The influence of adjacent molecular groups on the adsorption of ${\mathrm{H}}^{.}$, ${\mathrm{CH}}_3^{.}$, ${\mathrm{CH}}_2^{.}$, ${\mathrm{C}}_2$${\mathrm{H}}^{.}$, and ${\mathrm{C}}_2$${\mathrm{H}}_2$ on the diamond (111) surface has been investigated theoretically. Ab initio molecular-orbital theory was used in order to calculate the adsorption energies of these different species. The adsorption energies were found to be in the order ${\mathrm{C}}_2$${\mathrm{H}}^{.}$>${\mathrm{H}}^{.}$>${\mathrm{CH}}_2^{.}$>${\mathrm{CH}}_3^{.}$>${\mathrm{C}}_2$${\mathrm{H}}_2$ (onefold adsorption) for all the different types of neighboring environments investigated. The adsorbed species are here either surrounded by only ${\mathrm{H}}^{.}$ atoms or one of these ${\mathrm{H}}^{.}$ atoms is being removed, leaving a diradical site, or replaced by a ${\mathrm{CH}}_2^{.}$ or ${\mathrm{CH}}_3^{.}$ species, respectively. Generally, the sterically hindering effects of the branched neighboring species ${\mathrm{CH}}_2^{.}$ and ${\mathrm{CH}}_3^{.}$ will decrease the adsorption energy by about 60 and 100 kJ/mol, respectively. An exception is the ${\mathrm{C}}_2$${\mathrm{H}}_2$ species adjacent to a ${\mathrm{CH}}_2^{.}$ group, where the presence of the ${\mathrm{CH}}_2^{.}$ group increases the adsorption energy, due to a C-C bonding interaction between the two adsorbed species. This implies that ${\mathrm{C}}_2$${\mathrm{H}}_2$ adsorption under these circumstances will be energetically competitive with ${\mathrm{CH}}_3^{.}$ adsorption.

• Received 10 June 1994

DOI:https://doi.org/10.1103/PhysRevB.51.10003