Molecular dynamics calculation of carbon/hydrocarbon reflection coefficients on a hydrogenated graphite surface

Reflection coefficients for carbon atoms and hydrocarbon molecules on a carbon surface are critically needed for plasma-surface interaction analysis of carbon surfaces. These coefficients have been calculated with a molecular dynamics code using the Brenner hydrocarbon potential. The surface was prepared by bombarding a pure graphite lattice with energetic hydrogen, until a saturation was reached at ~0.42 H:C. Carbon atoms and several hydrocarbons (CH, CH₂, CH₃, and CH₄) were incident on this surface at different energies and angles. Typical results for carbon incident at 45° show reflection coefficients of 0.64±0.01 at thermal energy, decreasing to 0.19±0.01 at 10 eV. Hydrocarbons show more complicated behavior, tending to reflect as molecules at thermal energies and break up at higher energies, producing a spectrum of different reflected species. The total reflection of carbon via these fragments tends to decrease with incident energy, and increase with hydrogen content in the original molecule. The reflection coefficients, together with the energy and angular distribution of reflected particles, can be incorporated in erosion/redeposition codes to allow improved modeling of chemically eroded carbon transport in fusion devices.