BML-IRRAS

Although IRRAS is a well-established method for studying monolayers on transparent substrates, its sensitivity is almost an order of magnitude lower than on metals. At the same time, transparent IRRAS offers an important advantage that p- and j-polarized spectra of the film can be measured, which is extremely valuable for orientational studies (Section 3.11.5). One can combine the advantages of metalhc and transparent IRRAS by using a complex-substrate transparent layer on a metal (Fig. 2.16), rather than a single-substance substrate. The upper transparent layer, which imitates the surface chemistry of a bulk transparent substrate, is dubbed a buffer or interference layer. The technique that involves such a buffer layer-metal substrate is known as buried metal layer (BML)-IRRAS or interference underlayer IRRAS. 

Figures 2.19 and 2.20 show experimental spectra measured in the vCH region by BML-IRRAS of an octadecyltrichlorosilane (OTS) SAM on the Si-CoSi2 Si substrate with the Si buffer layer 200 and 70 nm thick, respectively [61]. A close inspection of these spectra reveals essential differences between them. First, for the 200-nm Si, the SNRs for s- and p-polarization are similar at the same angle of incidence, tending to increase at a lower angle of incidence. However, the SNR in the p-polarized spectra on a 70-nm Si is appreciably higher than that in the j-polarized spectra, and the p-polarized band intensities decrease with decreasing angle of incidence. Second, in contrast to the 200-nm buffer layer.

Figure 2.20. BML-IRRAS spectra of SAM of OTS on Si-CoSi2-Si substrate with 70-nm buffer layer as function of angle of incidence (a) p-polarization, (b) s-polarization. Reprinted, by permission, from Y. Kobayashi and T. Ogino, Appl. Surf. Sc/. 100/101, 407 (1996), p. 409, Fig. 1. Copyright 1996 Elsevier Science B.V.

The use of a buffer (interference) layer between a film and the metallic substrate (the BML-IRRAS technique) allows one to measure the 5-polarized spectra and to avoid the band distortions in the p-polarized spectra. At... 

A theoretical analysis of the system consisting of either the ZnSe or Ge IRE, an Fe or hematite substrate layer, an adsorbate layer, and a solution of methylene chloride has been performed by Loring and Land [88], The system consisting of the ZnSe IRE, AI2O3 intermediate layer, the sputtered Si substrate layer, and water has been analyzed within the framework of the Fresnel formalism by Sper-line et al. [89], Calculations also reveal that within a narrow wavelength range and at a certain ratio of the optical refractive indices, enhancement of intensities is observed in the spectrum of a given layer in an arbitrary two-layer structure located on an IRE. The ATR spectra of such structures were considered in detail in Ref. [66], This enhancement is attributed to interference of the radiation, as is the enhancement of the reflectivity in BML-IRRAS (Section 2.3.3). 

In the case of transparent substrates, the sensitivity may be significantly increased with the BML-IRRAS technique (Section 2.3.3) or simply by placing a flat mirror under the substrate while the IRbeam is incident on the other side [16]. The problem that can be met when measuring the IRRAS spectra of films on transparent substrates is interference fringes, which arise due to multiple reflections of the IR beam inside the substrate. To eliminate multiple reflections of the beam and to achieve the... 

T, quasielastic neutron scattering, dielectric relaxation measurements BML-IRRAS... 

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