Reactive and Inelastic Scattering Dynamics of Hyperthermal O and O2 from a Carbon Fiber Network

Abstract

The reactive and inelastic scattering dynamics of ground-state atomic and molecular oxygen from a carbon fiber network at 1023-1823 K was investigated with a molecular beam-surface scattering technique. A molecular beam containing hyperthermal O and O<sub>2</sub> with a mole ratio of 0.92:0.08 and nominal velocity of 8 km s<sup>-1</sup> was directed at the network, and time-of-flight distributions of the scattered products were collected at various angles with the use of a rotatable mass spectrometer detector. O atoms exhibited both impulsive scattering (IS) and thermal desorption (TD) dynamics, where the TD O-atom flux increased with surface temperature and the IS O-atom flux remained relatively constant. While the majority of the TD O atoms desorbed promptly after the beam pulse struck the network, signatures of thermal processes occurring over long residence times were also observed. Evidence of O<sub>2</sub> reactions was not observed, and the behavior of the inelastically scattered O<sub>2</sub> was invariant to the temperature of the network and showed both IS and TD dynamics. The dominant reactive product was CO, whereas CO<sub>2</sub> was a minor product. Both these products showed only TD dynamics. The observed flux of CO initially increased with temperature and then reached a plateau above which the flux no longer increased with temperature, over the temperature range studied. Thermally desorbed CO products exited the network promptly or after relatively long residence times, and two populations of CO with long residence times were distinguished. Hysteresis was observed in the temperature-dependent flux of thermally desorbed O and CO, with opposing trends for the two products. This work follows similar studies in our laboratory where the target materials were vitreous carbon and highly oriented pyrolytic graphite. The data suggest that the chemical reactivity of the three forms of <i>sp</i><sup>2</sup> carbon surfaces is similar and that the differences arise from the variations of the morphology. <br>

Air-Carbon Ablation Model for Hypersonic Flight from Molecular Beam Data

“…and FiberForm [19]. The main trends discussed above were consistent across each material, indicating that the measurements were general to many ablating carbon materials (i.e., those with significant roughness at the micron-scale and highly defected atomistic surface morphology).…”

“…Specifically, in our model, molecular oxygen must dissociatively chemisorb on the surface (reactions ( 16) and ( 19)) and the energy barrier is set to reduce the reactivity of O 2 to levels observed in experiments. Other than the dissociative chemisorption process, once the dissociated oxygen atoms are adsorbed on the surface in either O(s) (reaction ( 16)) or O * (s) (reaction (19)) states, the formation of CO and CO 2 proceeds via the same rates (i.e., the same mechanisms, pre-exponential factors, and activation energies) as the atomic-oxygen mechanisms as specificed by reactions (17), (18), and (20). Therefore, although the addition of molecular oxygen adds five reactions to the model, the only additional free parameter introduced is the activation energy for dissociative chemisorption.…”

“…The general experimental setup [12][16] [17] includes a rotatable mass spectrometer to monitor both the incident molecular beam and the volatile species that scatter from the surface. Experiments were performed at various surface temperatures for VC samples [12][13], and further experiments were performed for highly oriented pyrolytic graphite (HOPG) samples [18], and FiberForm samples [19]. The experiments provided a wealth of data regarding what reactions were dominant and the reaction probabilities for different products leaving the surface.…”