RESUMO
The classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.
RESUMO
We introduce the concept of Cavity Enhanced Self-Absorption Spectroscopy (CESAS), a new sensitive diagnostic tool for analyzing light-emitting samples. The technique works without an additional light source and its implementation is straight forward. In CESAS, a sample (plasma, flame, or combustion source) is located in an optically stable cavity consisting of two high reflectivity mirrors, and here it acts both as light source and absorbing medium. A modest portion of the emitted light is trapped inside the cavity, making 10(4)-10(5) cavity round trips while crossing the sample and an artificial augmentation of the path length of the absorbing medium occurs as the light transverses the cavity. Light leaking out of the cavity simultaneously provides emission and absorption features. The performance is illustrated by CESAS results on supersonically expanding pulsed hydrocarbon plasma. We expect CESAS to become a generally applicable analytical tool for real time and in situ diagnostics.
