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Bulk species

If it is required that the surface of the sample remains undisturbed during analysis, SIMS must be carried out at very low surface removal rates, typically about 10 monolayer/s. The terms static and dynamic are used to divide the sputtering rate of the sample into regimes where only surface species are observed (static SIMS) or where surface and bulk species are observed (dynamic SIMS). The static limit is usually considered to be <10 ions/cm impinging on the sample surface. Under these conditions, only about 1/1000 atoms on the surface of the sample are struck by a primary ion. [Pg.297]

At the extremes of pH it is common for the equilibrium to be driven completely to the left or right and then the electrode process becomes independent of pH since it is the bulk species which is electroactive. Thus the common shape of curves is shown in Fig. 7 (Zuman... [Pg.179]

Although there are no examples of wrell-resolved fine-structure spectra for surface complexes, polycrystaliine spectra for bulk species have been... [Pg.294]

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. [Pg.55]

In this chapter I explained how isotope ratios may be calculated from equations that are closely related, but not identical, to the equations for the bulk species. Extra terms arise in the isotope equations because isotopic composition is most conveniently expressed in terms of ratios of concentrations. I illustrated the use of these equations in a calculation of the carbon isotopic composition of atmosphere, surface ocean, and deep ocean and in the response of isotope ratios to the combustion of fossil fuels. As an alternative application, I simulated the response of the carbon system in an evaporating lagoon to seasonal changes in biological productivity, temperature, and evaporation rate. With a simulation like the one presented here it is quite easy to explore the effects of various perturbations. Although not done here, it would be easy also to examine the sensitivity of the results to such parameters as water depth and salinity. [Pg.97]

Any species in the solid below the gas-solid interface, is defined to be a bulk species. Chemical names of species in bulk phases might be annotated by (b). Each bulk phase must have at least one bulk species. A bulk phase with only one species is considered a pure bulk phase one containing several bulk species is considered a bulk mixture. The number of species in bulk phase n is termed Kb(n), and the species in that phase are numbered sequentially from the first species in the phase k[(n) to the last species Klb(n). The total number of bulk species in all phases is designated Kb. When we have accounted for all of species in the mechanism (gas + surface + bulk), the total number is... [Pg.449]

A chemical species on the top-most layer of the solid (i.e., a surface species) occupies a site. For example, an arsine molecule adsorbed on a surface occupies a site, and could be denoted AsH3(s). Another example of a surface species is a bare gallium atom, Ga(s), on the top layer of a gallium arsenide crystal. What happens if another species, say a gas-phase AsH3, lands on top of the Ga(s) As shown in Fig. 11.3, the gallium atom that was at the surface is buried it is no longer designated a surface species. In this nomenclature it has become a bulk species. [Pg.450]

In bulk phases the compositions of the mixtures are specified by the mixture mole fractions. The bulk species activities, ak, appear in the kinetic rate-of-progress expression, rather than mole fraction. For the sake of parallelism, we adopt the nomenclature for bulk species ... [Pg.452]

Thermochemical properties of gas-phase, surface, and bulk species are assumed to be available. This information is used in the calculation of the equilibrium constant, Eq. 11.110, and thus the reverse rate constant, Eq. 11.108. There is not a great deal of thermochemical data for species on surfaces, but techniques are becoming available for their calculation (e.g., Pederson et al. [310]). If surface reactions are specified to be irreversible, or if Arrhenius coefficients for the reverse rate constant are explicitly supplied, then the thermochemical data are not actually used. [Pg.469]

The summation on the right-hand side runs over all surface and bulk species. It is interesting to note that by substituting Eq. 11.123 into the flux term on the left-hand side, the energy balance can be written in a more compact form as... [Pg.473]

If the mass densities pk of the bulk-phase species are known, one may convert the surface reaction rate of production of a bulk species (mol/m2s) into a growth rate G (m/s). The needed relationship is... [Pg.473]

In this form it is apparent that the gas temperature is affected by the homogeneous and heterogeneous reaction rates and the specific enthalpies of the gas-phase species that are produced by these mechanisms. The gas temperature is also affected by heat transfer at the channel walls. It is interesting to note that the net production of surface or bulk species does not directly affect the fluid temperature. [Pg.660]

Index of the first bulk species in bulk phase n ... [Pg.868]

