Attenuated total internal reflection

The first HWGs confined light to their core either by total internal reflection (attenuated total reflection guides) or by reflection off a metallic surface (leaky guides) as discussed in Sect. 2.3.1. The latest generation of HWGs is based on PBG materials as discussed in Sect. 2.3.2. 

Total Internal Reflection (Attenuated Total Reflectance)... 

Attenuated total reflection (ATR), also called internal reflection, is based on the phenomenon of total internal reflection. In ATR the infrared beam is directed into an infrared-transmitting crystal so that it strikes the crystal surface at less than the critical angle and undergoes total internal reflection. 

Examples of nir analysis are polymer identification (126,127), pharmaceutical manufacturing (128), gasoline analysis (129,130), and on-line refinery process chemistry (131). Nir fiber optics have been used as immersion probes for monitoring pollutants in drainage waters by attenuated total internal reflectance (132). The usefulness of nir for aqueous systems has led to important biological and medical appHcations (133). 

Surface hydrogen is conveniently detected by MIR at attenuated-total-internal-reflection prisms of GaAs electrodes. Si-doped (100)-oriented n-GaAs single crystals were employed in the electrochemical cell illustrated schematically in Figure 3.3. 

Internal reflectance (attenuated total reflectance ATR ). The internal reflectance or, more usually, attenuated total reflectance (ATR), technique depends on the total reflectance of an IR beam at the internal face of an IR-transparent crystal of high IR refractive index, as shown in Figure 2.38. Medium 1 is a prism of such a material (for example, Si, Ge or KRS-5 [thallous bromide- iodide]), medium 2 is a thin coating of a metal (Au, Pt, Fe) which forms the working electrode and medium 3 is the electrolyte. The... 

Recently, polyethylene and Teflon mesh sample holders have been used. A drop of sample is placed on the mesh and spread to a relatively uniform thickness for analysis. These holders can often be rinsed and reused. A very convenient alternative to liquid sample holders is the technique called attenuated total reflection or ATR. The ATR cell is a crystal of gallium arsenide, GaAs and the infrared radiation enters one end of the trapezoidal crystal. With the angles adjusted to obtain total internal reflection, all of the IR radiation passes through the crystal and exits the other end as shown in Fig. 5.14. 

Infrared spectra of zeolitic samples can be measured in several different modes. These include transmission, diffuse reflectance, attenuated total internal reflection (ATR) and emission. Transmission and diffuse reflectance are by far the most widely used of these techniques. In the transmission mode, the sample is placed directly in the infrared beam of the instrument and the light passing through or transmitted is measured by the detector. This transmitted signal (T) is ratioed to the open beam (no sample) signal (To) to get the transmission spectrum of the sample. The transmission spectrum is converted to an absorbance spectrum ... 

Short path length flow cells may be used in lower volume, or lower flow situations. The extremely small gap between the optics of fibers limits them to these types of applications. It is possible to make flow cells with sample gaps as small as 25 um, so that they may be used with highly absorbing species. With even higher absorbing species, attenuated total internal reflection (ATR) probes may be appropriate (see below). 

Figure 4.2 Schematic of basic probe designs transmission cell, immersion probe (retroreflecting and transmission), attenuated total internal reflection.

As a result, the depth of penetration, or effective pathlength, will be higher the greater X or the smaller the frequency. Therefore, an interferogram (raw infrared spectrum) is a measure of the attenuation of a trans fat test sample of the totally internally reflected infrared light. The interferogram of a reference background material (trans-free fat) is similarly measured. These are subsequently used to obtain an absorption spectrum. 

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Attenuated total reflectance (atr) ftir spectroscopy is based on the principle of total internal reflection (40). Methods based on internal reflection in the uv and visible regions of the spectrum are also common in addition to those in the ir region. The implementation of internal reflection in the ir region of the spectrum provides a means of obtaining ir spectra of surfaces or interfaces, thus providing molecularly-specific vibrational information. 

Attenuated total internal reflection (ATR) probes offer several advantages over other probe types. ATR is a phenomenon that relies on a difference in the index of refraction of a crystal and that of the solution with which it is in contact to prevent light from escaping the crystal. Only the evanescent wave of the light interacts with the solution layer at the crystal face. The result is an optical pathlength of only a few microns. Typical designs make use of faceted crystals or hemispheres (see Figure 6.1). The most common ATR material in the UV-vis is sapphire. In rare cases, fused silica may be used. ATR allows spectra to be taken of neat samples with optical density (OD) of 500-1000... 

Nucleic acid biosensors based on optical modes of detection represent another common approach for generating analytical signals based on nucleic acid hybridization. The methods discussed herein are based on methods that are suitable for the study of materials on surfaces. There are a number of different optical methods that have been described, with the most common being attenuated total reflectance (ATR), total internal reflection fluorescence (TIRF) and surface plasmon resonance (SPR) [15]. All of these methods work... 

Sommer, A. J., Tsinger, L. G., Marcott, C. and Story, G. M. (2001) Attenuated total internal reflection infrared mapping microspectroscopy using an imaging microscope. Appl. 

The use of evanescent waves is very valuable to the study of interfacial properties. Techniques such as total internal reflection fluorescence (TIRF) and attenuated transmitted reflectance (ATR) use the energy of evanescent waves to probe thin regions in the vicinity of an interface to determine surface concentrations of interfacial species. 

Fig. 4. Schematic representation of surface diffraction from dielectric pattern, based on total internal reflection (TIR) and attenuated total reflection (ATR) coupling geometries, respectively.

The absorption is very intense, so direct measurement of absorption spectra requires very thin films. When the sample surface is smooth enough, the absorption can be calculated by Kramers-Kronig inversion of the reflectance, which can be done rather accurately for a well-isolated transition [113]. A strongly scattering sample must be studied by attenuated total internal reflectance. 

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