Probing Surface Chemistry with Ion Imaging
Chemical reactions at surfaces are of paramount importance to chemical and energy industries. Most large-scale chemical processes rely on heterogeneous catalysis, where reactants and catalysts are separated in different phases and the reaction takes place at the phase boundary. As computing power continues to increase exponentially, this holy grail of predicting reactivity seems to be within our grasp. Chemical kinetics and dynamics at surfaces can now be simulated with more detail and accuracy than ever, and thus require top-notch experimental results for comparison. The goal of researchers should thus be to design experiments which can be easily compared to theoretical calculations. This requires clear assignment of the observed processes, ideally elementary step processes, and the related rate coefficients and dynamical information. In order to correctly assign the observed processes, the experiment must thus be carried out for the simplest possible system. As inorganic catalysts possess different sites with distinct activity, such as closed-packed terraces, step edges or defect sites, reactions often take place at different sites. As the discrimination between the different pathways is extraordinarily difficult, most experiments measure a combination of reactions taking place at different active sites. Unless the reaction at other sites can be suppressed, this limits mechanistic understanding and makes meaningful comparison to theory impossible.
The ion imaging detection allows the simultaneous and resolved detection of several velocity components. For systems where reaction products from different sites exhibit a specific dynamical fingerprint in the velocity distribution, this allows us to measure active-site selected kinetics at surfaces. By measuring the velocity-resolved product flux as a function of molecular beam–laser delay we determine the exact time at which the reaction product desorbs from the surface. In combination with knowledge of the incident molecular beam arrival time on the surface we calculate the product flux as a function of reaction time on the surface, the kinetic trace.