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 (γw) over the temperature range 300-1119 K. The result expressed in Arrhenius form is γW(H on diamond) = 10-3.4±0.3 + 100.29±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 γw yielded the rate constant of reaction 1 (assuming the surface concentration of H atom, [Hs] = 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 γw(CH3 on diamond) = 100.1±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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry