Reststrahlen bands

Double and buried layers are readily detected by infrared ellipsometry if the film material is characterized by reststrahlen bands. The response is observed at the position... 

As with transmission analysis, DRIFTS test samples are generally ground and mixed with an IR transparent salt, such as potassium bromide (KBr), prior to sampling. However, to ensure that there is no catalytic effect, most systems use CaF2 or powdered crystalline carbon (diamond). One drawback is the presence of reststrahlen bands whose contrast can be reduced by... 

An interesting and useful relation can be derived between the frequencies and the real parts of the dielectric function on either side of the Reststrahlen band. The frequency oa, at which c vanishes is, from (9.21), given approximately by... 

Notice the second, weaker oscillator at 640 cm-1 from a quantum viewpoint, this oscillator is interpreted as the excitation of two phonons by a photon. In order for momentum to be conserved, two phonons with almost equal and opposite momenta must be created a photon has negligible momentum compared with phonon momenta. Such two-phonon transitions usually occur to a noticeable extent in ionic crystals, which necessitates a multiple-oscillator correction to the main one-oscillator Reststrahlen band. 

The principle problem with diffuse reflectance is that the specular component of the reflected radiation, that which does not penetrate the sample, is measured along with the diffuse reflected light which penetrates the sample. Generally, the change in specular reflection with frequency is small except in regions of strong absorption bands where the anomalous dispersion leads to Reststrahlen bands in the specular reflection spectrum. When the Reststrahlen bands are observed, the absorption bands can appear inverted at their center. This effect makes quantitative measurements on samples with strong absorptivity very difficult. 

In Fig. 3.8, typical IRSE spectra of a ZnO bulk sample and a ZnO film on sapphire are plotted. In the /- -spectrum of the ZnO bulk sample a plateau with S 45° can be seen, which corresponds to the bands of total reflection (reststrahlen bands), which occurs between the Ei(TO)- and E (LO)-mode frequencies [123]. The small dip within the plateau is caused by the loss in p-reflectivity, and localizes the Ti(LO)- and E (LO)-mode frequencies. The derivative-like structure in the / -spectrum of the bulk ZnO sample at u> 650cm-1 is caused by the anisotropy Re > Re j (Sect. 3.3) [38]. 

The first investigation of the phonon modes in binary InN was an extrapolation of the Gai-xInxN (0 < x < 1) alloy modes in reflection towards the binary compound [1], A typically high free carrier concentration in the mid 1020 cm 3 range controls the absorption (Drude absorption) in the infrared and must also account for the broadened Reststrahlen band in pure InN films (e.g. in [1]). In this case infrared active phonons couple to the plasma of the free electrons forming phonon-plasmon coupled modes [10,11], However, layers of low carrier concentration have been achieved and pure LO phonon energies have been derived in Raman spectroscopy. Resonant Raman spectroscopy at 514 nm has been performed, assigning five of the six Raman allowed zone centre phonon modes [8,9] (TABLE 1). 

Quite early in the history of infrared spectroscopy, reflections in form of reststrahlen bands were observed indicating strong absorption. Nowadays infrared spectroscopy is widely applied to organic compounds which are comparably weak absorbers. For their studies such reflection methods are favoured, whose results are interpretable in almost the same way as transmittance spectra are. However, there is an increasing interest in applying other reflection techniques also to such samples. Advantageous aspects are ... 

One example of a reflectivity measurement is shown in Fig. 26. The sample is CdCraSe4 and the reflection spectra exhibit the typical reststrahlen bands caused by the lattice vibrations. Another example (Fig. 27) is a transmission measurement of an absorption line due to antiferromagnetic resonance. This also shows how, with spectroscopy in the extreme far-infrared, the signal-to-noise ratio is rather... 

Table 7.23. Measured energies (meV) of different acceptor transitions from the IS3/2 (l s ) ground state in GaP, where the TO-LO one-phonon region (reststrahlen band) is between 45 and 50 meV...

The IRRAS spectra of ultrathin films can also be measured in the region of phonon absorption of the substrate. Figure 2.13 shows results of / -polarized ex situ IRRAS of calcite (CaCOs) after adsorption of oleate for 5 min in a 3.3 X 10 M oleate solution at pH 10, measured by Mielczarski and Miel-czarski [41] at different angles of incidence. It is important that although the absorption bands of adsorbed oleate at 1575, 1538, and 1472 cm lie within the region of the reststrahlen band of calcite (1600-1400 cm ), they have sufficient... 

Fig. 4.4. The reststrahlen bands of the quasi-binary amorphous system (ASjSjlx (ASjSej) i x(after Felty etal (1967)).

The reflection spectrum measured at room temperature from 90 to 600 cm" has a maximum near 130 cm (R 85%) and a minimum near 185 cm (R 1%). These two extrema were attributed to the lattice vibrations cojo and (Olq, respectively. Axe [1], see p. 187. Absorption measurements on an EuSe film on LiF at 2 K gave o)to = 134.0 cm" Ikezawa, Suzuki [2]. Bulk samples have their characteristic reststrahlen band between (Ojo and colo, see Fig. 116 which shows reflection and transmission spectra of stoichiometric EuSe obtained by sublimation for the paramagnetic state at 300 K and for the antiferromagnetic NNSS state at 3 K. The numerous intrinsic structures in T(v) (and weaker in R(v)) observed beside the reststrahlen band were explained by one-, two-, and three-phonon processes consistent with measurements of second-order Raman scattering (cf. p. 248). Absorption spectra calculated from R and T at 300, 80, and 3 K (K ax S x10" cm" at v 130 cm ) are given in the paper, Mutzenich etal. [3]. The reststrahlen wavelength is calculated by — 76 im... 

Besides the strong phonon-reststrahlen band in some nonannealed samples a free carrier contribution is observed at 300 K. This contribution disappears at low temperatures, but appears again in an external magnetic field. Fig. 117, p. 252, displays this behavior in the reflection spectra of as-grown Eu-rich samples at 4.2 K, in the NNSS state at H = 0 kOe, and by... 

Potter, A. E. Morgan, T. H. (1981). Observations of silicate reststrahlen bands in lunar infrared spectra. Lunar and Planetary Science Conference, 12B, 703-13. 

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