Atomic Oxygen Collisions Research Articles

Reactive, Inelastic, and Dissociation Processes in Collisions of Atomic Oxygen with Molecular Nitrogen.

We report the results of detailed calculations of reactive, inelastic, and dissociative processes in collisions of atomic oxygen with molecular nitrogen in their respective electronic ground states. Cross sections are calculated as a function of collision energy in the range 0.001-10 eV, considering the whole rovibrational ladder. Some problems related to the vibrational energy levels of the asymptotes of 3A″ and 3A' potential energy surfaces used in this work are solved by an appropriate scaling at the level of cross sections. The results are compared with data in the literature, obtaining excellent agreement with experimental thermal data for reactive processes on a very large temperature range, and reasonable agreement with indirect dissociative data. Significant discrepancies are observed with previous reactive state-to-state results calculated on less detailed potential energy surfaces. Inelastic results are compatible with extrapolation of experimental thermal rate coefficient for temperatures higher than 4500 K but completely fail to reproduce experimental data at room temperature. The issue is discussed, indicating the reasons and possible solutions to the problem, and a resonable rate coefficient is obtained combining experimental and theoretical results in the range 300-20000 K. Complete, accurate fits are provided for both reactive and dissociative state-to-state rate coefficients to use them in applicative numerical codes concerning air kinetics.

Direction dependent etching of diamond surfaces by hyperthermal atomic oxygen: A ReaxFF based molecular dynamics study

ReaxFF based reactive molecular dynamics simulations have been performed to gain an atomically detailed understanding of the etching of low index diamond surfaces exposed to energetic atomic oxygen collisions of the kind witnessed in the Low Earth Orbit (LEO) environment. ReaxFF simulations on small oxygen terminated diamond slabs indicated that a variety of functional groups such as ethers, peroxides, oxy radicals and dioxetanes can form on the surface, in agreement with earlier experiments and first principles based calculations. Successive oxygen collisions on larger reconstructed diamond surfaces showed that all the low index surfaces, namely diamond (100), (111) and (110) can be etched by hyperthermal atomic oxygen with diamond (100) showing the lowest etching rate and diamond (110) presenting the largest etching rate. The erosion yield of these surfaces is in good agreement with experimental results. The simulations performed here have been used to obtain an Arrhenius type rate law for the mass loss from these surfaces under such conditions. These simulations show that diamond thin films are promising materials for the surface of spacecraft exposed to LEO conditions and, in general, demonstrate the ability of ReaxFF to be used as an effective tool to screen or characterize materials for applications in extreme environments.

Large Scale Computational Chemistry Modeling of the Oxidation of Highly Oriented Pyrolytic Graphite

Large scale molecular dynamics (MD) simulations are performed to study the oxidation of highly oriented pyrolytic graphite (HOPG) by hyperthermal atomic oxygen beam (5 eV). Simulations are performed using the ReaxFF classical reactive force field. We present here additional evidence that this method accurately reproduces ab initio derived energies relevant to HOPG oxidation. HOPG is modeled as multilayer graphene and etch-pit formation and evolution is directly simulated through a large number of sequential atomic oxygen collisions. The simulations predict that an oxygen coverage is first established that acts as a precursor to carbon-removal reactions, which ultimately etch wide but shallow pits, as observed in experiments. In quantitative agreement with experiment, the simulations predict the most abundant product species to be O2 (via recombination reactions), followed by CO2, with CO as the least abundant product species. Although recombination occurs all over the graphene sheet, the carbon-removal reactions occur only about the edges of the etch pit. Through isolated defect analysis on small graphene models as well as trajectory analysis performed directly on the predicted etch pit, the activation energies for the dominant reaction mechanisms leading to O2, CO2, and CO product species are determined to be 0.3, 0.52, and 0.67 eV, respectively. Overall, the qualitative and quantitative agreement between MD simulation and experiment is very promising. Thus, the MD simulation approach and C/H/O ReaxFF parametrization may be useful for simulating high-temperature gas interactions with graphitic materials where the microstructure is more complex than HOPG.

