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, 72, 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 ν7′ = 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 72, 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.

Factors controlling the competition among rotational and vibrational energy transfer channels in glyoxal

“…He and among N 2 vs. CO vs. C 2 H 4 clearly show that the interaction potential plays only a secondary role in the channel competition. Other less well-defined studies suggest that this situation persists even for heavier and more disparate collision partners-e.g., Kr and cyclohexane (84 amu) (10). Some aspect of the collisional kinematics, yet to be identified, has by far the dominant influence on the relative sizes of the inelastic scattering cross sections.…”

“…Lower resolution fluorescence scattering spectra obtained in a preliminarily study (10) with two 84 amu collision partners, Kr and C 6 H 12 , also occur as a matched pair. While the individual K states were not resolved, it is clear that the general rotational scaling and particularly the rotational vs. vibrational competition are similar for Kr and C 6 H 12 .…”

“…In this paper, we discuss collisional energy transfer involving glyoxal (CHO-CHO), a 12-mode molecule that has been under study for some time. Crossed molecular beams coupled with S 1 -S 0 laser pumping and a dispersed fluorescence probe are used to observe state-to-state rotational and rovibrational inelastic scattering in the S 1 excited electronic state (6)(7)(8)(9)(10). The state resolution is sufficient to see the competition between many rotational and rovibrational channels.…”

“…He as well as with a pair of heavy partners Kr vs. C 6 H 12 (cyclohexane), both 84 amu, have been carried out at a fluorescence resolution (10 cm Ϫ1 ) where the individual rotational channels could not be seen (10). Those initial efforts suggested that the state-to-state channel competition is largely controlled by the kinematic effects.…”

H2 Production in the 440-nm Photodissociation of Glyoxal

“…23 Much of the recent interest in this molecule stems from its presence in the polluted tropospheric atmosphere 24,25 and as an intermediate in combustion systems. 26 Its well-characterized spectrum has led to further studies using this molecule to probe collisional energy transfer processes, [27][28][29][30][31][32][33][34][35][36] molecular alignment, 37 predissociation of van der Waals complexes, [38][39][40] and stimulated emission pumping. 39,41,42 Following the first observations of collisionless predissociation of glyoxal, [43][44][45] the dissociation dynamics of this molecule have also been examined.…”