Experimental and Theoretical Investigations of the Inelastic and Reactive Scattering Dynamics of O(³P) + D₂

Abstract

This paper presents a combined experimental and theoretical study of the dynamics of O((3)P) + D(2) collisions, with emphasis on a center-of-mass (c.m.) collision energy of 25 kcal mol(-1). The experiments were conducted with a crossed-molecular-beams apparatus, employing a laser detonation source to produce hyperthermal atomic oxygen and mass spectrometric detection to measure the product angular and time-of-flight distributions. The novel beam source, which enabled these experiments to be conducted, contributed unique challenges to the experiments and to the analysis, so the experimental methods and approach to the analysis are discussed in detail. Three different levels of theory were used: (1) quasiclassical trajectories (QCT), (2) time-independent quantum scattering calculations based on high-quality potential surfaces for the two lower-energy triplet states, and (3) trajectory-surface-hopping (TSH) studies that couple the triplet surfaces with the lowest singlet surface using a spin-orbit Hamiltonian derived from ab-initio calculations. The latter calculations explore the importance of intersystem crossing in the dynamics. Both experiment and theory show that inelastically scattered O atoms scatter almost exclusively in the forward direction, with little or no loss of translational energy. For the reaction, O((3)P) + D(2) --> OD + D, the experiment shows that, on average, approximately 50% of the available energy goes into product translation and that the OD product angular distributions are largely backward-peaked. These results may be interpreted in light of the QCT and TSH calculations, leading to the conclusion that the reaction occurs mainly on triplet potential energy surfaces with, at most, minor intersystem crossing to a singlet surface. Reaction on either of the two low-lying reactive triplet surfaces proceeds through a rebound mechanism in which the angular distributions are backward-peaked and the product OD is both vibrationally and rotationally excited. The quantum scattering results are in good agreement with QCT calculations, indicating that quantum effects are relatively small for this reaction at a collision energy of 25 kcal mol(-1).

A quantum wave-packet study of intersystem crossing effects in the O(P2,1,3,D21)+H2 reaction

“…1͑a͔͒ increases with increasing collision energy, and the reactive energy threshold of about 0.40 eV agrees well with a previous quantum single surface study on the O͑ 3 P͒ +H 2 reaction. 24 This is consistent with a barrier process. A similar increasing trend is found in the three cross sections caused by intersystem crossing ͓see Figs.…”

Crossed-Beams and Theoretical Studies of Hyperthermal Reactions of O(3P) with HCl

“…To obtain satisfactory fits to the experimental data, two separate sets of P(E) and T(θ) distributions were used in the forward-convolution fitting procedure, with one for forward scattering and one for backward scattering. As discussed at length previously, 41 the derived c.m. angular and translational energy distributions are not entirely unique because of the limited resolution of the data.…”

“…quantities from the laboratory TOF and angular distributions. [41][42][43][44] A supersonic expansion of HCl was created with a piezoelectric pulsed valve, with a nozzle diameter of 1.0 mm, from a stagnation pressure of 15 psig. The HCl beam passed through a 5-mm-diameter skimmer and a 3-mm-diameter aperture before crossing the O-atom beam.…”

Semiclassical nonadiabatic dynamics based on quantum trajectories for the O(P3,D1)+H2 system

“…Classical results reproduce the coupling effect fairly well, especially at high energies where quasiclassical TSH calculations indicated enhanced reaction probabilities ͑compared to the single surface probabilities͒. 10 Introduction of the linearized quantum force corrects single surface quantum effects in the dynamics and improves agreement with the QM probabilities. Inclusion of the exponential function into the basis gives accurate reaction probabilities.…”

“…7 Maiti and Schatz have used TSH to study spin-orbit interaction induced intersystem crossing effects in the O + H 2 reaction, including four PESs in this model. 8 This reactive system has been thoroughly investigated both experimentally and theoretically, [9][10][11][12] although the first quantum wave-packet study involving four coupled electronic surfaces has been reported only recently. 13 Accurate PESs are available due to Rogers et al 14 and Dobbyn and Knowles, 15 and the spin-orbit couplings have been determined by Hoffmann and Schatz.…”