Rotational Changes Accompanying Vibrational Transfer in Low-Energy Collisions between Benzene and H2, D2, and CH4

Abstract

We have examined the changes in rotation that occur simultaneously with a change in vibrational state within a large polyatomic. Specifically, a variety of rotational-level population distributions have been prepared in the 6 1 vibrational level of 1 B 2 u benzene, and the rotational distributions within 0° that result from collisions with H 2 , D 2 , or CH 4 have been observed by measuring the rotational contour of the 6 0 1 band in emission. The experiments were performed in the collision region of a supersonic free jet expansion at a translational temperature of 34 K (H 2 and D 2 ) or 18 K (CH 4 ). It is demonstrated that rotational-energy transfer within 6' and 0° does not obscure the rotational changes accompanying the vibrational change. For a particular collision partner, the final rotational distributions in 0° are essentially the same for all initial 6 1 distributions. However, the final 0° rotational distributions are quite different for the different collision partners. The observed 6 0 1 rotational contours are fit reasonably well by a thermal distribution with 0° rotational temperatures of 19, 41, and 93 K for H 2 , D 2 , and CH 4 , respectively. The 6 0 1 rotational contours were also fit using an exponentially decaying momentum gap model to evolve the initial 6 1 rotational distribution to the final 0° distribution. As an illustration, for an initial J, K distribution with average J and K values of 8 and 6, respectively, the changes in these average values are (written as (ΔJ, ΔK)) (3, 1), (8, 4), and (16, 9) for the thermal distribution fits for H 2 , D 2 , and CH 4 , respectively. The corresponding values for the momentum gap model are (4, 0), (10, 1), and (19, 8). The modeling shows that the ‖ΔK‖ changes are much larger for CH 4 than for H 2 and D 2 . A possible reason for the relaxed ΔK restriction with CH 4 may be that a broad range of collision geometries leads to 6' → 0 0 vibrational-energy transfer with this partner. The 0° distributions for D 2 are broader than for H 2 , illustrating that the reduced mass and rotational-level spacings play an important role in determining the rotational changes that occur.