Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of similar to 4 x 10(16) molecule cm(-3) in N-2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k(2) = (3.22 +/- 0.46) x 10(-11) and (2.12 +/- 0.12) x 10(-11) cm(3) molecule(-1) s(-1) at T = 69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633-5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k(3) < 1 x 10(-12) cm(3) molecule(-1) s (-1), consistent with the presence of a small activation barrier of similar to 5 kJ mol (-1) between the reagents and the OH-C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k(4) = (5.08 +/- 0.65), (5.02 +/- 1.11) and (3.11 +/- 0.09) x 10(-12) cm(3) molecule(-1) s(-1) were obtained at T = 69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k(1) = (7.8 +/- 2.5) x 10(-11) cm(3) molecule(-1) s (-1) was obtained for the reaction of OH with t-butyl-hydroperoxide at T = 86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors

Electronic quenching of OH A²Σ⁺ radicals in single collision events with H₂ and D₂: a comprehensive quantum state distribution of the OH X²Π products

A pump–probe laser-induced fluorescence technique has been used to examine the nascent OH X ²Π product state distribution arising from non-reactive quenching of electronically excited OH A ²Σ⁺ by molecular hydrogen and deuterium under single-collision conditions. The OH X ²Π products were detected in v″ = 0, 1 and 2; the distribution peaks in v″ = 0 and decreases monotonically with increasing vibrational excitation. In all vibrational levels probed, the OH X ²Π products are found to be highly rotationally excited, the distribution peaking at N″ = 15 when H₂ was used as the collision partner and N″ = 17 for D₂. A marked propensity for production of Π(A′) Λ-doublet levels was observed, while both OH X ²Π spin–orbit manifolds were equally populated. These observations are interpreted as dynamical signatures of the nonadiabatic passage of the OH + H₂/D₂ system through the seams of conical intersection that couple the excited state (2 ²A′) and ground state (1 ²A′) surfaces

Electronic quenching of OH A²Σ+ radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH X²Π products

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A &lt;sup&gt;2&lt;/sup&gt;Σ+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X &lt;sup&gt;2&lt;/sup&gt;Π population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A &lt;sup&gt;2&lt;/sup&gt;Σ+ (ν′ = 0, N′ = 0), which has a significantly reduced fluorescence lifetime due to quenching by H&lt;sub&gt;2&lt;/sub&gt;. The probe laser monitors the OH X &lt;sup&gt;2&lt;/sup&gt;Π (ν″, N″) population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N″) of the quenched OH X &lt;sup&gt;2&lt;/sup&gt;Π products observed in ν″ = 1 and 2 as well as a pronounced propensity for quenching into the Π(A′) Λ-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A &lt;sup&gt;2&lt;/sup&gt;Σ+ by H&lt;sub&gt;2&lt;/sub&gt; proceeds through conical intersections between two potentials of A′ reflection symmetry (in planar geometry) that correlate with the electronically excited A &lt;sup&gt;2&lt;/sup&gt;Σ+ and ground X &lt;sup&gt;2&lt;/sup&gt;Π states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H&lt;sub&gt;2&lt;/sub&gt;. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000) ], these calculations reveal a steep gradient away from the OH–H&lt;sub&gt;2&lt;/sub&gt; conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X &lt;sup&gt;2&lt;/sup&gt;Π products