DSMC analysis of molecular beam experiments for oxidation of carbon based Ablators

Direct simulation Monte Carlo (DSMC) is used to perform simulations of molecular beam scattering experiments of a hyperthermal O/O₂ beam striking a vitreous carbon surface. The current DSMC surface reaction model specifies the probabilities and characteristic frequencies associated with adsorption, desorption, Langmuir-Hinshelwood, and prompt thermal mechanisms according to reaction rate constants, sticking coefficients, and surface coverage. In addition to the macroscopic information (reaction mechanisms, rate constants, etc.), this DSMC model also includes microscopic information regarding detailed scattering of ablation products from a carbon surface (including sticking coefficients, desorption barriers, angular scattering, etc.). This DSMC solver is used to perform simulations of the molecular beam scattering from vitreous carbon with substrate temperatures ranging from 550-2000 K. Detailed analysis of the experimental time-of-flight (TOF) and angular distributions is used to propose modifications to the carbon surface oxidation model developed by Poovathingal et al., in order to better capture the features of the TOF distribution. We demonstrate that this revised model captures features in the TOF distributions, including the long tail in the CO distribution that indicates a slow CO production process and the fast component in the O distribution that corresponds to impulsively scattered and thermally desorbed O atoms, which were not captured with the previous model. Comparisons between the simulated (DSMC) and experimental TOF distributions, angular distributions, and relative product fluxes show excellent agreement. The resulting DSMC model is provided describing the detailed scattering information of the products, and the probabilities and characteristic frequencies of surface reactions based on finite rates.