Adsorbate-surface stretch modes

Figure 3 also contains an example of an ISER-flrel plot for a simple specifically adsorbed species, bromide on silver (solid curve). This plot was extracted from bromide coverage-potential data, obtained from differential capacitance measurements, along with the corresponding potential-dependent intensity of the SERS bromide-surface stretching mode at ca. 160 cm"1 (19.). In this case, the maximum (i.e. unity) value of 0r>1 corresponds to a close-packed bromide monolayer, ca. 1.4 x 10"9 mol cm 2. Again, the ISER-0t 1... 

Determination of surface functional groups, e.g., —OH, —C - C—, and >C = O, and identificadon of adsorbed molecules comes principally from comparison with vibrational spectra (infixed and Raman) of known molecules and compounds. Quick qualitative analysis is possible, e.g., stretching modes involving H appear for v(C—H) at 3000 cm and for v(0—H) at 3400 cm L In addition, the vibrational energy indicates the chemical state of the atoms involved, e.g., v(C=C) " 1500 cmT and v(C=0) " 1800 cm"L Further details concerning the structure of adsorbates... 

An aerosil sample was methoxylated at 400 C to examine the effect of surface composition on the infrared spectrum. The difference spectrum between the methoxylated silica and the dried silica is shown in Figure 2b. Comparing this with the difference spectrum for hydroxylated silica (2a) several changes are apparent. First, the band due to the hydroxyl stretches at 3744 cm is diminished and replaced by bands at 2958 and 2856 cm" due to the asymmetric and symmetric CH stretching modes of the adsorbed methoxy. 

The carbonate can also be compared with adsorbed formate species prepared by reacting methanol with the alumina surface at 350 C ( ). The spectrum for adsorbed formate. Figure 4d, shows the asymmetric carboxylate stretches at 1565 and 1440 cm l respectively, the CH stretch at 2832 cm , and the CH bending mode at 1505 cm . The Al-OC stretching mode is seen at 1060 cm", and the out-of-plane deformation at 750 cm . The signal to noise ratio in the low frequency end of the spectrum is insufficient to see the planar deformation, which should occur around 630 cm . It should be noted that the carbonate and formate species are very similar, the main distinction being the vibrations associated with the CH bond. 

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

In Figs. A(b) and A(c), spectra are shown which illustrate the irreversible change in the adsorbed layer as the surface is heated to room temperature. The low-frequency N-0 stretching mode disappears well before complete dissociation occurs. After this surface is heated to A07 K, dissociation is complete, as indicated by the sole loss feature at 573 cm-1. The intermediate state of the surface [Fig. A(c)] is characterized by a single N-0 stretching frequency at approximately 1782 cm-1 and a low-frequency mode at 538 cm-1, depending upon coverage. 

Finally, the perturbation induced on the stretching mode of adsorbed CO (expressed as the difference AD between the stretching frequency of CO in the gas phase and CO in the adsorbed state) is proportional to the electric field E at the equilibrium distance from the adsorbing site (55,56). It will be shown that the AD (CO) a E relationship can be used to establish a spectroscopic scale of the polarizing tendencies of the positive centers at surfaces of ionic solids and, indirectly, to construct a resonable scale of their acidic strengths. The spectrum of CO adsorbed on NaCl films (Fig. 3), consisting of several bands, can be used to show that fourfold coordinated Na+ ions on steps have enhanced polarizing character toward CO. 

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