Relative coverage measurement techniques

Changes in resistivity are restricted to thin films or thin wires [194] in which the surface layer contributes a measureable fraction of the overall conductivity and such measurements are often run alongside measurements of heats of adsorption [195]. Resistance changes due to adsorption have been attributed to a demetallisation of the surface layer of metals [196] and to diffuse scattering of metal conduction electrons from virtual 

Ellipsometry is an attractive non-destructive technique for monitoring surface coverage and can be applied to any surface with a significant reflection coefficient [199]. It is based on changes in the optical properties of a reflecting surface and has been specifically developed by Bootsma et al. [200] for coverage measurements. 

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This method is subject to the same problems as the closely related uptake technique described above in particular, exposures are seldom reported to an accuracy better than 50%, with little reliability in derived values of s. Nevertheless, since multiple pulses may be derived from different adsorbate states on the surface, desorption spectra provide the added advantage of providing a means of measuring the relative coverage in each state, in addition to the absolute total coverage, provided that the different pulses can be reliably deconvoluted. However, care must be taken in the interpretation interconversion between states can occur... 

In this paper it has been attempted to provide an introductory overview of some of the various nonlinear optical characterization techniques that chemists are likely to encounter in studies of bulk materials and molecular structure-property relationships. It has also been attempted to provide a relatively more detailed coverage on one topic to provide some insight into the connection between the macroscopic quantities measured and the nonlinear polarization of molecules. It is hoped that chemists will find this tutorial useful in their efforts to conduct fruitful research on nonlinear optical materials. 

Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

Appearance relative age The appearance of a surface darkens over time as varnish thickens and increases in coverage. However, there are too many exceptions to permit accurate or precise assignment of ages based on visual appearance. For example, varnish can form in under 100 years in selected microenvironments (Krumbein, 1969 Dorn and Meek, 1995). There is no known method that yields reliable results (Dorn, 2000), although individuals have tried to make visual measurements using different techniques... 

The degree to which l c(C. ) " penetrates the film is reflected in Ret, thus providing a quantitative measure of the relative DNA surface coverage. The EIS method has been applied to assay hybridization/dehybridization events of both pure DNA films as well as DNA/mercaptohexanol mixed monolayers (Figure 5-2). It has proven to be a simple and reliable technique to report on the composition of DNA films under a wide range of experimental conditions. 

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