Donna J. Garton

Reactive and inelastic scattering dynamics of hyperthermal oxygen atoms on a saturated hydrocarbon surface

The dynamics of the initial interactions of hyperthermal O atoms with a saturated hydrocarbon surface have been investigated by directing an O-atom beam at a continuously refreshed liquid squalane surface and monitoring time-of-flight and angular distributions of inelastically scattered O atoms and reactively scattered OH and H2O. These products are formed through thermal and nonthermal processes. The inelastic scattering processes may be described in terms of the limiting cases of direct inelastic scattering (nonthermal) and trapping desorption (thermal). The initial step leading to production of volatile OH and H2O is believed to be direct H-atom abstraction to form OH. Once formed, the OH may scatter directly into the gas phase before thermal equilibrium with the surface is reached, or it may undergo further collisions and reactions with the surface. These secondary interactions include trapping and desorption of OH and abstraction of a second hydrogen atom to form H2O. Interactions that occur before t...

A crossed molecular beams study of the O(3P)+H2 reaction: Comparison of excitation function with accurate quantum reactive scattering calculations

We present the first measurements of the relative excitation function for the O(3P)+H2 reaction, performed with the use of a crossed molecular beams apparatus in conjunction with a high-energy (laser detonation) source of O atoms. The results are in excellent agreement with accurate quantum wave packet calculations.

Comparative dynamics of Cl(2P) and O(3P) interactions with a hydrocarbon surface

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbon liquid, squalane, were investigated and compared to the results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as a function of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCl time-of-flight (from the surface to the detector) distributions revealed thermal and hyperthermal interaction channels, in analogy to the dynamical behavior of the O and OH signals observed in the previous study. The thermal HCl product may arise from two mechanisms: (i) desorption of trapped HCl product and (ii) reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O atoms with squalane led to a thermal OH signal, which could only come from desorption of trapped OH. The hyperthermal HCl s...

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.

Collision-Assisted Erosion of Hydrocarbon Polymers in Atomic-Oxygen Environments

Molecular beam–surface scattering experiments have been used to study the mechanisms of material removal when a hydrocarbon polymer surface erodes in the highly oxidizing environment of low Earth orbit or in a simulated space environment on Earth. During steady-state oxidation, CO and CO2 are produced. Formation of these volatile species is believed to account for a significant fraction of the mass loss of a polymer that is under atomic-oxygen attack. The rate of production of CO and CO2 is dramatically enhanced when a continuously-oxidized polymer surface is bombarded with Ar atoms or N2 molecules possessing translational energies greater than 8 eV. The yield of volatile products from the surface appears to increase exponentially with the collision energy of the inert atoms or molecules. Collisions of energetic inert species may accelerate the erosion of polymers in some exposure environments (e.g. in low Earth orbit, where N2 may strike oxidized surfaces with collision energies greater than 8 eV, and in certain atomic-oxygen test facilities that subject oxidized surfaces to bombardment by O2 molecules with average translational energies of approximately 10 eV).

Reactive scattering of ground-state and electronically excited oxygen atoms on a liquid hydrocarbon surface

We have directed a supersonic beam of atomic oxygen containing a large concentration of ground-state O(3P) and a small percentage of electronically excited O(1D) at a continuously refreshed liquid film of a long-chain saturated hydrocarbon, squalane (C30H62). Angularly resolved flux and energy distributions of reactively scattered products revealed that the dominant volatile reaction product is the OH radical, which can be formed by an Eley–Rideal direct-reaction mechanism or by a process that leads to trapping and desorption of the initial product. Both of these processes occur with comparable probabilities. A second product, H2O, is thought to be formed by abstraction of a hydrogen atom from the hydrocarbon chain by the primary OH product. The H2O product also exits the surface via non-thermal and thermal mechanisms, although the thermal mechanism dominates.

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%.

A flexible, highly efficient method for the preparation of homochiral oxazaphospholidine-boranes

Abstract An efficient and operationally simple procedure for the synthesis of 2-substituted 3,4-dimethyl-5-phenyloxazaphospholidine derivatives has been develop

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.

Dynamics of Atomic-Oxygen-Induced Degradation of Materials

We have investigated the mechanisms that lead to degradation of hydrocarbon-based materials that are subject to attack by atomic oxygen. Beams of energetic oxygen atoms were directed at liquid (squalane) and solid (Kapton) surfaces, and the reactive and nonreactive products that scattered from these surfaces were detected with a rotatable mass spectrometer detector. Angularly resolved flux and energy distributions of the products revealed that OH and H2O are the initial products of ground-state O(3 P) reactions. Subsequent reactions that become important on a static surface under continuous O-atom bombardment ultimately produce CO and CO2, All observed products exited the surface via thermal and nonthermal mechanisms, and the balance between these mechanisms was dependent on incident O-atom translational energy. Preliminary results suggest that concomitant surface bombardment by energetic atomic or molecular species can enhance the removal rate of CO and CO2, Protection of a polymer surface with a coating dramatically reduced the reactive signal that was detected and thus suggests an approach to materials testing in atomic oxygen environments that involves in situ monitoring of the process.