State-to-State Rotational Rate Constants for CO+He: Infrared Double Resonance Measurements and Simulation of the Data Using the SAPT Theoretical Potential Energy Surface

An extensive data set of 54 time-resolved pump-probe measurements was used to examine CO+He rotational energy transfer within the CO v=2 rotational manifold. Rotational levels in the range Ji=2-9 were excited and collisional energy transfer of population to the levels Jf=1-10 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions and ab initio theoretical calculations that employed the SAPT potential energy surface of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)]. Fitting laws based on the modified exponential gap (MEG), statistical power exponential gap (SPEG), and energy corrected sudden with exponential power (ECS-EP) models all yielded acceptable simulations of the kinetic data, as did the theoretical rate constants. However, the latter were unique in their ability to reproduce both our kinetic data and the pressure broadening coefficients for CO+He. These results provide an impressive demonstration of the quality of the symmetry adapted perturbation theory (SAPT) potential energy surface

State-to-State Rotational Relaxation Rate Constants for CO+Ne from IR-IR Double-Resonance Experiments: Comparing Theory to Experiment

IR-IR double-resonance experiments were used to study the state-to-state rotational relaxation of CO with Ne as a collision partner. Rotational levels in the range Ji=2-9 were excited and collisional energy transfer of population to the levels Jf=2-8 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions. Fitting laws based on the modifed exponential gap (MEG) and statistical power exponential gap (SPEG) models were used; the MEG model performed better than the SPEG model. A rate constant matrix was also generated from scattering calculations that employed the ab initio potential energy surface of McBane and Cybulski [J. Chem. Phys. 110, 11 734 (1999)]. This theoretical rate constant matrix yielded kinetic simulations that agreed with the data nearly as well as the fitted MEG model and was unique in its ability to reproduce both the rotational energy transfer and pressure broadening data for Ne-CO. The theoretical rate coefficients varied more slowly with the energy gap than coefficients from either of the fitting laws

SPECTROSCOPIC DETECTION AND CHARACTERIZATION OF THE SELENOKETENYL (HCCSe) RADICAL

$^{a}$S. L. N. G. Krishnamachari and T. V. Venkitachalam, Chem. Phys. Lett. 67, 69 (1979).Author Institution: Department of Chemistry, University of KentuckyIn 1979, Kirshnamachari and $Venkitachalam^{a}$ reported a new transient absorption spectrum observed in the flash photolysis of selenophene. They found a series of strong, sharp bands in the 418-397 nm region and tentatively ascribed them to a $C_{4}H_{4}$ species. We have detected the same band system by laser-induced fluorescence of the products of selenophene in a pulsed discharge jet experiment. Deuterium substitution experiments and the observed selenium isotope splittings show that the carrier of the spectrum contains a hydrogen and a selenium atom. We assign this band system to the previously unknown selenoketenyl or HCCSe radical. The B values for the ground and excited states have been determined by rotational analysis of high resolution spectra of the $0^{0}_{0}$ bands of HCCSe and DCCSe. A combination of LIF and wavelength resolved fluorescence studies are in progress in an attempt to understand the vibronic structure in the spectrum

RENNER-TELLER EFFECTS IN THE 2Πi^{2}\Pi_{i} GROUND STATES OF THE SiCH/SiCD and GeCH/GeCD RADICALS

Author Institution: Department of Chemistry, University of Kentucky; Chemistry Department, University of British ColumbiaExtensive emission spectra have been obtained by laser excitation of selected bands of the $\tilde{A}^{2}\Sigma^{+} - \tilde{X}^{2}\Pi_{i}$ systems of jet-cooled SiCH, SiCD, GeCh and GeCD. Up to four quanta of the Si-C or Ge-C stretching mode and even quanta of the bending mode have been observed by pumping upper state $\Sigma$ vibronic levels. Pumping vibronically induced transitions to upper state $\Pi$ vibronic levels has given further information about odd quanta of the bending mode. The assignments have been fitted using a vibronic coupling matrix that includes spin-orbit coupling. Renner-Teller effects. Fermi resonances, and the interaction between nearly degenerate bending levels of different $\nu_{2}$ and K. The emission spectra of SiCH and SiCD, with $A \approx \epsilon\omega$, are easily assigned and the Fermi resonance interaction between $2\nu_{2}$ and $\nu_{3}$ is found to be small. In the germanium species, the spin-orbit coupling is much larger than $\epsilon\omega$ and the spectra are more complicated. Levels of different $\nu_{2}$ with $\Delta\nu_{2} = \pm 1$ are mixed by a $\Delta P = 0, H_{RT} \times H_{SO}$ cross-term of substantial magnitude, disrupting the energy level pattern. Results from the vibronic analyses of all four species will be presented

SINGLE VIBRONIC LEVEL EMISSION SPECTROSCOPY OF JET-COOLED HALOSILYLENES (HSiF, HSiCl, and HSiBr)

$^{a}$H. Harjanto, W. W. Harper, and D. J. Clouthier, J. Chem. Phys. 105, 10189 (1996). $^{b}$W. W. Harper, D. A. Hostutler, and D. J. Clouthier, J. Chem. Phys. 106, 4367 (1997). $^{c}$W. W. Harper, and D. J. Clouthier, J. Chem. Phys. 106, 9461 (1997).Author Institution: Department of Chemistry, University of Kentucky, Lexington, KY; Department of Chemistry, Centre College, Danville, KY 40422-1309; Department of Chemistry, University of Wisconsin-Parkside, Kenosha, WIUsing the technique of single vibronic level emission spectroscopy, the ground state vibrational manifolds of HSiF, HSiCl, and HSiBr and their deuterated analogs have been studies. The silylenes were produced in a pulsed electric discharge jet with the appropriate halosilane $(HSiX_{3})$ as the precursor. The gas phase harmonic vibrational frequencies and anharmonicities have been determined for the first time and the ground state harmonic force fields have been obtained for all three species. Our previous ground state rotational $constants^{a b c}$ have been determined with the calculated harmonic contributions to the $\alpha$ constants to obtain average $(r_{z})$ structures and estimates of the equilibrium $(r^{z}_{e})$ structues. The reliability of the force constants has been evaluated by Franck-Condon simulations of the emission spectra and comparisons of the calculated and experimentally determined inertial defects