Frequency shifts coverage dependent

We have Independently observed larger values for the coverage dependence In various electrolytes, but also find evidence that the composition of the electrolyte affects the magnitude of the frequency shift (17). 

We recall that the coverage dependent frequency shifts on copper surfaces are small. In the light of the preceding discussion possible explanations for this difference from the behaviour of CO on platinum and palladium include ... 

Fourth, the vibrational frequency of the adsorbed species undergoes coverage-dependent shifts Av as a consequence of dynamic and static effects (see Section IV.A.4). The shift depends on the type of bonding and the density of cationic centers on the face. Several examples of coverage effects of this type are reported in the following. It will be shown later that this information also has morphological implications because different faces can be characterized by different bonds and cation densities (and hence by different static and dynamic shifts of the vibrational frequency). 

Ni2+ NO complexes on NiO(OOl) faces (284), which are stable at room temperature. These complexes are characterized by a coverage-dependent v(NO) stretching frequency in the 1805-1799 cm-1 range. A v(NO) value lower than that of NO gas [v0(NO) = 1876 cm-1] and the stability at RT imply that d-n overlap contributions must be substantial. The spectra of the NO/NiO system are illustrated in Fig. 12. As was observed for CO, the v (NO) band gradually shifts with coverage because of the static and dynamic dipole-dipole interactions (Avdyn = 32 cm-1 and Avst = -26 cm-1) (284). 

The coverage dependence of the CO stretching frequency has been studied extensively for the metal/UHV interface [21]. The coverage-induced frequency shift is due to increasing dipole-dipole coupling [72] and chemical interactions between adsorbed molecules. 

The large frequency shift (— 100 cm V ) observed for the 1200 cm band of adsorbed sulfate was attributed to a Stark effect by Kunimatsu et al. [36] and to a backbonding mechanism by Faguy et al. [141]. The potential-induced band shift in the case of sulfate ions must be analyzed in more detail. It was observed that the band center for the 1200 cm feature of adsorbed sulfate presents, additionally, a dependence on the degree of coverage [142, 165]. Since the latter is also a function of the potential, it is clear that a part of the frequency change can be due to lateral interactions, as discussed in Sec. 9.5 for sulfate ions adsorbed on Pt(lll). 

Fig. 18. FT-RAIRS spectra of CO adsorbed at 300K on IML and 20ML films of palladium deposited by metal vapour deposition on TiO2(110) [56]. The switch from a local titania dielectric (transmission band) to that of the metal (absorption band) takes place at about lOML of palladium. The singleton frequency and the coverage dependent dipole shift are similar for both palladium layers indicating little perturbation of the CO adsorption behaviour on the palladium by the Ti02(l 10) substrate.

As the concentration of CO approaches complete coverage, or 9— stretching peak for CO can be seen to be dependent on the CO surface concentration in Scheme 6. Since CO and CO molecules have different vibrational frequencies, the dynamic effect of the CO molecules on the shift of the CO frequency will be absent. Hence, for the ratio 12 88 (where CO molecules predominate over CO molecules), the peak shift for CO from 2152 cm (where CO molecules are too dilute to interact with their neighbors) to 2109 cm is essentially the result of static interaction of CO molecules with the CO molecules. On the other hand, when the CO molecule is siurounded by molecules with the same mass and vibrational frequency (ratio 99 1) both static and dynamic interaction occurs and the peak shifts from 2152 cm to 2136 cm In this way the static and total frequency shifts are determined from experiment to be —43 and —16 cm, respectively, so that the dynamic shift can be calculated as 27 cm (Scheme 6). 

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