Reactions of Gas-Phase Atomic Hydrogen with Chemisorbed Hydrogen on a Graphite Surface

The reaction of gas-phase hydrogen atoms H with H atoms chemisorbed on a graphite surface has been studied by the classical dynamics. The graphite surface is composed of the surface and 10 inner layers at various gas and surface temperatures (T_g, T_s). Three chains in the surface layer and 13 chains through the inner layers are considered to surround the adatom site. Four reaction pathways are found: H₂ formation, H-H exchange, H desorption, and H adsorption. At (1500 K, 300 K), the probabilities of H₂ formation and H desorption are 0.28 and 0.24, respectively, whereas those of the other two pathways are in the order of 10^-2. Half the reaction energy deposits in the vibrational motion of H₂, thus leading to a highly excited state. The majority of the H₂ formation results from the chemisorption-type H(g)-surface interaction. Vibrational excitation is found to be strong for H₂ formed on a cold surface (∼10 K), exhibiting a pronounced vibrational population inversion. Over the temperature range (10-100 K, 10 K), the probabilities of H₂ formation and H-H exchange vary from 0 to ∼0.1, but the other two probabilities are in the order of 10^-3