Results of crossed-molecular beam inelastic scattering experiments from two levels of 1Au (S1) trans-glyoxal in collisions with H2 (Ec.m.=80 meV, 650 cm−1) and He (Ec.m.=95 meV, 770 cm−1) are reported. Relative inelastic scattering cross sections with quantitative error bars are obtained. S1←S0 laser excitation was used to prepare either the 00 level or the 72 level (εvib=466 cm−1) with the high rotational selection of K′=0 and J′=0–10. The final high levels populated by rotationally and rovibrationally inelastic scattering were monitored by dispersed fluorescence with K′ state resolution. Fluorescence from only those molecules involved in inelastic scattering was obtained from the difference signal of spectra with the target gas beam (H2 or He) ON and OFF. Those dispersed fluorescence spectra were analyzed with a computer fitting procedure to extract 52 relative state-to-state cross sections for scattering from the (00, K′=0) level and 84 for the (72, K′=0) level. The cross sections have been compared quantitatively with the results of the three-dimensional quantal scattering calculations of Clary, Dateo, Kroes, and Rettschnick. The agreement between the experimental and theoretical cross sections is nearly quantitative for both target gases and both initial states. Disagreements between experiment and theory occur only in the details of K′ distributions within the rovibrational channels. The vibrationally inelastic scattering is extremely selective among the many accessible channels. The cross sections for Δυ7=±1 changes in the lowest frequency mode ν7′ = 233 cm−1, a CHO–CHO torsion, exceed those involving the other 11 modes by at least an order of magnitude. As judged by the relative magnitudes of rovibrational and rotational cross sections, rovibrational scattering is surprisingly efficient. In fact, when comparing cross sections for transferring energy by (T→R) vs (T→R,V) with similar ΔE, rovibrational cross sections are the greater in numerous examples.
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.
The SI vibrational predissociation (VP) dynamics and physical characteristics of the p-difluorobenzene-N2(~DFB-Nz) van der Waals complex are reported. The geometry of the complex is roposed to be similar to pDFB ring, and, by analogy to the benzene-NZ complex, with the NZ nearly freely rotating parallel to the aromatic molecular plane. Upper limits to the SI and So van der Waals binding energies of Do' I 240 cm-I and DO" I 213 cm-', respectively, were obtained. Only two of the nine observed SO pDFB ring modes (~6 " the symmetric ring stretch and v< the out-of-plane ring puckering mode) appear to be perturbed by complexation, and these only slightly. In SI, none of the observed ring levels appears significantly perturbed, but surprisingly, spectroscopic evidence concerning Y ( could not be obtained. In general, YS (or Y16a in Wilson notation) is the most perturbed vibration in aromatic-rare gas van der Waals complexes. Vibrational predissociation from four initial SI ring levels lying within the first 800 cm-I of the pDFB-NZ vibrational manifold was characterized using single vibronic level fluorescence spectroscopy. State-to-state dissociation from each level produces the pDFB product molecule in only a few of many accessible SI vibrational levels.Evidence of intramolecular vibration'al redistribution (IVR) within the pDFB-Nz complex is observed for one level. The dissociation is treated by preliminary modeling based on a serial mechanism (involving IVR within the complex followed by VP) that is related to that developed by Kelley and Bernstein [J. Phys. Chem. 1986, 90, 51641 for s-tetrazine-Ar VP. The modeling accounts for the final state selectivity and most but not all of the observed VP channels. The experimental and modeling results are compared with those of the pDFB-Ar complex whose vibrational level structure differs only modestly and in a known way from that of pDFB-N*.
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