In contrast to TEY detection which probes a 5 nm depth of the film, FY mode probes >50 nm (Kasrai et al., 1996). In Fig. 4.3 it was shown that the spectra measured for the model polyphosphates in TEY and FY modes were similar indicating that surface and bulk species are the same. Examining spectra F-H, it is immediately clear that the intensity of peaks a and b in (FY) spectra is much weaker than that in the corresponding TEY spectra A-C. [Pg.134]

As is widely known, bulk species which have chromophores that absorb in the UV-Vis can be analyzed quantitatively and qualitatively by this spectroscopy. The study of electrodes or species adsorbed as thin layers by UV-Vis is more difficult, due to sensitivity problems and the availability of the appropriate chromophores [50], Another use of this type of analysis is electroreflectance [51,52], Adsorption of species on reflective electrode surfaces changes their reflectivity. Thus, this method can indicate electroadsorption processes very sensitively, in situ, although it does not provide specific information on the structure and composition of surface layers. [Pg.123]

In the Reinert-Berg system, the homogeneous chemical reaction involves a bulk species, and there is no reaction layer (Sect. 2.2.6). [Pg.22]

Silver is an industrial oxidation catalyst that has attracted much attention regarding the nature of the surface oxygen species. Bulk AgO exhibits major Raman bands at 425 and 215 cm and a broad band centered at about 793 cm-1. Bulk silver oxide decomposes to the metallic silver phase at elevated temperatures (Millar et al., 1997). Oxygen exists in metallic silver as surface and bulk species. The most common Raman bands observed upon oxidation of metallic silver at elevated temperatures are located at 970,800,620,460, and 240 cm, and they do not correspond to the characteristic Raman bands of bulk AgO. [Pg.110]

If adsorption equilibrium is established rapidly and the adsorbed and bulk species remain in equilibrium throughout the reaction, cAads can be expressed in terms of a suitable isotherm. This allows the differential kinetic equation to be integrated. For example, if Henry s law adsorption is presumed to apply [43]... [Pg.77]

The main difficulty in surface characterization hes in the exceedingly low population of surface atoms (10 atoms cm ) relative to that of bulk species (10 atoms cm ). Experiments intended to examine the physical and chemical properties of surfaces must employ techniques that interact only with the outermost layers. For example, standard structural tools such as (nongrazing incidence) X-ray diffraction are not apphcable to single-crystal surfaces since X rays penetrate deeply into the material and yield information on the bulk rather than the surface. [Pg.6049]

The earliest quantitative theory of enzyme kinetics dates back to 1913, when Michaelis and Menten [27] succeeded in explaining a key feature of enzyme reactions with a very simple model. As an introduction and to establish the relationship between trace-level and bulk-species catalysis, this classical work and its subsequent refinements will now be reviewed. [Pg.206]

A comparison of eqn 8.23 for arbitrary distribution of catalyst material with the formula 8.2 developed for bulk-species catalysts is instructive. Application of the latter to the cycle 8.14 yields... [Pg.210]

In fast flames and shock tube flows such as are considered here, the concentration gradients in the recombination region are such that diffusion effects can be neglected. The recombination can also be considered as taking place in the presence of effectively constant concentrations of the bulk species Hj, HjO and N2 or Ar. As was first pointed out by Sugden and co-workers [133] the radical concentrations do not behave independently during the approach to full equilibrium. The observed relationships... [Pg.79]

Returning to our example solvent-separation problem, let us consider using liquid/liquid extraction to remove pentane from methanol and/or acetone. A suitable solvent is one that is immiscible with the bulk species, pentane. If at all possible, we would like the solvent to be present in the mixture so we do not have to introduce any other species into our separations problem. For our example problem, water is present. Noting the infinite-dilution A -values for water and pentane, we see that water will be highly immiscible with pentane. [Pg.124]

The current density corresponding to a Faradaic reaction can be expressed as a function of an interfacial potential V, as presented in equation (5.30), the surface concentration of bulk species Ci,o, and the surface coverage of adsorbed species 7jt as... [Pg.163]


See other pages where Bulk species is mentioned: [Pg.127]    [Pg.82]    [Pg.20]    [Pg.37]    [Pg.136]    [Pg.135]    [Pg.26]    [Pg.445]    [Pg.468]    [Pg.868]    [Pg.81]    [Pg.145]    [Pg.242]    [Pg.243]    [Pg.109]    [Pg.327]    [Pg.332]    [Pg.251]    [Pg.208]    [Pg.225]    [Pg.225]    [Pg.160]    [Pg.373]    [Pg.304]   
See also in sourсe #XX -- [ Pg.449 ]




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