Crossed-beams studies of the dynamics of the H-atom abstraction reaction, O(3P) + CH4 → OH + CH3, at hyperthermal collision energies.

The H-atom abstraction reaction, O((3)P) + CH(4) → OH + CH(3), has been studied at a hyperthermal collision energy of 64 kcal mol(-1) by two crossed-molecular-beams techniques. The OH products were detected with a rotatable mass spectrometer employing electron-impact ionization, and the CH(3) products were detected with the combination of resonance-enhanced multiphoton ionization (REMPI) and time-sliced ion velocity-map imaging. The OH products are mainly formed through a stripping mechanism, in which the reagent O atom approaches the CH(4) molecule at large impact parameters and the OH product is scattered in the forward direction: roughly the same direction as the reagent O atoms. Most of the available energy is partitioned into product translation. The dominance of the stripping mechanism is a unique feature of such H-atom abstraction reactions at hyperthermal collision energies. In the hyperthermal reaction of O((3)P) with CH(4), the H-atom abstraction reaction pathway accounts for 70% of the reactive collisions, while the H-atom elimination pathway to produce OCH(3) + H accounts for the other 30%.

Experimental and theoretical investigations of the inelastic and reactive scattering dynamics of O(3P) collisions with ethane.

Detailed experimental and theoretical investigations have been carried out for the reaction of O((3)P) with CH(3)CH(3) at collision energies in the range of 80-100 kcal mol(-1). Experiments were performed on a crossed molecular beams apparatus with a laser breakdown source (which produces beams of O((3)P) with average velocities of 6.5 to 8.5 km s(-1)) and a pulsed supersonic source of CH(3)CH(3) having an average velocity of approximately 0.8 km s(-1). A rotatable quadrupole mass spectrometer allowed universal detection, with angular and velocity resolution of products scattering from the crossing region of the two reagent beams. Theoretical calculations were carried out in two stages, (1) electronic structure calculations to characterize the stationary points associated with the title reaction and (2) direct dynamics calculations employing the MSINDO semiempirical Hamiltonian and density functional theory (B3LYP/6-31G**). The dynamics of O-atom inelastic scattering and H-atom abstraction to form OH + C(2)H(5) were clearly revealed by the experiment and were matched well by theory. Both of these processes favor high-impact parameters, with most of the available energy going into translation, indicating a stripping mechanism for H-atom abstraction. H-atom abstraction was the dominant reactive pathway, but H-atom elimination to form OC(2)H(5) + H was also inferred from the experimental results and observed in the theoretical calculations. This reaction proceeds through small-impact-parameter collisions, and most of the available energy goes into internal excitation of the OC(2)H(5) product, which likely leads to secondary dissociation to H(2)CO + CH(3) or CH(3)CHO + H. A relative excitation function for the H-atom elimination channel was also measured and compared to a calculated absolute excitation function. The theoretical calculations also identified several additional reaction pathways with low relative yields, including C-C bond breakage to form OCH(3) + CH(3). Interference from OC(2)H(5) decomposition products in the experiment inhibited the unambiguous observation of the low-yield reaction pathways that were identified by theory, although an upper limit for the relative yield of C-C bond breakage was determined.

Experimental and theoretical investigations of the inelastic and reactive scattering dynamics of O(3p) + D2.

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).

Crossed beams and theoretical studies of the O(3P)+CH4-->H+OCH3 reaction excitation function.

The excitation function for the reaction, O(3P)+CH4-->H+OCH3, has been measured in a crossed molecular beams experiment and determined with direct dynamics calculations that use the quasiclassical trajectory method in conjunction with a recently developed semiempirical Hamiltonian. Good agreement is found between experiment and theory, enabling us to address two fundamental issues for the O(3P)+CH4 reaction that arise for all O(3P)+saturated hydrocarbon reactions: (1) the importance of triplet excited states that correlate adiabatically to ground-state reactants and products and (2) the importance of intersystem crossing processes involving the lowest singlet surface [corresponding to reaction with O(1D)]. Our results indicate that the first excited triplet surface contributes substantially to the cross section when the collision energy exceeds the reaction barrier (approximately 2 eV) by more than 0.5 eV. Although triplet-singlet crossings may occur at all energies, we have found that their effect on the excitation function is negligible for the collision energies studied-up to 1.5 eV above threshold.

Measurements and simulations of high energy O(3P) + Ar(1S) angular scattering: single and multi-collision regimes.

We present differential angular cross sections for O(3P) + Ar(1S) scattering at collision energies near 90 kcal mol(-1) (approximately 8 km s(-1) relative velocity) from molecular beam measurements and high-level theoretical calculations. Beams of hyperthermal O(3P) are now being used to investigate novel gas-phase and gas-surface chemistries, and the comparison of theory and measurements on this simple system will be a stringent test of the experimental methodology. Potential energy curves were generated for O(3P) + Ar(1S) using a large cc-pVQZ basis within a valence multi-configuration plus perturbation theory treatment. These curves were then used in quantum scattering calculations to generate differential cross sections. Agreement between experiment and theory is excellent. In addition to these comparisons, the cross sections were used in direct simulation Monte Carlo calculations to investigate effects of increasing the Ar flux above the "single-collision" regime. As the Ar flux increases, the observed differential angular cross sections change in two ways. In addition to the main "single-scatter" peak along the incident O-atom beam direction, a secondary O-atom peak appears in the direction of the incident Ar beam, and the multiple-scattered O-atom translational energy starts to reflect the energy of the relatively slow moving Ar beam.

Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane.

Crossed molecular beams experiments and classical trajectory calculations have been used to study the dynamics of Ar+ethane collisions at hyperthermal collision energies. Experimental time-of-flight and angular distributions of ethane molecules that scatter into the backward hemisphere (with respect to their original direction in the center-of-mass frame) have been collected. Translational energy distributions, derived from the time-of-flight distributions, reveal that a substantial fraction of the collisions transfer abnormally large amounts of energy to internal excitation of ethane. The flux of the scattered ethane molecules increased only slightly from directly backward scattering to sideways scattering. Theoretical calculations show angular and translational energy distributions which are in reasonable agreement with the experimental results. These calculations have been used to examine the microscopic mechanism for large energy transfer collisions ("supercollisions"). Collinear ("head-on") or perpendicular ("side-on") approaches of Ar to the C-C axis of ethane do not promote energy transfer as much as bent approaches, and collisions in which the H atom is "sandwiched" in a bent Ar...H-C configuration lead to the largest energy transfer. The sensitivity of collisional energy transfer to the intramolecular potential energy of ethane has also been examined.