Surface selection rule SEIRA

Two other important characters of SEIRA should be addressed. First, the enhanced surface EM field around the metal particles is essentially polarized along the surface normal at any point on the particles, as shown by the small arrows in Fig. 8.3 [25]. Consequently, only the vibrations that give transition dipole components perpendicular to the local surface can be excited. The surface selection rule can be explained also by the interaction of the adsorbate dipole with its image induced in the metal the adsorbate dipole that is perpendicular to the surface constructively interacts with its image dipole to enhance the absorption, while the adsorbate dipole that is parallel to the surface destmctively interacts with its image dipole to reduce the absorption [26]. The surface selection rule is identical to that in IRAS [27] and the orientations of adsorbed molecules can be elucidated by using this rule, as will be described in more detail later. 

The phenomenon of surface-enhanced infrared absorption (SEIRA) spectroscopy involves the intensity enhancement of vibrational bands of adsorbates that usually bond through contain carboxylic acid or thiol groups onto thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule in the same way as reflection-absorption spectra of thin films on smooth metal substrates. When the metal nanoparticles become in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike surface-enhanced Raman scattering, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc. The mechanism of SEIRA is still being discussed but the enhancement and shape of the bands is best modeled by the Bruggeman representation of effective medium theory with plasmonic mechanism pla dng a relatively minor role. At the end of this report, three applications of SEIRA, namely spectroelectrochemical measurements, the fabrication of sensors, and biochemical applications, are discussed. 

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... 

Infrared ER spectrometry has no mechanism of intensity enhancement, in contrast to other methods such as RA spectrometry to be described in Chapter 10 and surface-enhanced infrared absorption (SEIRA), mentioned in Chapter 13. Nonetheless, infrared ER spectrometry provides a unique technique for utilizing s- and p-polarized radiations for obtaining information governed by the surface selection rule on the transition dipoles of molecular vibrations. Theoretical analysis of the information obtained by this technique has the possibility for elucidating molecular orientations in thin films on dielectric substrates and molecular interactions in a wide variety of materials, including liquid crystals. 

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