Construction of finite rate surface chemistry models from pulsed hyperthermal beam experimental data

A general approach for constructing finite rate surface chemistry models using time-of-flight (TOF) distribution data acquired from pulsed hyperthermal beam experiments is presented. First, a detailed study is performed with direct simulation Monte Carlo (DSMC) to analyze the TOF distributions corresponding to several types of reaction mechanisms occurring over a wide temperature range. This information is used to identify and isolate the products formed through different reaction mechanisms from TOF and angular distributions. Next, a procedure to accurately calculate the product fluxes from the TOF and angular distributions is outlined. Finally, in order to derive the rate constant of the reactions within the system, the inherent transient characteristic of the experimental pulsed beam set up must be considered. An analysis of the steady-state approximation commonly used for deriving the rate constants reveals significant differences in terms of the total product composition. To overcome this issue, we present a general methodology to derive the reaction rate constants, which takes into account the pulsed setup of the beam. Within this methodology, a systematic search is performed through the rate constant parameter space to obtain the values that provide the best agreement with experimentally observed product compositions. This procedure also quantifies the surface coverage that corresponds to the rates of product formation. This approach is applied to a sample system: oxidation reaction on vitreous carbon surfaces to develop a finite-rate surface chemistry model. Excellent agreement is observed between the developed model and the experimental data, thus showcasing the validity of the proposed methodologies.