Articles you may be interested inThe unimolecular dissociation of HCO. II. Comparison of calculated resonance energies and widths with high resolution spectroscopic data Theoretical stabilization and scattering studies of resonances in the addition reaction H+CO = HCO J. Chem. Phys. 94, 4192 (1991); 10.1063/1.460652Collision energy dependence of Penning ionization electron spectra in crossed supersonic beams: He*(21S)+N2We use dispersed fluorescence ͑DF͒ and stimulated emission pumping ͑SEP͒ spectroscopies on the B 2 AЈ -X 2 AЈ system of jet-cooled HCO to measure the vibrational energies, resonance widths, and relative fluorescence intensities of 73 bound and resonance states supported by the ground-state potential energy surface. The SEP experiments use both two-color resonant four-wave mixing ͑RFWM-SEP͒ and the more conventional technique in which SEP signals are obtained from fluorescence depletion ͑FD-SEP͒. Where applicable, RFWM-SEP provides superior spectra to those obtained with FD-SEP, which is susceptible to saturation broadening that can prevent accurate determinations of resonance widths. The observed bound and resonance states span an energy range of 2000-21 000 cm Ϫ1 and comprise a wide range of vibrational excitation among the three vibrational modes, including states with 1-12 quanta of excitation in the C-O stretch, 0-5 quanta of bending excitation, and 0-1 quanta of excitation in the C-H stretch. The widths are markedly mode-specific and often display striking, nonmonotonic variations with increasing C-O stretch excitation. We compare our results to those of previous DF and SEP studies and to recent dynamical calculations of resonance energies and widths that use realistic potential surfaces derived from ab initio calculations. The resonance widths are particularly sensitive gauges of the unimolecular dissociation dynamics and provide stringent tests of theoretical potential surfaces.

We measured absolute rate coefficients for the reactions of the hydroxyl radical with methane (kl) and methaned4 (k2) using the laser photolysis/laser-induced fluorescence technique. We characterized kl and k2 over the temperature range 293-800 K at pressures between 400 and 750 Torr of helium. We find excellent agreement between our results and the recent determinations of kl at lower temperatures by Vaghjiani and Ravishankara.The measured rate coefficients, in the units cm3 molecule-' s-l, fit well to the three-parameter expressions k l ( T ) = 9.65 X lO-2OP1.58 exp(-1082/T) and k2(79 = 8.70 X 10-22T3.23 exp(-1334/T). The kinetic isotope effect for abstraction of the H and D atoms varies from 6.75 at 293 K to 1.96 at 800 K. We compare our results to recently reported calculations by Melissas and Truhlar.

A model calculation for nonimpact four wave mixing AIP Conf. Proc. 216, 323 (1990); 10.1063/1.39930Field and pressure induced fourwave mixing line shapes AIP Conf. Proc. 172, 249 (1988); 10.1063/1.37365Fourwave mixing spectroscopy of state selective collisions in gases and solids AIP Conf.We present a combined theoretical and experimental study of the application of two-color resonant four-wave mixing ͑RFWM͒ to stimulated emission pumping ͑SEP͒ spectroscopy. The theoretical approach employs time-independent, diagrammatic perturbation theory and a spherical tensor analysis in an extension of a recent treatment of degenerate four-wave mixing ͓Williams, Zare, and Rahn, J. Chem. Phys. 101, 1072 ͑1994͔͒. The resulting signal expression for two-color RFWM separates the molecular properties from purely laboratory-frame factors determined by the polarizations of the input beams and the rotational branch types of the SEP PUMP and DUMP transitions. This expression is valid in the limit of weak fields and for molecules in which the total angular momentum ͑omitting nuclear spin͒ is a good quantum number. In addition, we demonstrate that the spectral response for tuning the DUMP laser is a simple Lorentzian in free-jet experiments. We test our theoretical results and demonstrate the applicability of RFWM-SEP to jet-cooled, transient species in experiments on C 3 and HCO. Using the well-studied à 1 ⌸ u -X 1 ⌺ g ϩ system of C 3 , we illustrate and compare the two possible schemes for RFWM-SEP. These are defined as 1 ϭ 2 ͑PUMP͒ and 3 ϭ 4 ͑DUMP͒ or 1 ϭ 4 ͑PUMP͒ and 2 ϭ 3 ͑DUMP͒, where 1 , 2 , and 3 are the input frequencies and 4 is the signal frequency. Using the B 2 AЈ -X 2 AЈ system of HCO, we obtain RFWM-SEP spectra that probe ground-state vibrational resonances lying above the low threshold for dissociation to HϩCO. Varying the polarization of the input beams or PUMP rotational branch produce dramatic effects in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well-described by our theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation are consistent with the theoretically predicted Lorentzian line shape; the full widths for these levels are in good agreement with those determined via unsaturated fluorescence depletion SEP.

We describe a laser photolysis/cw, laser-induced fluorescence kinetic study of the reaction between O H and 2-propanol, measured over the temperature range 293-745 K. The rate coefficient for hydrogen atom abstraction by OH from 2-propanol is best fit by the expression &(q = 1.044 X 10-17T1.86 exp(736/7) cm3 molecule-1 s-l.Chain-catalytic dehydration of 2-propanol by OH is an important component of the reaction mechanism. By using isotopic substitution, we determine, as a function of temperature, the branching ratio for H atom abstraction by OH from the &sites of 2-propanol. Between 500 and 600 K, biexponential [OH] decays result from the unimolecular decomposition of the H2CCH(OH)CH3 intermediate. We characterize the dissociation kinetics of this HO-propene intermediate by fitting biexponential [OH] decays to a reaction model. From these results and previously established kinetic and thermodynamic data, we estimate the strength of a methyl C-H bond in 2-propanol. Measurements above T = 600 K demonstrate a role for two minor reaction channels.

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