[en] A completely general canonical and microcanonical (energy-resolved) flexible transition state theory (FTST) expression for the rate constant is derived for an arbitrary choice of reaction coordinate. The derivation is thorough and rigorous within the framework of FTST and replaces our previous treatments [Robertson et al., J. Chem. Phys. 103, 2917 (1995); Robertson et al., Faraday Discuss. Chem. Soc. 102, 65 (1995)] which implicitly involved some significant assumptions. The canonical rate expressions obtained here agree with our earlier results. The corresponding microcanonical results are new. The rate expressions apply to any definition of the separation distance between fragments in a barrierless recombination (or dissociation) that is held fixed during hindered rotations at the transition state, and to any combination of fragment structure (atom, linear top, nonlinear top). The minimization of the rate constant with respect to this definition can be regarded as optimizing the reaction coordinate within a canonical or microcanonical framework. The expression is analytic except for a configuration integral whose evaluation generally requires numerical integration over internal angles (from one to five depending on the fragment structures). The form of the integrand in this integral has important conceptual and computational implications. The primary component of the integrand is the determinant of the inverse G-matrix associated with the external rotations and the relative internal motion of the fragments. (c) 2000 American Institute of Physics

[en] Wave packet calculations, using direct and damped-L² real propagation methods, of initial state-resolved and cumulative reaction probabilities for the O(³P)+HCl(¹Σ⁺)→OH(²Π)+Cl(²P) reaction are reported. Results are obtained using the recently developed ''S4'' potential surface of Ramachandran and co-workers and, for comparison, the earlier Koizumi, Schatz, and Gordon (KSG) potential energy surface. Most calculations are for total angular momentum J=0, although some J>0 centrifugal sudden results are also obtained. The thermal rate constant and the rate constant for HCl(v=1) are obtained from the J=0 cumulative reaction probability and J-K-shifting, using standard transition-state rotation constants. This type of shifting is justified by examining limited centrifugal sudden calculations. The S4 surface is shown to yield some surprising results. For example, despite a significantly higher ground state adiabatic barrier than the KSG surface, the thermal rate constant is not significantly different from one obtained with the KSG surface, although the one for the vibrationally excited HCl is. (c) 2000 American Institute of Physics

[en] Stellar reaction rates for proton captures on the nuclei 27Si, 31S, 35Ar, and 39Ca are estimated from the most recent nuclear structure information available. Reliable mirror-state correspondences are found by using the isobaric multiplet mass equation. An improved method for calculating proton partial widths is applied. Systematic comparisons of excitation energies, spectroscopic factors, proton partial widths, and γ-ray partial widths for states of the same isospin multiplet are presented. Stellar reaction-rate uncertainties are deduced, and our reaction rates are compared to previous estimates. Reaction network calculations are performed to investigate implications of the new reaction rates for nucleosynthesis in novae and X-ray bursts. Our recommended reaction rates are varied within their assigned uncertainties, and we find only minor effects on the nuclear energy generation and the final abundances after the outbursts. Thus, contrary to previous claims, we find no compelling reason for measuring the proton capture reactions on 27Si, 31S, 35Ar, and 39Ca by using radioactive ion beams. (c) (c) 1999. The American Astronomical Society

[en] Thermal wet oxidations of GaP and Al_0.4Ga_0.6P at 650 degree sign C for various times have been performed. Comparisons are made on oxidation rates and post oxidation morphology. Transmission electron microscopy shows that when oxidizing GaP, polycrystalline monoclinic GaPO₄·2H₂O forms without noticeable loss of phosphorus. Oxidation for 6 h or more leads to poor morphology resulting in cracks and detachment. A thickness expansion of about 2.5-3 times is noticed as a result of oxidation. In contrast, oxidized Al_0.4Ga_0.6P exhibits much better morphology without cracks or detachment from the substrate. The oxide has an almost amorphous-like microstructure. The oxidation process shows typical diffusion-limited reaction at long anneals. Preliminary work on the oxidation of AlP indicates that the reaction leads to formation of Al₂O₃ and possible volatile P₂O₅ diffusing out of the specimen. Thus, from the structural viewpoint, AlGaP forms a better oxide suitable for device needs. (c) 2000 American Institute of Physics

