Classical trajectory calculations were employed to study the reaction of acetylene with dimer sites on the Si(100) surface at 105 K. Two types of potential energy functions were combined to describe interactions for different regions of the model surface. A quantum mechanical potential based on the semiempirical AM1 Hamiltonian was used to describe interactions between C2H2 and a portion of the silicon surface, while an empirically parametrized potential was developed to extend the size of the surface and simulate the dynamics of the surrounding silicon atoms. Reactions of acetylene approaching different sites were investigated, directly above a surface dimer, and between atoms from separate dimers. In all cases, the outcome of C2H2 surface collisions was controlled by the amount of translational energy possessed by the incoming molecule. Acetylene molecules with high translational energy reacted with silicon dimers to form surface species with either one or two Si–C bonds. Those molecules with low translational energy either rebounded away from the surface or became trapped in a physisorbed state as evidenced by their bouncing motion above the surface. The reaction of C2H2 to form a bridge between dimers within the same dimer row was found to occur, while bridging between adjacent dimer rows appeared to be unlikely, the C2H2 molecule preferring to migrate to either of the dimers for direct reaction. A mechanism is proposed for chemisorption in which C2H2 first bonds to a dimer site in a mono-σ structure, subsequently attaining the more stable di-σ bonded state through radical–radical recombination. The simulations are consistent with C2H2 adsorption on Si(100) occurring through a mobile precursor mechanism.

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Classical trajectory calculations were employed to study diamond (100) surface reconstructions. Atomic forces were computed from two types of potential-energy functions. A quantum mechanical potential based on the semiempirical PM3 (parametric method number three) Hamiltonian was used to describe the central core of the surface model, while an empirically parametrized potential was developed to extend the size of the model surface. The results indicate that the most energetically favorable surface consists of alternating monohydride dimers and dihydride sites. The global topology of reconstructed (100) surfaces can include dimer rows and zigzag dimer sites. The two configurations are close in energy, for both dehydrogenated and monohydride surfaces, with the row configuration being slightly more favorable. The minimum-energy dehydrogenated (100) diamond surface was found to consist of dimers with biradical electronic structures. The presence of atomic-level steps was found to prevent the formation of nearby dimer bonds, even when each of the available carbon atoms has a free valence.

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Classical molecular dynamics simulations of diamond surface reactions were performed using quantum mechanically derived forces. A semiempirical AM 1 potential from the MOPAC quantum-chemistry program package was employed in the calculations. The evolving molecular geometry, energetics, bond orders, and vibrational frequencies were monitored. The conformation and bond breaking of a surface step, examples of reactions of gaseous species with the diamond ( 1 1 1) surface, and reactions of free gaseous analogs were examined. One of the main findings is the existence of a trapped, physisorption state for an acetylene molecule colliding with a diamond surface at a temperature typical of diamond CVD. The hypothesis of chemical similarity between the rate constants for gas-surface and analogous gas-phase reactions is addressed. The results obtained indicate that although a considerable similarity indeed exists, significant quantitative differences are induced by the confinement of the surface.