Hyperthermal Oxidation of Graphite and Diamond

Carbon materials have mechanical, electrical, optical, and tribological properties that make them attractive for use in a wide range of applications. Two properties that make them attractive, their hardness and inertness in many chemical environments, also make them difficult to process into useful forms. The use of atomic oxygen and other forms of oxidation has become a popular option for processing of these materials (etching, erosion, chemical functionalization, etc.). This Account provides an overview of the use of theory to describe the mechanisms of oxidation of diamond and graphite using hyperthermal (few electronvolts) oxygen atoms. The theoretical studies involve the use of Born-Oppenheimer molecular dynamics calculations in which on-the-fly electronic structure calculations have been performed using either density functional theory or density-functional-tight-binding semiempirical methods to simulate collisions of atomic oxygen with diamond or graphite. Comparisons with molecular-beam scattering on surfaces provide indirect verification of the results. Graphite surfaces become oxidized when exposed to hyperthermal atomic oxygen, and the calculations have revealed the mechanisms for formation of both CO and CO(2). These species arise when epoxide groups form and diffuse to holes on the surface where carbonyls are already present. CO and CO(2) form when these carbonyl groups dissociate from the surface, resulting in larger holes. We also discuss mechanisms for forming holes in graphite surfaces that were previously hole-free. For diamond, the (111) and (100) surfaces are oxidized by the oxygen atoms, forming mostly oxy radicals and ketones on the respective surfaces. The oxy-covered (111) surface can then react with hyperthermal oxygen to give gaseous CO(2), or it can become graphitized leading to carbon removal as with graphite. The (100) surface is largely unreactive to hyperthermal atomic oxygen, undergoing large amounts of inelastic scattering and supporting reactions that create O(2) or peroxy radicals. We did not observe a mechanism for the removal of carbon for this surface. These results are consistent with experimental studies that show formation of CO and CO(2) in graphite oxidation and preferential etching on (111) CVD diamond surfaces in comparison with (100) surfaces.

Quasiclassical Trajectory Dynamics Study of Atomic Oxygen Collisions on an O-Preadsorbed Graphite (0001) Surface with a New Analytical Potential Energy Surface

A new flexible periodic LEPS potential energy surface (FPLEPS) based on density functional theory data is constructed for the interaction of atomic oxygen with an O-preadsorbed graphite (0001) surface over a C–C bridge. New ingredients were added to the usual expression of the FPLEPS in order to take into account the entrance barriers, molecular orientation, and morphology of the surface. A total of 563 DFT points were used to fit the Eley–Rideal (ER) reaction channel, achieving a root-mean-square deviation of 0.120 eV for energies lower than 1 eV over reactants. A quasiclassical trajectory (QCT) dynamics study has been performed at several initial conditions: collision energies (0.01 ≤ Ecol ≤ 2.0 eV), incident angles (θv = 0°, 45°), and surface temperatures (100 ≤ Tsurf ≤ 900 K). Also quasithermal and hyperthermal (⟨Ecol⟩ = 5.2 eV) conditions were considered. Eley–Rideal reaction and O reflection were the main processes, keeping the formed O2 molecules translationally and internally excited via the ER pr...

Molecular-Dynamics-Based Study of the Collisions of Hyperthermal Atomic Oxygen with Graphene Using the ReaxFF Reactive Force Field

In this work, we have investigated the hyperthermal collisions of atomic oxygens with graphene through molecular dynamics simulations using the ReaxFF reactive force field. First, following Paci et al. (J. Phys. Chem. A 2009, 113, 4677 - 4685), 5-eV energetic collisions of atomic oxygen with a 24-atom pristine graphene sheet and a sheet with a single vacancy defect, both functionalized with oxygen atoms in the form of epoxides, were studied. We found that the removal of an O(2) molecule from the surface of the graphene sheet occurs predominantly through an Eley-Rideal-type reaction mechanism. Our results, in terms of the number of occurrences of various reactive events, compared well with those reported by Paci et al. Subsequently, energetic collisions of atomic oxygen with a 25-times-expanded pristine sheet were investigated. The steady-state oxygen coverage was found to be more than one atom per three surface carbon atoms. Under an oxygen impact, the graphene sheet was always found to buckle along its diagonal. In addition, the larger sheet exhibited trampoline-like behavior, as a result of which we observed a much larger number of inelastic scattering events than those reported by Paci et al. for the smaller system. Removal of O(2) from the larger sheet occurred strictly through an Eley-Rideal-type reaction. Investigation of the events leading to the breakup of a pristine unfunctionalized graphene sheet and the effects of the presence of a second layer beneath the graphene sheet in an AB arrangement was done through successive impacts with energetic oxygen atoms on the structures. Breakup of a graphene sheet was found to occur in two stages: epoxide formation, followed by the creation and growth of defects. Events leading to the breakup of a two-layer graphene stack included epoxide formation, transformation from an AB to an AA arrangement as a result of interlayer bonding, defect formation and expansion in the top layer, and finally erosion of the bottom layer. We observed that the breakup of the two-layer stack occurred through a sequential, layer-by-layer, erosion process.

