Transparent IRRAS

The ir spectra acquired in this way are extremely sensitive to the orientation of the surface molecules. Molecules must have a significant component of a molecular vibration perpendicular to the surface to be sensed by coupling with the highly directional electric field. Molecules whose dipole moments are perfectly parallel to the surface caimot couple to the existing electric fields, and therefore, are ir transparent by this method. This selectivity of the approach for molecule dipole moments perpendicular as opposed to parallel to the surface is known as the surface selection rule of irras. 

Transparent and Weakly Absorbing Substrates ( Transparent IRRAS)... 

IRRAS spectra of transparent layer with optical constants 02 = 1.5, fC2 = 0, and 0I2 = 10 nm on quartz surface. 

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. 

By making the buffer layer optically thin (< 70 nm thick) but chemically thick, the absorption due to perpendicular modes of ultrathin films on dielectrics can be measured in the whole IR range with surface sensitivity above that in the transparent IRRAS but below that in metallic IRRAS. 

When both bordering media are transparent, one can apply transmission spectroscopy in polarized radiation (Section 2.1) or, when there is a difference in the refractive indices of these media, the ATR method and IRRAS. For each type of solid-solid interface, except for the metal-metal interface, one can study the layers in the contact zone by IRRAS or ATR in the transparent spectral range of one of the media in the system. To choose the technique with which to investigate dielectric (semiconductor)-liquid, dielectric (semiconductor)-semiconductor, and dielectric-dielectric interfaces, several factors must be considered, including the region of transparency of the media under study and the relationship between their refractive indices. If the medium with the largest refractive index is the most transparent, one should use the ATR method otherwise IRRAS is more appropriate. 

Thus, for both metallic and transparent snbstrates, p-polarization, angles of incidence above cpc, and water layers thinner than 1-2 p,m are preferable. Under these conditions (p > (pc and t/2 < dp, where dp is the penetration depth), the cell windows act as IREs. In other words, the in situ IRRAS geometry under the optimum conditions is in fact the ATR geometry in Otto s configuration (Fig. 236d). 

In Fig. 3.17, the experimental spectra of an amorphous SiN layer deposited on a silicon wafer, measured by Yamamoto and Ishida [7] at various angles of incidence, are shown as an example of IRRAS of a strongly absorbing layer on a transparent substrate. A silicon wafer with no film was used as a reference. These spectra demonstrate two principal differences between IRRAS of a film on a metal substrate and a film on a transparent substrate. First, the vro band of SiNx at 840 cm is present in the / -polarized spectra, along with the vlo band, unlike with a metal substrate. Second, the signs of the i>ro and vlo bands in the -polarized spectra are opposite and change at a certain angle of incidence. 

It is of interest to elucidate how the absorption of a substrate affects the band shape in p-polarized IRRAS of ultratbin films on transparent substrate. A representative example is the IRRAS spectra of a 1-nm hypothetical organic layer whose dielectric function is characterized by 5 = 0.001, y = 10 cm", Vo = 2800 cm", and eoo = 1-7 (Fig. 3.28). The simulations are for refractive index ( 3 < 2) and for substrates with a high refractive index ( 3 = 2.0-4.0), the absorption bands are oppositely directed while in the intermediate case the band is distorted assuming a derivative-like shape. 

Figure 3.28. Band shape in p-polarized IRRAS spectra of hypothetical organic layer 1 nm thick as function of refractive index ns of transparent substrate (indicated in figure). Dielectric function of film was specified by S = 0.001, > = 10 cm", vo = 2800 cm", and Eoo = ,

It is noteworthy here that some bands of anisotropic films on transparent substrates can vanish from the jp-polarized IRRAS [69,70] or polarization modulation (PM) IRRAS [71, 72] spectra. This phenomenon is a consequence of the SSR for dielectrics (see Section 3.11.4 for more detail). 

Thus, the difference between the band intensities in the experimental spectra measured by IRRAS and the calculated ones based on the sharp interface model can be connected with the existence of the optical property gradient in real optical systems, which is most clearly manifested in IRRAS of strong absorbers on the surface of transparent and weakly reflecting substrates. 

Optimum conditions in the case of Ag films on the Si or ZnSe IREs and the ATR geometry were found [403] to be the film thickness of 9 nm and the angle of incidence of 30°-40°. As in ordinary spectra obtained by transparent IRRAS (Section 2.3.1), the j-polarized IRRAS-SEIRA bands are negative, independent of the angle of incidence, while the / -polarized bands are negative at and positive at (pi > cpB, where (Pb is the Brewster angle of the substrate [384]. The maximum band intensity is observed at small and oblique angles of incidence in s- and p-polarization, respectively [384], However, the maximum absorption of 5-polarization is always smaller than that for p-polarization, independent of the metal film thickness and the optical properties of the substrate [359, 360, 384]. The enhancement factor increases as the refractive index of the substrate decreases [350, 384],... 

It follows that to distinguish the contribution of the surface to the reflectivity, the dielectric function of the bulk substrate should be known. However, in the case of a transparent substrate at energies less than Eg, 3 = 0 and the reflectivity becomes dAs". For transitions with energies hv > Eg, the second term in Eq. (3.41) cannot in principle be neglected. However, it was found that B 0 at energies lower than 3.2 eV for Si, 2 eV for Ge, 2.8 eV for GaAs, and 3.5 eV for GaP. When 0, the IRRAS spectrum is related to both and s" (see also Section 6.5). 

Since the substrate may influence the anisotropic optical properties of the overlying film [595], the method of Buffeteau et al. [247, 566-568, 593] is conceptually more reliable when the MO is studied on solid transparent substrates, whereas the initial anisotropic optical constants are extracted from normal- and oblique-incidence transmission or polarized reflection of the same film on the same substrate. In the case when different substrates participate into the measurements (e.g., when MO in monolayers at the AW interface is studied), the comparison of the simulated and experimental spectra can be used for distinguishing chemical effects generated by specific film-substrate interactions [568b]. In particular, the kmm values derived from spectra of monolayers at the AW interface obtained by IRRAS are usually larger than those obtained by eUipsometric measurements of thin films on solid supports [247]. This difference has been attributed to a gradient in the optical properties of the interfacial water [71]. 

Chernyshova and Rao [558] snggested to characterize the MO and MP from IRRAS of ultrathin films on transparent substrates by fitting the DR calculated... 

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... 

Adsorbed layers on flat, transparent substrates can be studied by transmission, IRRAS, and ATR methods. Historically the first measurements were by the... 

As pointed out in Section 3.11.4, the IRRAS method offers an intrinsic advantage in MO measurements on transparent substrates. Namely, the electric field component perpendicular to the surface (which is the most sensitive to the tilt angle) is comparable in magnitude to that parallel to the surface, unlike in ATR measurements, in which the angle of incidence exceeds the critical angle by more than 10°. This advantage has been exploited in a number of structural studies, including those of LB films [453,454], SAMs [455-457], adsorbed surfactants [325, 372, 451], and L monolayers at the AW interface [458-460]. 

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