Surface Species Naming Conventions

Each surface species occupies one or more surface sites. A site is considered to be a location or position on the surface at which a species can reside. A site does not necessarily have to be a particular atom or have a composition itself. The total number of sites per unit area is considered a property of the material surface, and is often assumed to remain constant. 

We require that a given surface species only resides in one particular surface phase. For example, the properties of a hydrogen atom adsorbed on a step site might be different from a hydrogen atom adsorbed on a terrace site, so they could reasonably be considered different species (even though their elemental composition is the same). The number of species in surface phase n is termed Ks(n), and the species in that phase are numbered sequentially from the first species in the phase k (n) to the last species Kls(n). The total number of surface species in all surface phases is designated Ks. 

As a simple example, consider the case of the adsorption of a gas-phase molecule, A, on a surface. The surface is composed of either open sites or adsorbed molecules. In this formalism, there are two surface species one corresponding to the adsorption location, the open site, designated O(s), and the adsorbed molecule, A(s). The site fractions of O(s) and A(s) surface species must sum to unity. There is one surface phase in this case. In this trivial example, such overhead and formal definitions are unnecessarily complicated. However, in complex systems involving many surface phases and dozens of distinct surface species, the discipline imposed by the formalism helps greatly in bookkeeping and in ensuring that the fundamental conservation laws are satisfied. 

Researchers have had different views of naming the species that participate in reactions on surfaces. We highlight two conventions in particular, which we designate the atomic site convention and the open site convention. These two conventions are equally valid ways of describing surface reactions. Personal preference or, perhaps, the nature of a particular problem might dictate using one over the other in a given situation. 

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The use of optical methods which probe interface electronic and vibrational resonances offers significant advantages over conventional surface spectroscopic methods in which, e.g. beams of charged particles are used as a probe, or charged particles emitted from the surface/interface after photon absorption are detected. Recently, three-wave mixing techniques such as second-harmonic generation (SHG) have become important tools to study reaction processes at interfaces. SHG is potentially surface-sensitive at nondestructive power densities, and its application is not restricted to ultrahigh vacuum (UHV) conditions.However, SHG suffers from a serious drawback, namely from its lack of molecular selectivity. As a consequence, SHG cannot be used for the identification of unknown surface-species. 

Conventional approaches to microbiological examination of specimens require that they be cultured to assess the total numbers of specific groups of microorganisms or to determine the presence or absence of particular named species. The majority of samples taken for examination contain mixtures of different species, so simple plating onto an agar surface may fail to detect an organism that is present at < 2% of the total viable population. Various enrichment culture techniques may therefore be deployed to detect trace numbers of particular pathogens, prior to confirmatory identification. 

A key issue in the activity of these catalysts concerns the specific role of the Cu and Cr active sites which are expected to be located on the surface of the catalyst particles. Since both Cu(II) and Cr(III) are paramagnetic, EPR spectroscopy can be used to identify the nature of the heterometallic species within the catalyst matrix after different thermal and chemical treatments. However, a detailed picture of the local environment of tlie transition metal center camiot be obtained by conventional continuous wave EPR spectroscopy alone. These can, nevertlieless, be obtained by pulsed EPR methods, namely the electron spin echo envelope modulation (ESEEM) tecluiique. 

Nn,i is the component of the molar flux density of species in the direction normal to the surface at the point being studied. Therefore, =N,x n. Nn,i is a scalar quantity. By convention, the normal is oriented from phase a to phase We will use ws. to symbolize the algebraic production rate of species i per unit area. This species is produced or consumed locally by heterogeneous chemical reactions occurring in the interfacial zone. is the surface concentration of species f, in mol m , namely the... 

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