Energy Dependence of Hyperthermal Oxygen Atom Erosion of a Fluorocarbon Polymer: Relevance to Space Environmental Effects

The origin of the inconsistency in the erosion phenomena of fluorocarbon polymers between a ground-based atomic-oxygen test environment and the low Earth orbital space environment has been investigated. A detailed experiment was performed in order to study the effect of the high-energy component in the atomic-oxygen beam pulses on the erosion. The experiment was carried out with the combination of a polymer-coated quartz crystal microbalance and a high-speed chopper wheel installed in a laser-detonation atomic-oxygen facility. It was clearly observed that the mass-loss rates of both polyimide and fluorocarbon polymers depend on the impact energy of atomic oxygen. However, the energy dependence is more significant on the fluorocarbon polymer compared with that on polyimide. Collisions of atomic oxygen with translational energies higher than ∼5 eV induce a significant mass loss on the fluorocarbon polymer. These experimental findings agree with earlier measurements of the volatile products released during h...

Synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure study on oxidative etching of diamond-like carbon films by hyperthermal atomic oxygen

Surface structural changes of a hydrogenated diamond-like carbon (DLC) film exposed to a hyperthermal atomic oxygen beam were investigated by Rutherford backscattering spectroscopy (RBS), synchrotron radiation photoelectron spectroscopy (SR-PES), and near-edge X-ray absorption fine structure (NEXAFS). It was confirmed that the DLC surface was oxidized and etched by high-energy collisions of atomic oxygen. RBS and real-time mass-loss data showed a linear relationship between etching and atomic oxygen fluence. SR-PES data suggested that the oxide layer was restricted to the topmost surface of the DLC film. NEXAFS data were interpreted to mean that the sp 2 structure at the DLC surface was selectively etched by collisions with hyperthermal atomic oxygen, and an sp 3-rich region remained at the topmost DLC surface. The formation of an sp 3-rich layer at the DLC surface led to surface roughening and a reduced erosion yield relative to the pristine DLC surface.

Eley–Rideal reaction dynamics between O atoms on β-cristobalite (1 0 0) surface: A new interpolated potential energy surface and classical trajectory study

We present a theoretical study of the collisions of atomic oxygen with O-precovered β-cristobalite (1 0 0) surface. We have constructed a multidimensional potential energy surface for the O 2/β-cristobalite (1 0 0) system based mainly on a dense grid of density functional theory points by using the interpolation corrugation-reducing procedure. Classical trajectories have been computed for quasithermal (100–1500 K) and state-specific (e.g., collision energies between 0.01 and 4 eV) conditions of reactants for different O incident angles ( θ v). Atomic sticking and O 2(adsorbed) formation are the main processes, although atomic reflection and Eley–Rideal (ER) reaction (i.e., O 2 gas) are also significant, depending their reaction probabilities on the O incident angle. ER reaction is enhanced by temperature increase, with an activation energy derived from the atomic recombination coefficient ( γ O( θ v = 0°, T)) equal to 0.24 ± 0.02 eV within the 500–1500 K range, in close agreement with experimental data. Calculated γ O( θ v = 0°, T) values compare quite well with available experimental γ O( T) although a more accurate calculation is proposed. Chemical energy accommodation coefficient β O( T) is also discussed as a function of ER and other competitive contributions.