[en] The quantum yield for the formation of HCN from the photodissociation of pyrazine excited at 248 nm and 266 nm is determined by IR diode probing of the HCN photoproduct. HCN photoproducts from excited pyrazine are produced via three different dissociation channels, one that is extremely ''prompt'' and two others that are ''late.'' The total quantum yield from all reaction channels obtained at low quencher gas pressures, φ=1.3±0.2 for 248 nm and 0.5±0.3 for 266 nm, is in agreement with preliminary studies of this process as well as recent molecular beam studies. To investigate if HCN production is the result of pyrazine multiphoton absorption, this photodissociation process has been further studied by observing the HCN quantum yield as a function of total quencher gas pressure (10 mTorr pyrazine, balance SF₆) and as a function of 248 nm laser fluence from 2.8 to 82 mJ/cm2. At the highest SF₆ pressures, the HCN quantum yield shows strong positive correlation with laser fluence, indicating that the ''prompt'' channel is the result of multiphoton absorption; however, at low pressure, the HCN quantum yield is affected little by changing laser fluence, indicating that the majority of the HCN photoproducts at low pressure are produced from pyrazine which has absorbed only one UV photon. At the lowest pressures sampled, HCN produced from the one-photon ''late'' process accounts for more than 95% of all HCN formed (at low laser fluence). At high pressures the single photon ''late'' pyrazine dissociation is quenched, and HCN produced at high quencher gas pressures comes only from the multiphoton absorption channel, which can be clearly observed to depend on laser fluence. The HCN quantum yield as a function of laser intensity at high pressure has been fit to a quadratic function that can be used to determine the amount of ''prompt'' ''unquenched'' HCN produced from multiphoton photodissociation. Additionally, the information theoretic prior functions for energy disposal in the 248 nm photodissociation of pyrazine to form HCN have also been developed. Prior functions for one, two, and three-photon absorption indicate that only HCN with near room temperature translational energy comes from the one-photon process and that all HCN molecules with large amounts of translational energy are produced by multiphoton processes. Finally, analysis of the quenching data within the context of a strong collision model allows an estimate of the rate constant for HCN production from pyrazine for the major ''late'' channel, k_d1s=1.69x10⁵ s^-1, for 248 nm excitation, and k_d1s=1.33x10⁴ s^-1 for 266 nm excitation. After 266 nm excitation, pyrazine produced by the major one-photon channel lives for almost an order of magnitude longer than after 248 nm excitation. (c) 2000 American Institute of Physics

[en] An energy transfer probability distribution function, P(E,E), for the collisional relaxation of a highly vibrationally excited donor molecule (C₆F₆, pyrazine) is constructed for the first time from experimental data on the bath (CO₂) energy gain. A prescription for mapping bath quantum state resolved data onto P(E,E) is described in detail. Analysis of earlier experimental data allows a calculation of the high ΔE=E-E region (-7000 cm^-1< E-E<-1500 cm^-1) of P(E,E) for the above systems. Comparison of the P(E,E) functions reveals that C₆F₆ is a more efficient donor molecule than pyrazine, in agreement with previous experiments and trajectory calculations. In addition, resonance like structures in the P(E,E) functions arising from long range force mediated, V endash V excitation of the carbon dioxide ν₃ mode are discussed. These results indicate that accurate P(E,E) functions can be determined from experiments involving probes of the bath energy gain. This technique can be expected to provide stringent tests of current energy transfer theory and can, in principle, be used in conjunction with measurements of thermal kinetics to obtain energy dependent unimolecular rate constants, k_E. copyright 1997 American Institute of Physics

