Angular Restrictions Research Articles

Monte Carlo simulation of RRKM unimolecular decomposition in molecular beam experiments. V. product ox angular and energy distributions from O( 3P)+X 2 (X = Br, I)

Statistical simulation was applied to the unimolecular decomposition of the collision complexes formed in the crossed beam experiments on O( 3P) + Br 2 by Fernie et al. and O( 3P) + I 2 by Durkin et al. The simulation procedure used the fundamentals of RRKM theory and included exact angular momentum conservation. The impact parameter distributions were varied to obtain the best fits. Good agreement with experimental laboratory angular distributions measured with O atoms seeded in both He and Ne was found for impact parameter distributions which were peaked at quite small values, in most cases between 2 and 3 Å. Product OX molecules were found to be rotationally excited and inverted with a mean rotational energy close to twice the value expected without angular momentum restrictions. The differences found between the calculated and the experimental angular distributions do not support any assumptions about osculating or short-lived complexes. The normal exoergicity Δ D 0 of 27 kJ/mol for the O + I 2 reaction agrees well with the experiments by Dunkin et al.

Monte Carlo simulation of O(1D) + H2 and O(1D) + HCl — rotational excitation of product OH radicals

Experimental studies with molecular beam and LIF techniques have independently shown that the reaction O( 1 D) + H 2 → OH + H passes through a long-lived complex and gives products with small translational and large rotational excitation. We have previously published a statistical algorithm, based on ordinary RRKM theory with angular momentum restrictions included, which was designed to simulate molecular beam experiments. It has now been modified and applied to simulate the experimental rotational OH distributions from O( 1 D)+H 2 , measured by Luntz et al. The present study also includes simulation of similar results by Luntz for O( 1 D) + HCI → OH + Cl. The purely statistical algorithm successfully simulates the apparently non-statistical experimental rotational distributions. For these reactions the total angular momentum conservation. which is applied at the transition state, proves to be decisive for the product energy distributions.

Experimental observations of excited dissociative and metastable states of H3 in neutralized ion beams

Electron transfer reactions for a fast beam of H3+ ions with Mg and K atoms have been investigated by neutral beam scattering techniques. Reactions with Mg and K targets form H3 molecules in the dissociative 2p 2E′ ground state and predissociative 2s 2A1′ and 2p 2A2″ excited states, respectively. Fragmentation energies, obtained from beam scattering measurements, allow the scaling of these electronic states of H3 with respect to their dissociation products. A metastable form of H3 observed in the H3+/K reaction is identified as the nonpredissociating, nonrotating molecule in the 2p 2A2″ electronic state. The cross section for the state-to-state process H3+(X 1A1′, N=1, K=0)+K(g)→H3*(2p 2A2″, N=K=0)+K+ for a 6 keV ion beam is 7.0±1.0 Å2. Total ion beam attenuation cross sections for the species H3+, H2D+, D2H+, and D3+ with K targets are in the relative order 1.0, 0.59, 0.58, 0.53. The higher cross section observed for the H3+/K reaction is partially accounted for by an usually high cross section for the near resonant process H3+(X 1A1′, N=1, K=0)+K(g)→K++H3 (predissociative states). The branching ratio for the formation of metastable/predissociative states from the H3+(X 1A1′, N=1, K=0)/K reaction is about 0.012. The relative intensity of metastable D3 neutrals is at least an order of magnitude lower than that of H3 for identical experimental conditions. This effect is discussed in reference to the Pauli principle and the statistical weights of nuclear spin states of D3+(D3) and H3+(H3). The importance of angular momentum restrictions on electron transfer cross sections is also discussed.

Specific rate constants k(E, J) for unimolecular bond fissions

The specific rate constants k(E, J) for unimolecular bond fission reactions are calculated explicitly by a simplified statistical adiabatic channel model. The input data are reactant and product parameters, together with the ratio α/β of a global looseness parameter α and a Morse parameter β characterizing the intermediate part of the potential surface. Angular momentum coupling restrictions are being accounted for. The approach includes rigid RRKM and loose phase space theory as limiting cases for α/β=0 and α/β≳1, respectively.

On statistical models and prior distributions in the theory of chemical reactions

Statistical models which yield rate expressions involving the asymptotic channel states are classified according to three main characteristics: (i) The ensemble in which framework the basic statistical assumption is formulated. This assumption states that the initial and final states of the collision are, partly or completely, independent. (ii) The “measure in phase” associated with the product state distribution. (iii) The extent to which dynamical considerations are incorporated into the model. In this connection the discussion will be confined to the effects of angular momentum restrictions. The diversity in statistical models is demonstrated by comparing the product vibrational rotational distributions obtained from several models for atom—diatom exchange reactions. Numerical results are presented for two exoergic reactions which involve different mass combinations. Particular attention is paid to a comparison between the objectives, assumptions and the final expressions for product state distributions corresponding to the so called phase space theory on the one hand and the prior (zero surprisal) limit of the information theoretic approach to molecular dynamics on the other.

Excited states of30P investigated through the29Si(3He,d)30P reaction

The Oxford multigap spectrometer has been used in a study of the 29Si(3He,d)30P reaction at an incident 3He energy of 15 MeV. Twenty-eight states in 30P have been identified up to an excitation energy of about 6 MeV. Parity assignments have been made from a DWBA analysis of the angular distributions spin restrictions have been placed on some levels while the 3.731 MeV level has been assigned Jpi =1+. Spectroscopic factors have been obtained and compared with theoretical values. The results are discussed with respect to model calculations.

Elastic Scattering ofO18byO18

The excitation function at 90\ifmmode^\circ\else\textdegree\fi{} c.m. has been measured between 7.5 and 27.5 MeV c.m. The prominent gross structure observed in $^{16}\mathrm{O}$- $^{16}\mathrm{O}$ is not seen in the present case, and the absolute cross section falls to much lower values at high energies. The results of the comparison of the two systems are discussed in terms of angular momentum restrictions on configurations directly coupled to the elastic channel.

Nucleon Exchange Effects in theB10(d, p)Stripping Reaction

The consequences of an exchange contribution to a specific stripping reaction are worked out in some detail. The particular transition considered is that which leads to the first excited state of B/sup 11/ and which is forbidden by the angular momentum restrictions of ordinary stripping theory. The exchange calculation provides a fair measure of agreement with experiment on the angular distribution, yield, and energy dependence of the reaction. The relation of the present analysis to treatments of heavy-particle and spinflip stripping is briefly discussed. (auth)