Internal-reflection element refractive index

The real utility of d comes in the analysis of thin films. Consider a substrate of refractive index supporting a thin film of thickness d and refractive index in contact with an internal reflection element (the prism) of refractive index as shown in Figure 24. In this case, d depends on the polarization of the incident light beam and is given by... 

In this method, the sample (such as a polymer film) is pressed against a transparent material having a high refractive index (called the internal reflection element (IRE)). The IR light beam passes through this material, rather than air, before and after reflecting from the sample, hence the reason for describing... 

Fig. 24. Attenuated total reflectance of thin film of thickness d and refractive index n2 on a substrate of refractive index 3 at the surface of internal reflection element (IRE) of refractive index nv Decay of evanescent field beyond thickness of thin film indicated.

Attenuated total reflection FTIR is a well-established technique for obtaining absorbance spectra of opaque samples. The mode of interaction is unique because the probing radiation is propagated in a high index-of-refraction internal reflection element (IRE). The radiation interacts with the material of interest, which is in close contact with the IRE, forming an interface across which a nonpropagating evanescent field penetrates the surface of the material of interest to a depth in the order of one wavelength of the radiation. The electric field at the interface penetrates the rarer medium in the form of an evanescent field whose amplitude decays exponentially with distance into the rarer medium. 

Attenuated total-reflectance (ATR) spectroscopy is a widely used sampling technique, in which a sample is placed in contact with a reflecting medium (a plate or prism shaped material called an internal reflectance element). A beam of radiation entering the prism is reflected internally if the angle of incidence at the interface between sample and prism is greater than the critical angle (a function of the refractive index of the sample and the prism). 

The sample is placed in contact with the internal reflection element, the light is totally reflected, and the sample interacts with the evanescent wave resulting in the absorption of radiation by the sample. The internal reflection element is made from a material with a high refractive index, e.g., zinc selenide (ZnSe) or silicon (Si). 

It can be seen from Equations 1.2 and 1.3 that the spatial resolution of infrared microspectroscopy can be improved by immersing the sample in a medium of high refractive index. This exactly what is done in attenuated total reflection (ATR) spectroscopy using a single-reflection hemispherical internal reflection element (IRE). Eor example, if a germanium n = 4.0) hemispherical IRE is used, not only... 

Sapphire was selected as an internal reflection element with a high value of refractive index (n. = 1.81 at 313 nm). It is transparent down to 200 nm, mechanically hard, chemically stable, and has a small birefringence. The dimension of the used plate... 

The polymer properties in thin films are then compared with the bulk as measured by FTIR attenuated total reflection spectroscopy (ATR). A modest refractive index of the internal reflection element (IRE made of ZnSe, n=2.43 at 2000 cm ) and an incident angle of 65° - still low but well above the critical angle of total reflection - are chosen for the p-polarized light in order to obtain an information depth of several microns. Hence, the ATR spectra provide the bulk properties of the polymer sample. 

Where A is the wavelength of the radiation, ni is the refractive index of the IRE (ATR crystal), and n2 refractive index of the sample, and 0 is the angle of incidence. Figiue 3 illustrates the evanescent wave formed at the internal reflection element- sample interface. 

The MSEF formalism was used for deriving the formulas for estimating the adsorption density at the internal reflection element (IRE)-solution interface [153, 154, 166-174], Assuming that (1) species at the solid-solution interface are steplike distributed (with the maximum at the surface), (2) the refractive index of the adsorbed species is close to that of the solution, and (3) the absorption indices of the adsorbed and solvated species are close to each other, Tompkins [166] derived heuristically the following equation that describes the absorbance per reflection in the multiple internal reflection (MIR) spectrum of the species adsorbed at the IRE ... 

Representative results are shown in Figure 12.3, with ATR spectra of the same polymers being shown for comparison [8], although it should be noted that the ATR spectra were measured at higher resolution than the photothermal spectra. The distortion of the stronger bands in the ATR spectra of the more polar polymers is caused by the effect of anomalous dispersion when an internal reflection element (IRE) with a relatively low refractive index, presumably ZnSe, was used. Remarkably the highest quality of photothermal spectrum was measured in the case of polypropylene, which has a relatively weak spectrum, and the lowest quality spectrum was measured in the case of Nylon 6, where the effect of photoacoustic saturation [10] is clearly evident. It is interesting to speculate on whether this spectrum and that of polycarbonate would have been improved had the velocity of the interferometer mirror been increased. Spikes in some of the spectra at 1082 and 1804 cm were attributed to supply frequency harmonics. 

For any material, n( is determined by Snell s law. A few materials have no significant absorption in the mid- and near infrared. Those materials with low refractive index (1.45 high refractive index (2.4 internal reflection elements (see Chapter 15). For organic and inorganic molecules whose spectra exhibit typical absorption bands, the refractive index changes across the absorption band. A typical refractive index spectrum has the appearance shown in Figure. Aa. This... 

The chalcogenides are all insoluble in water and other common solvents. ZnSe and CdTe have excellent transmission characteristics. The only problem with these materials is their high refractive index, which leads to high front-surface reflectance (see Section 13.2.2), so that transmission spectra of liquids held in cells fabricated from these materials often give rise to interference fringes (see Section 11.1.3). These materials aU make excellent internal reflection elements. AMTIR (amorphous material that transmits infrared radiation) is a mixture of several chalcogenides. Many optical fibers used for mid-infrared spectrometry are made from this material (see Section 15.4). 

As implied by Eq. 15.2, all the materials in Table 15.1 must have higher refractive indices than the material with which they are in contact. Materials that are commonly used as windows in the mid-infrared [e.g., KBr n = 1.53) and KCl ( i = 1.45)] are not included in this list, as their refractive indices are too low for use as IREs. Because the refractive indices of KBr and KCl are roughly equal to the refractive index of organic compounds, total internal reflection will not be observed. In this case, radiation passes directly through the IRE and sample without regard to the angle of incidence. In other words, these materials are better suited for use as infrared-transmitting windows than as internal reflection elements. 

Attenuated Total Reflection Measurements 183 Table 13.1 High refractive-index materials used for the internal reflection element (IRE)... 

In order to enhance spatial resolution, it is necessary to make the NA of the objective larger, as is clear from Equation (17.3) that is, either n or 6, or both of them should be increased. Due to the optical geometry of a microscope, there is an upper limit for 0. On the other hand, it is possible practically to increase n by introducing an attenuated total reflection (ATR) accessory into a microscope (see Chapter 13 for the ATR method this is a frequently used accessory for many recent infrared microspectroscopy absorption measurements). The refractive index n of Ge, which is a commonly used material for an internal reflection element (IRE) in the ATR method, is about 4, and the NA when using a Ge IRE exceeds 2. This means that, if an ATR accessory with a Ge IRE is combined with a microscope, the theoretical spatial resolution is enhanced about four times that of the conventional reflection measurement. In fact, in the FT-IR microspectroscopic imaging measurement with a Ge ATR accessory, it has been confirmed that a spatial resolution comparable to the infrared wavelength used for the measurement is realized, and thus a higher spatial resolution may be attainable. 

There are many applications for internal reflectance spectroscopy, and only a few will be mentioned here. Internal reflectance spectroscopy can be used to obtain the spectra of rubbery materials that are hard to grind. The rubbery material is simply pressed against the internal reflectance plate, and it is ready to run. Carbon-fllled rubber or other polymers may be run using a high index of refraction germanium as the internal reflectance element. Internal reflectance is used to obtain selectively the top few micrometers of a sample surface where the composition may be different than that further down. It is also good for water solutions because the controlled penetration keeps the effective sample thickness small. 

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