has been, and remains, a valuable technique for the investigation of vibrational energy transfer in polyatomic gases. The celebrated theory of Schwartz, Slawsky, and Herzfeld (SSH) has proven to be greatly successful in predicting the efficiency of V--T processes, particularly for self-collisions of diatomic species or between these species and the noble gases. For more complex molecules, particularly those containing hydrogen atoms, the role of vibrational amplitudes and of rotational (R--T and V--R,T) energy transfer effects may complicate the picture. Moreover, the presence of multiple relaxation processes may cause further departure from single relaxation steps in the sound velocity measurements. Nevertheless, the basic approach of SSH theory remains qualitatively correct in many cases, once the proper relaxation process and the interaction potential are identified. Vibrational collisions numbers Z[sub 10], in polyatomic molecules, i.e., the average number of collisions required for the vibrational transition v=1->v=0, will be discussed both for room temperature ultrasonic measurements and, for some species, over a range of temperatures. Various predictive schemes will be reviewed, and some recent results on substituted ethanes and their mixtures will be presented. It will be shown that the SSH formulation gives agreement with these experiments when a very steep repulsive potential is employed.