Surface concentrations intensity data

Quantification at surfaces is more difficult, because the Raman intensities depend not only on the surface concentration but also on the orientation of the Raman scat-terers and the, usually unknown, refractive index of the surface layer. If noticeable changes of orientation and refractive index can be excluded, the Raman intensities are roughly proportional to the surface concentration, and intensity ratios with a reference substance at the surface give quite accurate concentration data. 

One of the distinct advantages of XPS is that, through analysis of the corelevel intensities, it can provide quantitative data on adsorbate concentrations. The following equation relates the surface concentration a to the intensities of... 

Figure 6P.2 contains the Raman spectra of pyridine adsorbed on silica gel at three temperatures as reported by Schrader and Hill [Rev. Sci. Instrum, 46 (1335), 1975]. Four bands are apparent at 991, 1006, 1032, and 1069 cm F If the band intensities are proportional to surface concentrations and if each band is associated with one vibrational degree of freedom of an adsorbed species, what is your interpretation of these data ... 

III. Calculation of Surface Concentrations from Photoelectron Intensity Data... 

The detailed structure of the radioactive cloud after stabilization has occurred (within 10 minutes) has not been studied extensively. Figure 2 shows some relative radiation intensity data obtained by rockets following the Zuni explosion (3.5-megatons, coral island surface). The data pertain to the situation 15 minutes after the explosion. The evidences of a toroidal structure are still marked. Note that the bulk of the radioactivity is in the core of the cloud and that the distribution, either horizontally or vertically, is far from uniform. The aircraft samples on Zuni were collected at 41 kft., well below the core of concentration as found at 15 minutes. Since only the lower fringes of the nuclear cloud were penetrated, analyses probably do not represent fairly the nuclear cloud as it existed at the time of sampling. 

These data can be compared with those for a/w films shown in Figure 15. Such comparison suggests that there is substantially less protein at the interface in o/w thin films, indeed almost five times less. However, care needs to be exercised when equating surface concentration to fluorescence intensity. It is possible that the fluorophore is located in different environments in the two types of thin film and that the difference in fluorescence intensity is a fluorescence quantum yield effect. However, this is unlikely since the surface concentration, as judged by the surface fluorescence signal at which surface diffusion is first observed, in both a/w and o/w films is very similar at approximately 600 counts per channel. It is reasonable to assume that the structure of the adsorbed layer is similar at the point where surface diffusion is first observed. The presence of similar surface counts indicates that the quantum yield of fluorescence is similar at both o/w and a/w interfaces. Thus, this strongly supports the... 

Nearly all theories to date predict that IETS intensities should be proportional to n, the surface density of molecular scatterers. Langan and Hansma (21) used radioactively labeled chemicals to measure a surface concentration vs solution concentration curve ( Fig. 10 ) for benzoic acid on alumina using the liquid doping technique. The dashed line in Fig. 10 is a 2 parameter fit to the data using a simple statistical mechanical model by Cederberg and Kirtley (35). This model matched the free energy of the molecule on the surface with that in solution. The two parameters in this model were the surface density of binding sites ( 10" A )... 

Figure 38 illustrates accumulated surface scans in the rhodium 3d and phosphorus 2p region taken from granules of the rhodium anchored catalyst. The surface concentration is low enough that scan accumulation was necessary to detect these elements. These particles were oxygen plasma etched for thirty minutes and Figure 39 includes a survey spectrum as well as Rh 3d and P 2p spectra taken from the sample after OPE. The intensity of the rhodium and phosphorus lines is enhanced considerably as a result of etching. To investigate the depth of penetration of the anchored metal into the surface of the particles, surface spectra were obtained as a function of OPE times. This data is given in Table VIII and the phosphorus and rhodium spectra as a function of etch time in minutes is shown in Figure 40. The intensity of the rhodium and phosphorus lines increases up to twenty minutes of etching or equivalent to penetration of 160 nm into the surface of the particles. This analysis indicates that rhodium is fairly uniformly distributed into the particles at least 160 nm into the interior.

It is of interest to examine quantitatively such potential-dependent redox equilibria as determined by SERS in comparison with that obtained by conventional electrochemistry. Figure 1 shows such data determined for Ru(NH3 )6 3" 2+at chloride-coated silver. The solid curves denote the surface concentrations of the Ru(III) and Ru(II) forms as a function of electrode potential, normalized to values at -100 and -500 mV vs SCE. These are determined by integrating cyclic voltammograms for this system obtained under conditions [very dilute (50 yM) Ru(NH3)63 +, rapid (50 V sec-1) sweep rate] so that the faradaic current arises entirely from initially adsorbed, rather than from diffusing, reactant (cf. ref. 6b). The dashed curves denote the corresponding potential-dependent normalized Ru(III) and Ru(II) surface concentrations, obtained from the integrated intensities of the 500 cm 1 and 460 cm-1 SERS bands associated with the symmetric Ru(III)-NH3 and Ru(II)-NH3 vibrational modes.(5a)... 

FIGURE 2.8. Square root of the sum-ftequency intensity of the phenyl mode V2 as a function of surface concentration for (a) DBS at the CCU/water interface with 0.1M NaCl. (The line is a fit to the data.) (b) DBS at the air/water interface. The solid data points coirespond to surface concentrations at and near fiiU monolayer coverage. Adapted firom Ref. [40]. 

