Two-region model of CH4, CO, CO2, and H2O formation in graphite due to simultaneous irradiation by O+ and H+

Based on our experimental studies of the simultaneous ${\mathrm{O}}^{+}$ and ${\mathrm{H}}^{+}$ irradiation of graphite, symbolically represented as the ${\mathrm{O}}^{+}-{\mathrm{H}}^{+}\to \mathrm{C}$ reaction system, we have developed a semiempirical kinetic model for describing the reactions leading to the formation of the reaction products ${\mathrm{CH}}_4,$ CO, ${\mathrm{CO}}_2,$ and ${\mathrm{H}}_2\mathrm{O}.$ The model builds on previously proposed concepts for methane formation via intermediate precursor generation and a “two-region” description of the graphite microstructure. These concepts have been extended to include intermediate precursor-containing steps for the formation of CO, ${\mathrm{CO}}_2,$ and ${\mathrm{H}}_2\mathrm{O},$ and the breakup of ${\mathrm{CH}}_4$ by the incident ions. Model predictions of the flux and energy dependence of the ${\mathrm{CH}}_4,$ CO, and ${\mathrm{CO}}_2$ yields were generally in good agreement with experimental results for the ${\mathrm{H}}^{+}\to \mathrm{C}$ and ${\mathrm{O}}^{+}\to \mathrm{C}$ reactions. For the ${\mathrm{O}}^{+}-{\mathrm{H}}^{+}\to \mathrm{C}$ reaction, the model agrees quite well with the flux ratio dependence of the ${\mathrm{H}}_2\mathrm{O}$ yield and the corresponding reductions in the CO, ${\mathrm{CO}}_2,$ and ${\mathrm{CH}}_4$ yields.