Infrared refractive indices

Little is known about infrared refractive indices of organic compounds, and only very few such studies related to liquid crystals are reported. To some extend this is due to the fact that special techniques and even dedicated equipment are required. On the other hand birefringence can be derived from the polarization pattern produced by the phase difference between the ordinary and the extraordinary beam. This experiment had been outlined by Born and Wolf (1980) and was applied to liquid crystals by Wu et al. (1984). The procedure is primarily suitable in transparent regions, for a more comprehensive optical characterization it should be extended to complete ellipsometry (Reins et al., 1993). Results obtained by infrared-spectroscopic ellipsometry are shown in Figs. 4.6-5 and 4.6-6. 

Although UV detection is most commonly used in the quahty control of drug substances, other detectors such as fluorescence, electrochemical, near infrared, refractive index, evaporative light scattering, or mass spectrometry may be used as appropriate. 

Volz, F. E., 1972. Infrared refractive index of atmospheric aerosol substance, Appl. Opt., 11, 755-759. 

The electron effective mass in GaN is now reasonably well established by cyclotron resonance measurements [14-16] asm, = (0.22 0.0 l)m, and the low frequency dielectric constant (appropriately averaged spatially) e(0) = 9.5 0.2, from infrared refractive index and optic phonon energy measurements [17]. We can therefore derive a reliable value for the hydrogenic donor ionisation energy of EDH = (33.0 1.5) meV which compares well with IR absorption measurements, giving Ed = (35 1) meV [18] (see below). The discrepancy is readily explained in terms of a small chemical shift. 

The attenuated total reflectance (ATR) technique is used commonly in the near-infrared for obtaining absorption spectra of thin Aims and opaque materials. The sample, of refractive index i, is placed in direct contact with a material which is transparent in the region of interest, such as thallium bromide/thallium iodide (known as KRS-5), silver chloride or germanium, of relatively high refractive index so that Then, as Figure 3.f8... 

Optical Properties. Teflon FEP fluorocarbon film transmits more ultraviolet, visible light, and infrared radiation than ordinary window glass. The refractive index of FEP film is 1.341—1.347 (74). 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Table 4 lists the specifications set by Du Pont, the largest U.S. producer of DMF (4). Water in DMF is deterrnined either by Kad Fischer titration or by gas chromatography. The chromatographic method is more rehable at lower levels of water (<500 ppm) (4). DMF purity is deterrnined by gc. For specialized laboratory appHcations, conductivity measurements have been used as an indication of purity (27). DMF in water can be measured by refractive index, hydrolysis to DMA followed by titration of the Hberated amine, or, most conveniendy, by infrared analysis. A band at 1087 cm is used for the ir analysis. 

The PLM can be used in a reflection or a transmission mode. With either mode, light of various wavelengths from ultraviolet to infrared, polarized or unpolarized, is used to yield a wide variety of physical measurements. With just ordinary white light, a particle or any object detail down to about 0.5 p.m (500 nm) in diameter can be observed to detect shape, size, color, refractive index, melting point, and solubiUty in a group of solvents, all nondestmetively. Somewhat larger particles yield UV, visible, or IR absorption spectra. 

Additions to the PLM include monochromatic filters or a monochromator to obtain dispersion data (eg, the variation in refractive index with wavelength). By the middle of the twentieth century, ultraviolet and infrared radiation were used to increase the identification parameters. In 1995 the FTIR microscope gives a view of the sample and an infrared absorption pattern on selected 100-p.m areas (about 2—5-ng samples) (37). 

The dielectric constant is a measure of the ease with which charged species in a material can be displaced to form dipoles. There are four primary mechanisms of polarization in glasses (13) electronic, atomic, orientational, and interfacial polarization. Electronic polarization arises from the displacement of electron clouds and is important at optical (ultraviolet) frequencies. At optical frequencies, the dielectric constant of a glass is related to the refractive index k =. Atomic polarization occurs at infrared frequencies and involves the displacement of positive and negative ions. 

Infrared ellipsometry is typically performed in the mid-infrared range of 400 to 5000 cm , but also in the near- and far-infrared. The resonances of molecular vibrations or phonons in the solid state generate typical features in the tanT and A spectra in the form of relative minima or maxima and dispersion-like structures. For the isotropic bulk calculation of optical constants - refractive index n and extinction coefficient k - is straightforward. For all other applications (thin films and anisotropic materials) iteration procedures are used. In ellipsometry only angles are measured. The results are also absolute values, obtained without the use of a standard. 

Determination of the optical constants and the thickness is affected by the problem of calculating three results from two ellipsometric values. This problem can be solved by use of the oscillator fit in a suitable wavenumber range or by using the fact that ranges free from absorption always occur in the infrared. In these circumstances the thickness and the refractive index outside the resonances can be determined - by the algorithm of Reinberg [4.317], for example. With this result only two data have to be calculated. 

In order to understand RAIR spectroscopy, it is convenient to model the experiment (see Fig. 4). Consider a thin film with refractive index n =n ik and thickness d supported by a reflecting substrate with refractive index ni = ri2 — iki- The refractive index of the ambient atmosphere is o- Infrared radiation impinges on the film at an angle of incidence of 6 . The incident radiation can be polarized parallel to or perpendicular to the plane of incidence. 

Select the detector. To acquire molecular weight distribution data, use a general detector such as a refractive index detector. To acquire structural or compositional information, employ a more selective detector such as an ultraviolet (UV) or infrared (IR) detector. Viscometric and light-scattering detectors facilitate more accurate molecular weight measurement when appropriate standards are not available. 

An infrared beam is directed through a crystal of refractive index (ni) onto a sample of smaller refractive index (n2). The intensity of the reflected beam is monitored as a function of the wavelength of the incident beam. These absorptions are used to identify the chemical structure. ATR has a sampling depth of about 0.3-3.0 microns. 

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