Crossed beam rovibrational energy transfer from S1 glyoxal. III. Quantitative H2 and He cross sections for (0, K′=0) and (72, K′=0) and comparison with theory

Abstract

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.

Factors controlling the competition among rotational and vibrational energy transfer channels in glyoxal

“…A comparison of our H 2 and He cross sections with the theoretical predictions has been shown elsewhere (9). The quality of the theoretical fit is high, with mismatches not generally exceeding the 20-30% experimental error bars.…”

“…The key aspect of data analysis involves extracting relative cross sections for the state-to-state inelastic scattering from the dispersed fluorescence (9). We acquire a set of cross sections that defines the competition among inelastic channels by computer simulation of the spectrum.…”

“…A full description of the experimental approach is given elsewhere (7,9,11). The basic elements are molecular beams intersecting at 90°as shown in (1997) expansion allows isolation of the fluorescence from only those molecules that have undergone inelastic scattering.…”

“…In this paper, we discuss collisional energy transfer involving glyoxal (CHO-CHO), a 12-mode molecule that has been under study for some time. Crossed molecular beams coupled with S 1 -S 0 laser pumping and a dispersed fluorescence probe are used to observe state-to-state rotational and rovibrational inelastic scattering in the S 1 excited electronic state (6)(7)(8)(9)(10). The state resolution is sufficient to see the competition between many rotational and rovibrational channels.…”

“…Experiments using the collision partner He have already been reported for each of the initial levels shown in bold in Fig. 1 (8,9). They include the zero point level, both in-plane and out-of-plane fundamentals and an overtone of the CHOCHO torsion, 7 .…”