CROSSED-BEAM ROVIBRATIONAL ENERGY-TRANSFER FROM S-1 GLYOXAL .4. REDUCED MASS EFFECTS AND AN OVERVIEW OF THE INELASTIC-SCATTERING CHARACTERISTICS FROM 4 INITIAL LEVELS

Crossed molecular beam studies of rotationally and rovibrationally ine lastic scattering of S-1 glyoxal from H-2 and He have been extended to one additional light gas, D-2, and to two heavy gases of identical ma sses, Kr and cyclohexane, C6H12 (84 amu). Laser excitation was used to prepare glyoxal in its 0(0) level with K'=0 and 0 less than or equal to J'less than or equal to 10, Dispersed fluorescence detection was us ed to observe the final K' and vibrational states of the inelastic sca ttering. The relative scattering cross sections for D-2 and He collisi ons are identical to within experimental error and differ substantiall y from those of H-2. The Kr and C6H12 cross sections are also a matche d set. These results show that the competition among the approximately 25 observable scattering channels is far more sensitive to the reduce d mass of the collision than to variation in the intermolecular potent ial or even the internal structure of the target gas. An overview of r otational and rovibrational scattering in glyoxal from four vibrationa l levels (0(0), 7(2), 5(1), and 8(1)) extending to epsilon(vib)=735 cm (-1) is used to uncover generalities and insights about the energy tra nsfer. For all four initial levels the vibrational state changes are h ighly selective. The detectable channels are always limited to +/-1 qu antum change in only one of the 12 modes, specifically nu(7)' = 233 cm (1), the lowest frequency mode. The cross sections for vibrational sta te change are surprisingly large relative to those for pure rotational ly inelastic scattering. Many cases occur with the light target gases where the Delta K resolved cross sections for rovibrational interactio ns are nearly equal to those for pure rotationally inelastic scatterin g with equivalent energy transfer Delta E. Scattering from 7(2), K'=0 glyoxal contains examples with both H-2 and He where the rovibrational cross sections actually exceed those for rotational scattering. Plots of the entire set of cross sections [rotational (Delta K) plus rovibr ational (Delta upsilon(7) = +1)] against Delta E are essentially super imposible for He scattering from 0(0), 5(1), and 8(1) glyoxal. In cont rast, scattering from 7(2) glyoxal with the active mode initially exci ted is distinctive. For all initial levels, the distribution of cross sections for different Delta K within rotational channels differs from that within rovibrational channels. It is further seen in these compa risons that the change in angular momentum Delta K rather than Delta E controls the relative sizes of cross sections within these channels. The theoretical predictions of Clary, Kroes, and Rettschnick are in ac cord with these trends and distinctions, agreeing even on some rather subtle points.