Qualitatively, it can be readily gleaned from the data in Fig. 5 and Fig. 6 that, in the alloyed state generated at 1000 K, the elemental composition at the outermost layer, as measured by the LEISS Pt Co peak-intensity ratio, is vastly different from that in the bulk. The peak intensities can be quantitatively converted to surface concentrations through the use of Eqs. (6) and (7) where the value offit-co was determined from experiment to be equal to 0.5. 

The total surface concentration and intensity distribution of acidic and basic active sites are presented in Fig. 7.10. The total height of the stacked bars represents the total surface concentration of the acidic and basic active sites in millimoles per gram. The individual parts of the stacked bar correspond to the intensity distribution. As shown in Fig. 7.10, these data indicate that magnesium silicate has a total acidic and basic site concentration of 1.8 and 2.3 mM/g, respectively [17]. In comparison with other types of adsorbents used in frying oil (activated carbon, alumna [basic], alumina [neutral], alumina [acidic], bleaching earth, dia-tomaceous earth, and silica), magnesium silicate shows the highest values of total acidic and basic sites. 

In a practical sense the effects described in the foregoing discussion place a severe limitation on the applicability of spectral studies of adsorbed molecules to the detailed elucidation of the adsorption process and of the stereochemistry involved in surface catalysis. Since the absorption intensity may be either enhanced or decreased as a result of adsorption on a surface, and may either increase or decrease with variation in surface coverage, it becomes very difficult indeed to use spectral data as a measure of the surface concentration of adsorbed species. This is of particular importance when more than one species occupies the surface e.g., physisorbed and chemisorbed species. In this case the absolute concentration of either species on the surface cannot be measured directly nor can it be reliably inferred from a comparison of the intensity of the bands corresponding to these two species. Moreover, in the identification of an adsorbed species the relative intensities of two or more bands characteristic of that species e.g., the CH stretching and the CH deformation frequencies for adsorbed hydrocarbons, cannot be used as evidence for the structure of the adsorbed species since the absorption coefficients of the individual bands may change in opposite directions as a function of surface coverage. Thus the relative intensities of such bands cannot be compared to the relative intensities of the same bands observed in solution or in the gas phase. A similar difficulty arises when attempts are made to use the electronic spectra of adsorbed molecules to complement the infrared spectra for identification purposes. 

Fig. 17 Comparison of the intensity of the benzoate anion solution loss feature at 1547 cm ( ) with the surface concentration of the adsorbed benzoate determined from chronocoulometric data. (From the work of Lipkowski and coworkers, Ref [155].)...

To understand how the conformational order of the alkyl chains varies with surface concentration, the ratio of the intensities of the symmetric methyl and symmetric methylene stretch modes is used in conjunction with the interfacial tension data. Previous studies have demonstrated that the ratio of the intensities of the symmetric... 

The aim of this paper is to summarize the results obtained on the microstructure and positional correlations of the polymeric micelles at concentrations for beyond the micellization boundary. Polymer concentration at the upper boundary of the disordered micellar phase is about 20 wt%. When the hydration effect of the copolymer is taken into account, the volume fraction of the micelles can be as high as 40 to 50%. The traditional methods in surfoce science, such as surface tension measurements, are useful in detecting the unimor-to-micelle transition boundary, but are very limited in higher concentration dispersions. On the other hand, the scattering techniques have been proven to be powerful even at higher concentrations (4, 20, 21). We have developed practical methods for analyzing absolute intensity data from both... 

The surface mass spectrum characterises the surface chemical structure. The spectral intensities can be used to determine the relative surface concentrations of the different surface species. Both positive and negative ion detection modes are possible in SIMS, as in all mass spectrometry techniques. A comparison of the positive and negative ion spectra can often substantially improve the analysis of the results. In SIMS, the charged fraction of the secondary particle flux is very small (10 ). Moreover, the number of sputtered ions per incident primary ion (i.e. the secondary-ion yield) is matrix dependent. With such yield variations direct quantification of surface species based on the number of desorbed secondary ions (i.e. from the SIMS data) is generally impossible [123]. Wucher et al. [143] have recently described a method to determine the secondary ion formation probability, i.e. the ionisation probability of sputtered particles in a direct and quantitative manner. 

Surface concentration data show that the effective thickness of /Mactoglobulin on gold is approximately 15 A. A reduced intensity by a factor of two of the Amide II bands in the titanium spectrum indicates that even thinner films are formed on this surface. 

There arc two principal ways in which surface concentration data may be obtained by the evaluation of XPS intensities. One relies on a first-principle description of photoelectron emission from a solid surface, the other on empirically determined elemental. sensitivity factors (cf. Chapter 4 and 5). Either approach can be used in its simplest form to estimate elemental surface concentration ratios from XPS intensity ratios for a catalyst surface, provided the sample is homogeneous and isotropic, i.e.. all elements are uniformly distributed in the surface layer sampled by XPS. However, a heterogeneous catalyst is in fact just that—heterogeneous it can be multipha.se and of complex structure. Operative words here are porosity, inner and outer surface, texture, segregation, etc. Nevertheless the simple procedures have been used extensively in catalyst characterization studies for straightforward interpretation of XPS in-... 

---