Effect of Four-Quantum Vibrational-Vibrational Coupling on Sound Absorption and Dispersion Curves

Previous measurements of sound absorption due to vibrational relaxation in CO2/D2S mixtures indicate a vibrational-vibrational (v-v) coupling between the bending mode of CO2 and the stretching modes of D2S. A theory by Tanczos was used to analyze the results, but this theory did not allow for quantum changes greater than two in either one of the colliding molecules. Theoretical curves were calculated, therefore, for a v-v coupling in which a two-for-one quantum exchange took place between the bending mode in CO2 and the stretching modes in D2S. The theory has now been extended to treat cases where the v-v coupling involves a three-for-one quantum exchange between modes. Using this theory, there is a slight improvement in the theoretical fit to the experimental absorption data on CO2/D2S mixtures. This is to be expected because of the near resonance between the 0310 level of CO2 and the 100 and 001 levels of D2S.