Silicon dimer formation by three-body recombination

Abstract

The rates and dynamics of three-body thermal recombination of silicon atoms to form dimers is investigated at temperatures of 800, 1000, and 1200 K with Ar and Si atoms acting as the third body. A previously reported global potential-energy surface fitted to the results of ab initio calculations at the MP4/6-31G* level and experimental data are employed for the [Si,Si,Si] system. A simple, pairwise potential is used for the [Ar,Si,Si] system. The calculated rate coefficients for the [Ar,Si,Si] system all lie in the range of 1.34–1.46×1016 cm6/mol2 s. If rotationally trapped dimers are included, the results are in the range of 2.51–2.68×1016 cm6/mol2 s. The weak temperature dependence is characterized by an activation energy of 1.2 kcal/mol. When silicon is the third body, the rates are more than an order of magnitude larger due to the increased interaction and the opening of a complex formation channel for recombination. Four mechanistic pathways leading to recombination are identified. These are direct energy exchange, direct atom exchange, complex formation, and metastable formation due to a rotational barrier. For the [Si,Si,Si] system at 800 K, the contributions of these pathways to the total recombination rate are: direct energy and atom exchange (65.5%), complex formation (6.5%), and metastable formation (28%). Internal energy distributions for product Si2 dimers are reported. In every case, these distributions exhibit a prominent maximum at the Si2 dissociation threshold. The falloff at energies below the maximum reflects the expected exponential distribution of translational energies in unimolecular dissociation processes. The distributions for the [Si,Si,Si] system are broader than those obtained when Ar is the third body. This increased breadth is interpreted to be due to the increased interaction and complex formation that is not present for the [Ar,Si,Si] system.