State-to-State Inelastic Scattering from S1 Glyoxal with the Rare Gas Series: Uniform Rotational vs Changing Vibrational Channel Competition

Abstract

To provide data for the complete series of rare gases, relative cross sections are obtained for the crossed molecular beam state-to-state rotationally and rovibrationally inelastic scattering of S1 (1Au) glyoxal (CHO−CHO) in its 00, K‘ = 0 states by Ne, Ar, and Xe. When added to cross sections from a new analysis of Kr data and to published data for H2 and He, sets of cross sections for the entire rare gas series are available that show the competition among more than 25 rotational and rovibrational channels. The latter all involve Δυ7‘ = +1 where ν7‘ = 233 cm-1 is the lowest frequency mode. Despite large variations in the collisional kinematics and in the interaction potential energy surfaces, the competition among rotationally inelastic channels is essentially identical for the gases Ne, Ar, Kr, and Xe. In turn, those cases differ from H2 and He solely by the fact that orbital angular momentum constraints with the light gases limit scattering to only those states with ΔK ≲ 15. In contrast, the competition between rotational and rovibrational scattering changes with the collision partner to the extent that state-to-state resolution of rovibrational scattering is not possible for Ar, Kr, and Xe. Previous theoretical predictions for Ar inelastic scattering are consistent with earlier arguments that this competition is dominated by kinematic factors rather than by variations in the interaction potential. The relative cross sections are obtained from experiments in which a laser prepares S1 glyoxal in the 00 K‘ = 0 state with J‘ ≈ 0−10. Dispersed S1−S0 fluorescence is used to monitor the inelastic scattering to more than 25 destination states with ΔK‘ resolution. Inelastic cross sections are extracted by computer simulation of the fluorescence spectra.