Quasiclassical trajectory studies of the H+H2 reaction on an accurate potentialenergy surface. III. Comparison of rate constants and cross sections with experiment Quasiclassical trajectory studies of the H+H2 reaction on an accurate potential energy surface. II. Effect of initial vibration and rotation on reactivity Quantum mechanical calculations are compared to quasiclassical trajectory forward (QCT) calculations for the collinear. endoergic reaction Hlnl)+I->H+HI for two different potential energy surfaces, a rotated-Morse-curve (RMC) surface and the semiempirical valence-bond surface of Raff et al. Vibrationally stateselected reaction probabilities and rate constants and Arrhenius parameters are presented. Thermally averaged rate constants and their Arrhenius parameters are also given. For one of the potential energy surfaces, quasiclassical trajectory reverse histogram (QCTRH) calculations were also performed. The results show that classical mechanics and quantum mechanics are in significant qualitative agreement for state-selected properties. Specifically, for the n I = 0 state of the Raff et al. surface the quantum mechanical reaction probabilities are very small (less than 0.005) and the QCT method predicts this state to be totally non-reactive. For all other states on both surfaces quantum mechanics and QCT and QCTRH results all agree that reaction probabilities attain much higher values (up to 0.85). For both surfaces quantum mechanical and QCT results predict that excited vibrational states make significant contributions to the thermal reaction rates, although the methods disagree as to which vibrational state is quantitatively most important. Quantitative agreement with quantum mechanical results is obtained only with the QCTRH method for thermally averaged rate constants (agreement within 2%) and with both QCT and QCTRH methods for the Arrhenius parameters (agreement within a few tenths kcal mol-1 for activation energy). However, to achieve such agreement the QCT method had to be suitably modified to correct unphysical discrepancies in the threshold energy region. We present tables of these and many other results as a function of temperature. These should be useful in assessing the validity of trajectory studies of various kinds of reaction attributes under conditions where they are used to interpret experiments.

A potential energy surface for the Ne–H2+ reaction has been obtained in the LCAO–MO–SCF approximation. Analysis of the surface indicates that the reaction Ne+H2+→NeH++H should proceed with an endoergicity of 12 kcal/mole, in agreement with the experimental results of Chupka and Russell. Several procedures for parameterizing a diatomics-in-molecules (DIM) representation of the NeH2+ surface are considered. The results show that an accurate representation of the SCF surface can be obtained from the DIM model using a minimum of diatomic and triatomic data.

Atomic photoionization cross sections and asymmetry parameters are calculated using a simple oneelectron model potential consisting of the positive nuclear charge at the origin surrounded by a series of negatively charged spherical shells. The radial Schrodinger equation is solved exactly for unbound states of the potential using Whittaker functions. Parameters for the model potential (shell radii and charges) are fitted using SCF charge densities. Applications are made to inner-shell ionizations of atoms and molecules as observed in ESCA (Mg K a and A1 K a photon energies), with emphasis on 1s ionizations for boron through neon and 2s and 2p ionizations for aluminum through argon. The periodic variation in cross section for ionization of a given orbital is discussed in terms of the photoelectron kinetic energy. Effects due to core relaxation are considered for the neon atom. The calculations are compared with experimental X-ray absorption coefficients and photoelectron spectral intensities.hotoelectron spectroscopy using soft X-ray sources P has recently become a widely used experimental technique for studying atoms, molecules, and solids. The basic experiment consists of bombarding a sample to be studied with nearly monoenergetic photons and measuring the properties of the ejected photoelectrons. Although most of the emphasis has centered around the energetics of the photoionization process leading to a determination of electron binding energies, with recent advances in the experimental technique it has become feasible to determine the intensity3 of photoelectron peaks as well as the angular (spatial) distri-bution4l5 of the ejected photoelectrons. With these additional parameters the experimentalist can begin to unravel more complex photoelectron spectra.6 The in-( I ) (a)

Collinear quantum mechanical calculations are reported for the inelastic scattering of He by H+2 below the reaction threshold. The inelastic transition probability curves show a severe oscillatory behavior similar to that recently observed in reaction probability curves for this same system. Perturbation calculations and considerations of the channel phase shifts indicate that the resonance structure is caused by the existence of quasibound (resonance) states of HeH+2. A simple picture is presented which accounts for these quasibound states.

Theoretical studies of photoionization cross sections and angular distributions are reported for the ground state of the hydrogen molecule in the photon energy range 20–40 eV. Results are obtained using an integral equation technique recently developed by Chapman and Hayes. The method employs a partial wave expansion for the continuum wavefunction to generate a set of coupled equations which are solved exactly using an integral equation technique. Single center expansions are used for the initial and final electronic wavefunctions and exchange is treated iteratively. The results agree rather well with earlier studies of Kelly, of Ritchie, and of Hirota. Studies of the dependence of the differential cross section on molecular orientation are reported. These studies are of potential interest as a simple model for photoionization of a molecule adsorbed on a surface.