Heterogeneous reactions of H atoms and CH₃ radicals with a diamond surface in the 300-1133 K temperature range

The heterogeneous reactions of H atoms and CH₃ radicals with a polycrystalline diamond surface have been studied using discharge flow and pulsed excimer-laser photolysis techniques coupled with photoionization mass spectrometry. A random walk Monte Carlo solution of the three-dimensional diffusion equation was used to interpret the experimental data and to obtain the collisional efficiency for removal of hydrogen atoms by the diamond surface (γ_w) over the temperature range 300-1119 K. The result expressed in Arrhenius form is γ_W(H on diamond) = 10^-3.4±0.3 + 10^0.29±0.15 exp(-(6020 ± 410)cal/RT). The second term is assigned to reaction 1, surface H atom abstraction by gas-phase H atoms, followed by the fast recombination reaction 2 of gas-phase H atom with the surface sites created by reaction 1. The experimentally measured γ^w yielded the rate constant of reaction 1 (assuming the surface concentration of H atom, [H_s] = 1.8 × 10¹⁵ molecule cm^-2, and k₂ ≫ k₁): k₁ = 10^-10.06±0.15 exp(-(6680 ± 470)cal/RT) cm³ molecule^-1 s^-1. This expression for the rate constant is in good agreement with that for the gas-phase abstraction of tertiary hydrogen atoms. The CH₃(g) radical decay above the diamond film was studied over the temperature range 739-1133 K. The measured collision efficiency for the removal of CH₃(g) radicals by the diamond surface in this temperature range is γ_w(CH₃ on diamond) = 10^0.1±0.7 exp(-(10640 ± 3420)cal/RT), which, if interpreted as reaction 3 (surface H atom abstraction by gas-phase methyl radicals) with subsequent fast reaction (4) of gas-phase methyl radical with the surface sites created by reaction 3, yields k₃ = 10^-10.8±0.7 exp(-(11540 ± 3420)cal/RT) cm³ molecule^-1 s^-1. The absolute value of k³ over the temperature range studied (derived assuming the same surface density of hydrogen atoms) is 6 times higher than that of the analogous gas-phase processes.