Rovibrational energy transfer from the (0, K′=0) and (72,K′=0) levels of S1 glyoxal in crossed beam collisions with H2

Abstract

Crossed molecular beams have been used to study rotationally and vibrationally inelastic scattering from the (72, K′=0) and the (00, K′=0) levels of 1Au (S1) trans-glyoxal in collision with H2. The center-of-mass collision energy was approximately 80 meV (650 cm−1). Laser excitation was used to select each of the initial vibrational levels with rotational states limited to J′=0–10, K′=0. Dispersed fluorescence was used to monitor quantitatively the final rotational and vibrational levels populated in single collision scattering. Vibrationally inelastic scattering from each initial level has an absolute cross section large enough to allow the process to be observed easily with resolution of accompanying ΔK rotational state changes. Although many final vibrational levels are energetically accessible and in principle observable, only those reached by ±1 quantum changes in the lowest frequency mode, the OHC–CHO torsion ν7, are seen. From the initial level 72, the cross sections for Δυ7=+1 and Δυ7=−1 are nearly identical, mirroring the symmetry of UP and DOWN vibrational transitions recently observed in I*2 inelastic scattering from H2, D2, and He. The total cross section for pure rotationally inelastic scattering from 00 is about nine times larger than that for rovibrationally inelastic scattering, whereas the two cross sections are nearly equivalent for 72. With resolution of ΔK rotational state changes, it is seen that the cross sections for these processes scale approximately exponentially with the amount of energy ΔE transferred between translational motion and the internal degrees of freedom of the glyoxal molecule. For the initial level (00, K′=0), not only do these rotational and rovibrational cross sections obey the same energy scaling law, but where similar ΔE occurs, they are the same size. In contrast, the ΔK rotationally state-resolved cross sections for transitions from (72, K′=0) fall off much more rapidly in the vibrationally elastic (Δυ7=0) channel than in the vibrationally inelastic (Δυ7=±1) channels. Consequently, some rovibrational transitions have cross sections comparable to the pure rotational transitions with the same value of ΔK. These relationships contradict the conventional rule of thumb prescribing large rotational and small vibrational cross sections.