[en] The unimolecular dissociation of formaldehyde to H₂+CO was studied using extended basis set calculations and a variety of medium-to-high accuracy correlation recovery techniques. These included second and fourth order perturbation theory, multireference configuration interaction wave functions, coupled cluster theory with perturbative triples and full iterative triples, and estimated full configuration interaction wave functions. The intrinsic error of the electronic structure methods was assessed by extrapolating total energies to the complete basis set limit. Our best estimate of the barrier height, including zero point vibrational effects, is 81.9±0.3 kcal/mol, almost 3 kcal/mol larger than the experimental value of 79.2±0.8 kcal/mol. This estimate includes corrections for the effects of finite basis set truncation (which is negligible at the quintuple zeta level), higher order correlation recovery, core/valence correlation, and scalar relativistic effects. Using the same theoretical approach, we estimate the exothermicity of the dissociation reaction to be -1.6 kcal/mol, compared to experimental values in the -0.4 to -2.2 kcal/mol range. New calculations of the unimolecular dissociation rate constants using a variety of techniques failed to reconcile theory and experiment. (c) 2000 American Institute of Physics

[en] We present an efficient algorithm for generating semiglobal potential energy surfaces of reactive systems. The method takes as input molecular mechanics force fields for reactants and products and a quadratic expansion of the potential energy surface around a small number of geometries whose locations are determined by an iterative process. These Hessian expansions might come, for example, from ab initio electronic structure calculations, density functional theory, or semiempirical molecular orbital theory. A 2x2 electronic diabatic Hamiltonian matrix is constructed from these data such that, by construction, the lowest eigenvalue of this matrix provides a semiglobal approximation to the lowest electronically adiabatic potential energy surface. The theory is illustrated and tested by applications to rate constant calculations for three gas-phase test reactions, namely, the isomerization of 1,3-cis-pentadiene, OH+CH₄→H₂O+CH₃, and CH₂Cl+CH₃F→CH₃Cl+CH₂F. (c) 2000 American Institute of Physics

[en] A new potential energy surface is reported for the gas-phase reaction Cl+CH₄→HCl+CH₃. It is based on the analytical function of Jordan and Gilbert for the analog reaction H+CH₄→H₂+CH₃, and it is calibrated by using the experimental thermal rate coefficients and kinetic isotope effects. The forward and reverse thermal rate coefficients were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200-2500 K. This surface is also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate coefficients. We find that excitation of C-H stretching modes and Cl-H stretching modes enhances the rate of both the forward and the reverse reactions, and excitation of the lowest frequency bending mode in the CH₄ reactant also enhances the rate coefficient for the forward reaction. However, the vibrational excitation of the CH₃ umbrella mode (lowest frequency mode in products) slows the reaction at temperatures below 1000 K, while above 1000 K it also accelerates the reaction. (c) 2000 American Institute of Physics

[en] A model of collisional processes of hydrocarbons in hydrogen plasmas has been developed to aid in computer modeling efforts relevant to plasma-surface interactions. It includes 16 molecules (CH up to CH₄, C₂H to C₂H₆, and C₃H to C₃H₆) and four reaction types (electron impact ionization/dissociative ionization, electron impact dissociation, proton impact charge exchange, and dissociative recombination). Experimental reaction rates or cross sections have been compiled, and estimates have been made for cases where these are not available. The proton impact charge exchange reaction rates are calculated from a theoretical model using molecular polarizabilities. Dissociative recombination rates are described by the equation A/T^B where parameter A is fit using polarizabilities and B is estimated from known reaction rates. The electron impact ionization and dissociation cross sections are fit to known graphs using four parameters: threshold energy, maximum value of the cross section, energy at the maximum, and a constant for the exponential decay as energy increases. The model has recently been used in an analysis of the Joint European Torus [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)] MARK II carbon inner divertor using the WBC Monte Carlo impurity transport code. The updated version of WBC, which includes the full set of hydrocarbon reactions, helps to explain an observed asymmetry in carbon deposition near the divertor. (c) 2000 American Institute of Physics