Normand C. Blais

A double many‐body expansion of the two lowest‐energy potential surfaces and nonadiabatic coupling for H3

We present a consistent analytic representation of the two lowest potential energy surfaces for H3 and their nonadiabatic coupling. The surfaces are fits to ab initio calculations published previously by Liu and Siegbahn and also to new ab initio calculations reported here. The analytic representations are especially designed to be valid in the vicinity of the conical intersection of the two lowest surfaces, at geometries important for the H+H2 reaction, and in the van der Waals regions.

Trajectory‐surface‐hopping study of Na(3p 2P) +H2 → Na(3s 2S)+H2(v′, j′, θ)

Trajectory‐surface‐hopping calculations involving the three lowest‐energy 2A′ potential surfaces are reported for Na(3p 2P) collisions with H2(v=0, low j) at 0.9 kcal/mol relative translational energy. In addition to the total quenching cross section, we report distributions of final translational energy, final vibrational and rotational quantum numbers, internal energy, scattering angle, and collision time for the quenching collisions. We also report the opacity function, the correlation of scattering angle with impact parameter, and separate product translational spectra for the forward and backward scattered halves of the quenched ensemble. These results provide a detailed picture of the chemical dynamics of a typical quenching system proceeding through a quasibound intermediate configuration with large ionic character.

Monte Carlo trajectories: Dynamics of the reaction F+D2 on a semiempirical valence‐bond potential energy surface

A semiempirical valence‐bond calculation was carried out for the potential energy surface of H2F treating explicitly seven valence electrons (2pF51sHa1sHb). The integrals were evaluated from diatomic potential energy curves using the Cashion‐Herschbach method. Using this potential energy surface, the chemical reaction F+D2→FD+D was studied by Monte Carlo calculations of quasiclassical trajectories. Cross‐section calculations were carried out for initial conditions in these ranges: relative translational energy, 1.56–19.3 kcal/mole; rotational angular momentum (in units of ℏ), 0–5; vibrational quantum number, 0–1. In addition, distributions of internal energy and scattering angles for the molecular product were calculated. The results were compared with those of previous theoretical studies and the comparison indicates that subtle features (not well understood) of the potential surface may be important for obtaining correct results. The results were also compared with molecular beam, chemical laser, and in...

Monte Carlo trajectory study of Ar+H2 collisions. I. Potential energy surface and cross sections for dissociation, recombination, and inelastic scattering

Modified statistical electron–gas calculations using the methods of Gordon, Kim, Rae, Cohen, and Pack are carried out to obtain the interaction energy of Ar with H2 as a function of geometry. The results are combined with the accurate pairwise interactions, the long‐range nonpairwise interaction, and the potential LeRoy and van Kranendonk fit to spectral data on the van der Waals’ complex to obtain a potential energy surface which is as accurate as possible at all geometries. This surface and the pairwise additive surface are then used in a Monte Carlo quasiclassical trajectory study of the cross sections (under shock‐tube high‐energy collision conditions) for complete dissociation, for production of quasibound states of H2, and for V–T, R–T, and V–R–T energy transfer. Except for R–T energy transfer, the accurate surface yields smaller cross sections than the pairwise additive surface does. The cross sections for dissociation are much smaller than predicted by the available‐energy hard‐sphere model but ar...

Monte Carlo Calculations. II. The Reactions of Alkali Atoms with Methyl Iodide

A large number of trajectories for the reactions of alkali metal atoms with methyl iodide were calculated with a very fast digital computer, and the results compared with those of molecular‐beam experiments on the same reactions. Agreement was found to be very good. The calculations also show the relative influence of various collision parameters on the distributions of product internal energy and angle of emission, and the effect of certain general features of the assumed potential energy surface. The calculated distributions of product rotational energy indicate that approximately all the initial angular momentum appears as product rotation.

Molecular beam photoionization study of acetone and acetone‐d6

High resolution photoionization efficiency curves have been obtained for CH3COCH3+ and CD3COCD3+ using supersonic molecular beam sampling. As a result of adiabatic cooling during the nozzle expansion, sufficient concentrations of (CH3COCH3)2, (CD3COCD3)2, (CH3COCH3)3, and (CH3COCH3)4 were formed to permit the study of their photoion yield curves as well. Appearance potential curves have been determined for CH3CO+, CD3CO+, and (CH3COCH3) ⋅CH3CO+ fragments. The measured ionization potentials of acetone and acetone‐d6 monomers are 9.694±0.006 and 9.695±0.006 eV, respectively. Transitions to higher vibrational levels in CH3COCH3+ are seen at 320, 695, and 930−1370 cm−1 above threshold. The effect of perdeutero substitution is to reduce these frequencies to 260 and 660–1100 cm−1. Appearance potentials of CH3CO+ and CD3CO+ fragments are observed at 10.52±0.02 and 10.56±0.02 eV, respectively. The measured ionization energies for (CH3COCH3)n, n=1–4, are found to decrease linearly as a function of 1/n. Observed io...

Calculated product-state distributions for the reaction H + D2 → HD + D at relative translational energies 0.55 and 1.30 eV

Abstract We have used the quasiclassical trajectory method and the most accurate ab initio potential energy surface to calculate product vibrational-rotational distributions for H + D2 → HD + D at two energies for comparison with two new experiments.

Legendre moment method for calculating differential scattering cross sections from classical trajectories with Monte Carlo initial conditions

We present a method for obtaining continuous differential cross sections for molecular collisions from trajectories with initial conditions selected by Monte Carlo methods. It is a moment method and we represent the differential scattering cross section in terms of the moments of the normalized Legendre polynomials of the cosine of the scattering angle. The method is applied to five examples, for four of which we evaluate the necessary moments exactly. The fifth is a realistic example of an experimentally obtained differential cross section. For the first four cases we can obtain satisfactory convergence with as few as 400 trajectories with the proper choice of the highest order moment used in the expansion. As a working criterion we select as the highest order coefficient the highest order moment whose absolute value is larger than 0.05. Generally, the method does not converge any more rapidly than does the histogram method. It does provide a simple way of deciding the available angular resolution in the differential cross section for a given number of trajectories.

Thermal Conductivity of Helium and Hydrogen at High Temperatures

A steady‐state hot wire method for measuring the thermal conductivity of light gases in the temperature range 1200° to 2100°K is described. In contrast to other methods, free convection currents and large temperature gradients occur; convection effects are shown to be negligible, and the experimental procedure for eliminating the large gradient effects is described. The thermal conductivity of helium is found to follow the equation λ×106=991+0.678(T—1200) cal/sec cm deg over the temperature range covered. That for hydrogen is λ×106=1434+1.257(T—1200) cal/sec cm deg.

The effect of a conical intersection on cross sections for collision‐induced dissociation

The cross section for H+H2(v, j)→3H, where v and j denote selected vibrational and rotational quantum numbers, is calculated by the quasiclassical trajectory method, using trajectory surface hopping to include the effect of the first excited electronic state which has a conical intersection with the ground state. The excited electronic state allows for collision‐induced dissociation by the process H+H2(X 1Σ+g)→H3(1 2A’)→H3(2 2!iA’) →H +H2(b 3Σ+u) →3H, where the various transitions all occur in the course of a single collision. This new surface hopping mechanism increases the cross sections and rate constants for production of unbound states by 2%–44% for the conditions examined.