Lisa A. Pederson

A global A-state potential surface for H2O: Influence of excited states on the O(1D)+H2 reaction

In this article a global potential energy surface for the 1A′′ state of H2O based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio results is presented. The resulting 1A′′ surface is used in conjunction with a previously determined 1A′ surface to study the O(1D)+H2(HD,D2) reaction dynamics, with emphasis on the influence of the 1A′′ excited state on measurable properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of about 2 kcal/mol on the 1A′′ surface, and even at 5 kcal/mol, the 1A′′ reactive cross section is only a small fraction (∼20%) of the barrierless 1A′. However, the 1A′′ surface populates very specific product vibrational states (v=3–4) and gives strongly backward peaked differential cross sections, so certain types of measurements are quite sensitive to the presence of this excited state. In particular, better agreement is found with experimental vibrational and angul...

POTENTIAL ENERGY SURFACE AND QUASICLASSICAL TRAJECTORY STUDIES OF THE N(2D)+H2 REACTION

We present a global potential energy surface for the 1A″ state of NH2 based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio (multireference configuration interaction) results. Extensive quasiclassical trajectory calculations are performed on this surface to study the N(2D)+H2/D2 reaction dynamics. Comparison is made with calculations on the lower level [first order configuration interaction (FOCI)] surface of Kobayashi, Takayanagi, Yokoyama, Sato, and Tsunashima (KTYST). We find a saddle point energy of 2.3 (1.9) kcal/mol for the perpendicular approach for the second order configuration interaction (SOCI) (SOCI with Davidson correction) surfaces, and a collinear stationary point energy of 5.5 (4.6) kcal/mol. The ordering of these stationary points is reversed compared to the corresponding FOCI results, and the only true reaction path on our surface is perpendicular. The primary reaction mechanism is determined to be C2v insertion to produce short lived...

Exploring the reaction dynamics of nitrogen atoms: A combined crossed beam and theoretical study of N(2D)+D2→ND+D

In the first successful reactive scattering study of nitrogen atoms, the angular and velocity distribution of the ND product from the reaction N(2D)+D2 at 5.1 and 3.8 kcal/mol collision energies has been obtained in a crossed molecular beam study with mass spectrometric detection. The center-of-mass product angular distribution is found to be nearly backward–forward symmetric, reflecting an insertion dynamics. About 30% of the total available energy goes into product translation. The experimental results were compared with those of quasiclassical trajectory calculations on an accurate potential energy surface obtained from large scale ab initio electronic structure computations. Good agreement was found between the experimental results and the theoretical predictions.

Reaction dynamics of O(1D)+HD. I. The insertion pathway

The doubly differential cross sections (angle and speed) for the reactions O(1D)+HD→OH(OD)+D(H) at Ec=2.05 kcal/mol were directly mapped out using the Doppler-selected time-of-flight technique in a crossed-beam experiment. Except for the previously noted discrepancy in the H/D isotope branching ratio, good agreement with QCT (quasiclassical trajectory) results on a high quality ab initio surface are found for all dynamical features, including the product angular and translational energy distributions and the very detailed angle-specific translational energy release.

Reaction dynamics of O(1D)+HD. II. Effects of excited surfaces

To explore the effects of excited surfaces on the title reaction, an in-depth crossed-beam investigation was carried out at two collision energies, 3.7 and 4.6 kcal/mol. In combination with the results obtained at the lower energy, 2.05 kcal/mol (the preceding paper), the extent of abstraction contribution to total reactivity was deduced from the doubly differential cross-section measurements and its dominant dynamical features were revealed for the first time. The former was found to be in reasonable agreement with our previous estimate based on the excitation function data. We also present quasiclassical trajectory results based on accurate potential energy surfaces for the 1 1A′ and 1 1A″ surfaces. The trajectory results give approximately the same breakdown into insertion and abstraction components as in the experiments, and some aspects of the product distributions, such as the angular distributions, are in rough agreement. However, other aspects of the product distributions, such as the abstraction ...

Adiabatic and non-adiabatic dynamics studies of O(1D)+H2→OH+H

This paper presents a detailed study of the title reaction, with emphasis on the contribution of excited electronic states, and on coupling between ground and excited states, in the reaction dynamics. Most of the calculations are based on diatomics-in-molecules (DIM) potential energy surfaces and couplings, but to provide calibration we compare adiabatic DIM results for the two lowest surfaces with corresponding results for high quality abinitio surfaces that were developed using the reproducing kernel Hilbert space interpolation method. These comparisons indicate that the DIM surfaces are close to being correct, with the most serious deficiencies occurring in the long range behavior of the 1A′ surface (too attractive), and the barrier height of the 1A″ surface (too high). The effect of non-adiabatic coupling has been described using Tullys fewest switches method and the DIM surfaces and couplings. We find that the surface hopping trajectories for collisions starting on the ground (1A′) surface show slightly colder product vibrations and rotations, and less backscattering than the equivalent adiabatic calculations, while the overall integral cross-sections are not significantly changed. For trajectories that start on the 2A′ surface, where reaction cannot occur in the absence of surface hopping, we find that the cross-section is often about half that associated with the 1A″ surface (which has the same collinear barrier but does not require hopping for reaction to occur). Product state distributions on the 2A′ surface are intermediate in character between 1A′ and 1A″.