Transmission, ATR, and IRRAS Spectra of Ultrathin Films on Polarization Berreman Effect

DEPENDENCE OF TRANSMISSION, ATR, AND IRRAS SPECTRA OF ULTRATHIN FILMS ON POLARIZATION (BERREMAN EFFECT) 

As shown in Chapter 2, to optimize the contrast in the IR spectrum of an ultrathin film, it is necessary in many cases to use nonnormal angles of incidence and p-polarized radiation, which creates specific difficulties in the interpretation of the spectrum. The problems stem from the appearance of additional bands in the spectra of samples that are small relative to the wavelength these bands are due to the surface charges resulting from the polarization of the samples. The dependence of the transverse vibrational frequency of a polar crystal on the crystal size, called the size effect, was discovered by Frohlich [2]. A convincing explanation of this effect in the IR spectra of thin films was presented in 1963 by Berreman [3] while studying the transmission of 325-348-nm LiF layers. Consequently, this size effect in the IR spectra of ultrathin films became known as the Berreman effect by Harbecke et al. [4]. 

In this section the manifestation of the Berreman effect in experimental work will be illustrated by some typical examples and interpreted qualitatively. In the following section this effect will be considered from the viewpoint of Maxwell s 

Such a strong dependence on polarization in the IR spectra of ultrathin films may suggest a molecular-level, structural interpretation or, because of the contrast with ordinary transmission spectra, orientational effects. However, this is not the 

A qualitative understanding of the appearance of the absorption band near vlo in p-polarized spectra can be reached by considering the electric polarization of a slab-shaped cubic ionic crystal. If the external electric field Eq is perpendicular to the slab, the positive ions are displaced to one side of the slab and the negative ions to the other (Fig. 3.4). The excess charge on the surfaces results in a polarization P perpendicular to the slab plane and given by 

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