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.

The role of angular momentum in collision-induced vibration–rotation relaxation in polyatomics

“…͑For this molecule the equivalent rotor bond length evaluated from the rotational constant is very close to that calculated from the molecular dimensions͒. Figures 4͑a͒ and 4͑b͒ show experimental data on glyoxal-Ne 16 together with the results of equivalent rotor calculations for RT and for VRT. Calculated data were normalized to the K f ϭ3 probability.…”

“…Experimental uncertainties are not reproduced here, but very few calculated points lie outside the experimental error bars. 16 The level of agreement obtained is quite encouraging, particularly since the data reported by these authors are the most extensive collision-induced cross sections available for a polyatomic molecule. Use of an equivalent rotor bond length also gives good results.…”

“…Parmenter and co-workers, 16 in a series of crossed beam experiments, convincingly demonstrated the central role played by kinematic factors in RT and VRT involving glyoxal. These results suggest that the AM model of inelastic processes, 7,8 developed initially for diatomics, might be useful in the analysis of collision-induced transfer in larger molecules.…”

“…Data from two experiments are used here, the first being the extensive K-changing RT and VRT cross sections for glyoxal-Ne collisions reported by Clegg et al 16 the second being J-changing RT rate constants in acetylene-He encounters of Henton et al 22 Here equivalent rotor model predictions are compared with experiment and the method then used to predict rotational distributions for 6 1 →0 0 VRT transfer for benzene. VRT data in benzene are less well resolved than those for glyoxal or acetylene but despite this, valuable insights are obtained.…”

“…16 The equivalent rotor for glyoxal was determined as in Sec. III, though in this case the ''bond length'' was calculated from molecular dimensions, giving a value of 2.7 Å.…”