Adsorbate coverage, measurement

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

For other purposes, obtaining a measure of the adsorbate surface density directly from the experiment is desirable. From this perspective, we introduce a simple model for the variation of the surface nonlinear susceptibility with adsorbate coverage. An approximation that has been found suitable for many systems is... 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

It is useful to define the tenns coverage and monolayer for adsorbed layers, since different conventions are used in the literature. The surface coverage measures the two-dimensional density of adsorbates. The most connnon definition of coverage sets it to be equal to one monolayer (1 ML) when each two-dimensional surface unit cell of the unreconstructed substrate is occupied by one adsorbate (the adsorbate may be an atom or a molecule). Thus, an overlayer with a coverage of 1 ML has as many atoms (or molecules) as does the outennost single atomic layer of the substrate. 

In conclusion, TDS of adsorbates on single crystal surfaces measured in ultrahigh vacuum systems with sufficiently high pumping speeds provides information on adsorbate coverage, the adsorption energy, the existence of lateral interactions between the adsorbates, and the preexponential factor of desorption, which in turn depends on the desorption mechanism. Analysis of spectra should be done with care, as simplified analysis procedures may easily give erroneous results. 

Microcantilever deflection changes as a function of adsorbate coverage when adsorption is confined to a single side of a cantilever (or when there is differential adsorption on opposite sides of the cantilever). Since we do not know the absolute value of the initial surface stress, we can only measure its variation. A relation can be derived between cantilever bending and changes in surface stress from Stoney s formula and equations that describe cantilever bending [15]. Specifically, a relation can be derived between the radius of curvature of the cantilever beam and the differential surface stress ... 

Differential enthalpy changes of hydrogen and isobutylene adsorption were measured, as described elsewhere (36, 42). Figure 3 shows differential heat versus adsorbate coverage for hydrogen and isobutylene adsorption on Pt/Sn/Si02. Values of the differential enthalpy changes of adsorption,... 

Majda and co-workers provided an elegant demonstration of through-bond and chain-chain hopping processes as parallel mechanisms for electron transfer across SAMs [46, 133]. In n-alkanethiol monolayers formed on hanging drop mercury electrodes, the adsorbate coverage T is measured by chronocoulometry of the mercuric thiolate formed upon adsorption. For the as-adsorbed films, geometric con-... 

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