Gas-Surface Reactions Proceeding via a Strongly Adsorbed Precursor

Gas-Surface Reactions Proceeding via a Strongly Adsorbed Precursor 

Once the thermodynamic parameters of stable structures and TSs are determined from quantum-chemical calculations, the next step is to find theoretically the rate constants of all elementary reactions or elementary physical processes (say, diffusion) relevant to a particular overall process (film growth, deposition, etc.). Processes that proceed at a surface active site are most important for modeling various epitaxial processes. Quantum-chemical calculations show that many gas-surface reactions proceed via a surface complex (precursor) between an incident gas-phase molecule and a surface active site. Such precursors mostly have a substantial adsorption energy and play an important role in the processes of dielectric film growth. They give rise to competition among subsequent processes of desorption, stabilization, surface diffusion, and chemical transformations of the surface complex. 

Available calculated ab initio quantum-chemical data are not sufficient for a purely ab initio calculation of the rates of all participating processes. Therefore, a model approach is desirable that incorporates the available quantum-chemical data and depends on a small set of physically reasonable parameters. The model must describe the competing processes of adsorption, desorption, chemical transformations, and energy relaxation into the bulk. A possible model of this type will be described below. 

Quantum-chemical calculations show that the reaction path profiles Urp(qr) (where qr is a reaction coordinate) for exothermic reactions of a gas molecule B with a surface center A/s/ via a surface precursor AB/s/ with the formation of a gas molecule D and a new surface center C/s/ have 

The evaluation of macroscopic rate constants of adsorption (kads) and direct reactions (kr+, kt) involves the following steps  

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