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The product state-resolved dynamics of the photon-initiated reaction HϩN 2 O→OH ϫ( 2 ⌸ 3/2 ,vЈ,NЈ)ϩN 2 has been studied using Doppler-resolved laser induced fluorescence ͑LIF͒ at a mean collision energy of 143 kJ mol Ϫ1 ͑ϵ1.48 eV͒. Nascent OH(vЈϭ0,1) rovibrational population measurements indicate that only a small fraction of the available energy is channeled into the internal modes of the OH reaction products, as is consistent with previous work at other collision energies. State-resolved angular scattering distributions have been determined and are found to depend sensitively on product OH rovibrational quantum state. For the vЈϭ0 products, the angular scattering distributions are forward-backward peaking at low NЈ, changing to sideways peaking at high NЈ. OH products born in the vЈϭ1,NЈϭ6 state possess forward-backward peaking angular scattering distributions, similar to the OH(vЈϭ0) products born with intermediate NЈ. In addition to these findings, the experiments have allowed the precise determination of the OH quantum state-resolved distributions of kinetic energy releases and, hence, by energy balance, of internal energies accessed in the N 2 co-products. The product state-resolved kinetic energy disposals are found to broaden somewhat, and to favor higher kinetic energy disposal, as the internal energy of the OH is increased. The internal energies accessed in the OH and N 2 products are therefore ͑anti-͒correlated. More interestingly, the kinetic energy distributions are bimodal, particularly for OH(vЈϭ0) fragments born in high NЈ, and for those born in vЈϭ1. This finding is attributed to the operation of two microscopic reaction mechanisms, which are probably associated with H atom attack at the two ends of the NNO target molecule. The results are discussed in the light of previous experimental and theoretical work.

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The OH state-resolved angular momentum polarization generated by the H+N2O reaction has been investigated at a mean collision energy of 1.5 eV. The data were obtained under room temperature bulb conditions using 225 nm photolysis of H2S to generate translationally excited H atoms, and employed Doppler-resolved laser induced fluorescence to probe the nascent OH reaction products. The measurements revealed the OH rotational angular momentum, j′, to be aligned in the scattering plane (i.e., in the plane containing the reactant and product relative velocity vectors, k and k′). Furthermore, j′ was found to be preferentially aligned parallel to k′, particularly for lower OH rotational states. Out-of-plane torsional forces have been shown, therefore, to play an important role in generating OH rotation as the fragments separate. The new data are discussed in light of previously published studies of the title reaction, both from our own laboratory, and from those of other workers. Insight into the reaction mechanism is provided by comparison with the photodissociation dynamics of HN3, which helps, in particular, to clarify the origin of the propeller-like OH rotational angular momentum polarization.

Rate constants have been determined for the reaction between CH radicals and N atoms at temperatures between 216 and 584 K. Discharge-flow methods were used to generate known steady-state concentrations of N atoms in a cell which could either be heated within an oven or cryogenically cooled. CH radicals were produced, in concentrations much smaller than those of the atomic radicals, by pulsed laser photolysis (PLP) of a small concentration of CHBr, introduced into the gas flow, and their first-order kinetic decays were observed using the time-resolved laser-induced fluorescence (LIF) technique. The rate constants show only a slight dependence on temperature in the range covered in the present experiments and they are fitted by the functional form: k1(T)/cm3 molecule-1 s-' = (l.66 k 0.12) x 10-'0(T/298)(-0~09*0~2) where the errors are single standard deviations. If corrections are made to allow for the effects of electronic degeneracies and near degeneracies (Smith and Stewart, J . Chem. SOC. Faraday Trans., 1994,90, 3221), rate constants, k;(T), can be derived for just that fraction of collisions which occur on the potential-energy surfaces assumed to lead to reaction. The values of k;(T)/cm3 molecule-' s-' fit the expression (3.4 & 0.2,) x 10-'o(T/298)(0.04*0.2), suggesting that the rate of reaction is determined by 'capture' on the long-range potentials which correlate adiabatically with reagents and products. It is proposed that the value k1(T)/cm3 molecule-' s-' = 2.0 x lo-'' is used in chemical models of dense interstellar clouds.Gas-phase reactions between free radicals are important in a number of complex environments: for example, in the chemistry of planetary atmospheres' and of interstellar clouds,* and in combustion system^.^ They are also fundamentally interesting; in part, because they often occur across potentialenergy surfaces without any potential-energy barrier along the minimum-energy path leading from reagents to product^.^,^ Consequently, there is no well defined transition state. Rather, one must adopt a microcanonical version of transition-state theory, summing and averaging over initial reagent states to estimate thermal rate constants. In recent years, radicalradical reactions have received considerable attention from theoreticians. Two related approaches which are both based on a microcanonical approach are the statistical adiabatic channel model proposed by Troe and co-workers6 and the various adiabatic capture methods described by Clary7 and by Phillips.'Reactions between radical atoms and diatomic radicals are, at least in one sense, the simplest class of radical-radical reactions. Recently, we have reportedg low-temperature measurements on the reactions between OH radicals and N atoms (down to 103 K) and between OH and 0 atoms (down to 153 K). These experiments extended earlier work on these two reactions by Howard and Smith" which covered the temperature ranges 220-500 K for the reaction involving N atoms and 250-500 K for that involving 0 atoms. The present paper reports rate consta...