Hydrogen desorption from a diamond-like carbon film by hyperthermal atomic oxygen exposure

The densities of hydrogen and carbon atoms in a hydrogenated diamond-like carbon (DLC) film exposed to a hyperthermal atomic oxygen beam were investigated by Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA). The hydrogen density in DLC decreased upon atomic oxygen beam exposure with collision energy as low as 2 eV, whereas an exposure greater than 3 eV was necessary to remove carbon atoms. A high-collision energy also led to hydrogen desorption in the deeper region of DLC. The surface density of hydrogen decreased 6% by atomic oxygen exposure, and was independent of the collision energy. Additionally, the non-bonded hydrogen, which could diffuse in DLC, was desorbed by the energy transfer from the atomic oxygen collisions.

Modeling of Radiation from a Side Jet Atmospheric Interaction at High Altitudes

Highly nonequilibrium radiation transitions from electronically excited states generated by collisions of atomic oxygen with reaction control system thruster plumes is modeled for different altitudes and vehicle velocity conditions. The flowfield results are obtained using a Navier-Stokes solver to calculate the flow inside the nozzle and the direct simulation Monte Carlo method (DSMC) is used to simulate the chemically reacting, 3-D plume-atmospheric interaction. The radiation rate is compared for two types of overlay methods and with direct simulation in DSMC. The distribution function for collisional relative velocities is explored and used to interpret the sensitivity of the radiation rate to freestream altitude, temperature, velocity, and to the radiation mechanism activation energy.

Degradation on a mechanical property of high-modulus aramid fiber due to hyperthermal atomic oxygen beam exposures

High-modulus aramid fiber was exposed to energetic (5 eV) atomic oxygen, which simulates the space environment in low Earth orbit. It was found that Young’s modulus and tensile strength decreased significantly due to the atomic oxygen exposures. X-ray photoelectron spectroscopy showed that the decomposition of amid group occured at the exposed surfaces. Scanning electron microscopy revealed that the diameter of the fiber was not influenced so much by the attack of atomic oxygen, but the exposed fiber became an extremely rough surface. The change in mechanical properties has been explained by the formation of surface texture that is characteristic to the high-energy atomic oxygen collisions.

Lower thermospheric infra-red emissions from minor species during high-latitude twilight—A. Experimental results

During the DYANA campaign, thermospheric infra-red emissions of CO 2( v 2), CO 2( v 3), O 3( v 3) and NO ( ΔV = 1) were measured by a rocket-borne liquid helium-cooled spectrometer on 6 March 1990 above Kiruna, Sweden. The measured radiances are compared with the results obtained during other flights of the same instrument. According to the measured data the 15 μm band of CO 2 is in local thermodynamic equilibrium up to about (100 ± 5) km. Recently proposed high rate constants for the excitation of CO 2( v 2) in atomic oxygen collisions are verified by the measured data. The nitric oxide cooling rate in the lower thermosphere is found to be remarkably constant during different rocket soundings. A comparison between the measurements and a non-LTE model is given in part B of this paper; ( Ratkowski et al., 1994, J. atmos. terr. Phys. 56, 1899–1914).

Pancake cyclones and anti-cyclones in the ionosphere

The formation of pancake cyclones and anticyclones in the E-region of the Earth's ionosphere is considered. It is shown that the vortices can be maintained by the neutral winds or by chemical reactions including the energy release caused by the triple collisions of atomic oxygen with neutrals. It is found that the variations of the magnetic fields induced by the vortices are not localized and decrease slowly far from the vortex core. They can be easily detected by ground based magnetometers or by facilities on board the low-orbiting satellites.

Atomic oxygen emission at 63 µm as a cooling mechanism in the thermosphere and ionosphere

FAR IR emission by atomic oxygen fine structure transitions is considered an important cooling mechanism in the Earth's upper atmosphere and ionosphere as well as in several astronomical objects. The cooling rates so far have only been calculated theoretically on the basis of local thermodynamic equilibrium. No experimental or theoretical determination of the fine structure excitation or quenching cross sections for mutual neutral atomic oxygen collisions has been reported. We report here the first spectrometrically measured atomic oxygen 63-µm emission profile at thermospheric altitudes (90–180 km). An energy-averaged quenching cross section for the O(3P1) level was deduced. The importance of the 63-µm emission as a cooling process for both the neutral thermosphere and the ionosphere is much less than predicted.