IR Surface-Enhanced Spectra

Enhancement of absorption bands in the IR spectra of ultrathin films in the presence of discontinnons (islandlike) nnder- and ovemanolayers of Ag and An was discovered by Hartstein et al. [356] in the early 1980s. Although these researchers believed that they observed an increase in the vCH band intensities for p-nitro-benzoic acid (p-NBA), benzoic acid, and 4-pyridine-COOH films, it was recently shown [350] that the spectra reported are in actual fact due to fully saturated hydrocarbons (possibly vacuum pump oil). In any case, this discovery has stimulated various research activities and led to the development of surface-enhanced IR absorption (SEIRA) spectroscopy. To date, the SEIRA phenomenon has been exploited in chemical [357] and biochemical IR sensors (see [357-360] and literature therein), in studying electrode-electrolyte interfaces [171, 361-365], and in LB films and SAMs [364, 366-370]. Other metals that demonstrate this effect are In [371] and Cu, Pd, Sn, and Pt [372-375]. The metal films can be prepared by conventional metal deposition procedures such as condensation of small amounts of metal vapor on the substrate, spin coating of a colloidal solution, electrochemical [388], or reactive deposition [299] (see also Section 4.10.2). 

The SEIRA phenomenon can be illustrated by the / -NBA-Ag system, which has been extensively studied [382, 384, 390-393] since the work of Hartstein et al. [356]. Curve a in Fig. 3.61 shows the SEIRA spectrum of / -NBA adsorbed from an acetone solution onto a CaF2 window coated by an 8-nm Ag islandlike layer, after removal of physisorbed / -NBA by washing with acetone. This spectrum was recorded in the transmission configuration at (p =0°. For comparison, the /7-polarized spectrum obtained by IRRAS (cpi = 80°) of / -NBA adsorbed on a thick Ag layer and the transmission spectrum of / -nitrobenzoate potassium salt in a KBr pellet are also shown. The intensity of the SEIRA spectrum is enhanced about 10-fold relative to the spectrum obtained by IRRAS of this enhancement, a factor of 3 was attributed to the increase in the total surface area. Compared to the transmission spectrum at normal incidence, the enhancement is 200. 

It is seen from Fig. 3.61 that the SEIRA spectrum is identical to the one obtained by IRRAS. Only the synunetric (ai) modes (1352 and 1413 cm for CO2 and NO2 groups, respectively) appear, while the antisynunetric (( 1) modes (1528 and 1592 cm , respectively) are practically absent in both spectra. This can be understood assuming that p-NBA is adsorbed at the Ag surface as the p-nitrobenzoate ion with its C2 axis normal to the metal surface, as sketched in Fig. 3.62, provided charge transfer does not occur [390]. If this is the case, the dynamic dipole moment of the symmetric and antisymmetric CO2 and NO2 stretches is directed perpendicular and parallel to the metal surface, respectively. From this observation, Osawa and Yoshii [361] concluded that the surface selection rule (SSR) for metal surfaces is also valid for SEIRA, which was confirmed by Zhang et al. [367], In fact, analysis of the polarizability tensor of a molecule adsorbed on a metal particle [392, 394] has confirmed the dominance of the component, where the z-axis is normal to the surface at the adsorption site. Greenler et al. [395] have performed both classical and quantum-mechanical calculations for the interaction of the electromagnetic field with particles of varying sizes and arrived at the conclusion that the SSR should only be applied to 

As shown in Chapter 2, the electromagnetic enhancement can be generated within a film that has a smooth metal under- or overlayer when there is a specific combination of the refractive indices of the various components of the system. This effect is described by the Fresnel formulas (Sections 1.5-1.7) when the surfaces of the metal film are assumed to be smooth [400, 401]. 

Yakovlev and coworkers [402a, 402b] reported the IR absorption enhancement by mote than one order of magnitude for hydrocarbons adsorbed inside porous silicon, and assigned this effect to photon confinement in the microcavity acting like a multipass (Fabry-Perot type) cell. Recently, Jiang et al. [402c] observed a 50 times enhancement in the in situ IRRAS of CO adsorbed on Pd nanoparticles synthesized in cavities of Y-zeolite, as compared to the cases when the supports were ultrathin Pd films deposited directly on the zeolite or on amorphous alumosiUcate layer. 

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