Crossed beam rovibrational energy transfer from S₁ glyoxal. IV. Reduced mass effects and an overview of the inelastic scattering characteristics from four initial levels

Abstract

Crossed molecular beam studies of rotationally and rovibrationally inelastic scattering of S1 glyoxal from H2 and He have been extended to one additional light gas, D2, and to two heavy gases of identical masses, Kr and cyclohexane, C6H12 (84 amu). Laser excitation was used to prepare glyoxal in its 00 level with K′=0 and 0<J′<10. Dispersed fluorescence detection was used to observe the final K′ and vibrational states of the inelastic scattering. The relative scattering cross sections for D2 and He collisions are identical to within experimental error and differ substantially from those of H2. The Kr and C6H12 cross sections are also a matched set. These results show that the competition among the approximately 25 observable scattering channels is far more sensitive to the reduced mass of the collision than to variation in the intermolecular potential or even the internal structure of the target gas. An overview of rotational and rovibrational scattering in glyoxal from four vibrational levels (00, 7 2, 51, and 81) extending to εvib=735 cm-1 is used to uncover generalities and insights about the energy transfer. For all four initial levels the vibrational state changes are highly selective. The detectable channels are always limited to ±1 quantum change in only one of the 12 modes, specifically ν′7 = 233 cm1, the lowest frequency mode. The cross sections for vibrational state change are surprisingly large relative to those for pure rotationally inelastic scattering. Many cases occur with the light target gases where the ΔK resolved cross sections for rovibrational interactions are nearly equal to those for pure rotationally inelastic scattering with equivalent energy transfer ΔE. Scattering from 7 2, K′=0 glyoxal contains examples with both H2 and He where the rovibrational cross sections actually exceed those for rotational scattering. Plots of the entire set of cross sections [rotational (ΔK) plus rovibrational (Δυ7=+1)] against ΔE are essentially superimposible for He scattering from 00, 51, and 81 glyoxal. In contrast, scattering from 72 glyoxal with the active mode initially excited is distinctive. For all initial levels, the distribution of cross sections for different ΔK within rotational channels differs from that within rovibrational channels. It is further seen in these comparisons that the change in angular momentum ΔK rather than ΔE controls the relative sizes of cross sections within these channels. The theoretical predictions of Clary, Kroes, and Rettschnick are in accord with these trends and distinctions, agreeing even on some rather subtle points. © 1994 American Institute of Physics.