Rovibrational energy transfer from the (0⁰, K′=0) and (7²K′=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 (S 1) 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 ΔΚ 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 7 2, the cross sections for Δυ7 = + 1 and Δυ7 = -1 are nearly identical, mirroring the symmetry of UP and DOWN vibrational transitions recently observed in I2* 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 ΔΚ rotational state changes, it is seen that the cross sections for these processes scale approximately exponentially with the amount of energy ΔΕ 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 ΔΕ occurs, they are the same size. In contrast, the ΔΚ 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 ΔΚ. These relationships contradict the conventional rule of thumb prescribing large rotational and small vibrational cross sections. © 1988 American Institute of Physics.