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A collaborative experimental and theoretical study of rotationally inelastic collisions of CH(A 2Δ,v=0) resolved rotational/fine-structure levels with argon is presented. Experimental state-to-state rate constants were extracted from CH A 2Δ→X 2Π fluorescence spectra upon laser excitation to individual levels in the A 2Δ state in the presence of Ar. Fluorescence detection of the collision-induced population permits resolution of the fine-structure levels at low N, but no Λ-doublet discrimination. For the lowest value of N (N=2), the dominant process is the ΔN=0 fine-structure-changing transition, and the efficiency of this transition decreases markedly with increasing N. There is an increasing preference for conservation of the fine-structure label in ΔN≠0 transitions as N increases. These rate constants have been compared to and interpreted with theoretical rate constants computed through quantum coupled-states calculations of cross sections based on ab initio CH(A 2Δ)–Ar potential energy surfaces determined by Kaledin and Heaven (to be published). The tendency to conserve the fine-structure label is attributed to the Hund’s case (b) nature of the CH(A 2Δ) state, for which the electron spin is a spectator in the collision.

The kinetic behavior of OH radicals was studied in the presence of a large excess of NO at 5–82 torr He. Hydroxyl radicals were produced by pulsed vacuum-uv photolysis of H2O and were monitored directly by a resonance fluorescence method. The rate constant for the reaction OH+NO was nonlinearly dependent on the concentration of diluent He in this pressure range. The high pressure limit of the bimolecular rate constant was determined to be (2± 1)× 10−12cm3molecule−1· sec−1 at 300°K.

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Articles you may be interested inFine-structure state resolved rotationally inelastic collisions of CH (A 2 Δ,v=0) with Ar: A combined experimental and theoretical study Quantum scattering studies of inelastic collisions of NH(A 3Π) with helium: Finestructure and Λdoublet propensities J. Chem. Phys. 95, 5036 (1991); 10.1063/1.461792Inelastic collisions of OH (2Π) with H2: Comparison between theory and experiment including rotational, fine structure, and Λdoublet transitions A comprehensive set of single-collision state-to-state rate constants for the relaxation of NH͑A 3 ⌸, vϭ0,N,F i ,e/ f ͒ levels in collisions with helium are presented. These rate constants were extracted from measured fluorescence intensities of isolated A→X rotational lines in the presence of He subsequent to laser excitation of individual levels in the A state. There is no indication of the collisional propensities predicted for a Hund's case ͑a͒ 3 ⌸ state, most likely because NH(A) represents an intermediate coupling case. With increasing N, NH(A) rapidly approaches the Hund's case ͑b͒ limit. For high initial N, rotational relaxation proceeds predominantly through ⌬NϭϪ1 fine-structure conserving transitions to ⌳ doublet levels with ⌸͑AЈ͒ reflection symmetry. In addition, a propensity to conserve the ⌸(AЈ)/⌸(AЉ) symmetry of the initial level is found for ⌬Nϭ0 fine-structure changing transitions. The observed propensities have been interpreted by comparison with full quantum close-coupling and coupled-states calculations of cross sections based on ab initio potential energy surfaces ͑PES's͒. The propensity for forming ⌸͑AЈ͒⌳ doublet levels in ⌬NϭϪ1 collisions of high N initial levels is found to be facilitated by approach in a ''helicopterlike'' orientation on the more attractive HeNH(A)AЈ PES, followed by curve crossing to the more repulsive AЉ PES which correlates adiabatically to the next lower rotational manifold. In addition, thermal rate constants derived from the calculated cross sections agree extremely well with those obtained from a deconvolution of the experimental spectra.