Multiple-reflection elements

This situation is obviously useless for spectroscopy. Hence, valuable films are very thin, on the order of few nanometers, depending on the material. Figure 4 shows that even for a thin metal film the overall reflectivity of the system can be severely reduced. This characteristic is even more important for multiple reflection elements, for which the total reflectivity Rtot is the reflectivity for a single reflection R to the power of the number of reflections N. Rtot = R - For a reflectivity R of 90% (90% of the incident intensity is reflected), only 35% of the incident intensity will reach the detector after 10 reflections (N — 10). The smaller the reflectivity Rtot> the smaller is the signal at the detector and the larger the noise. On the other hand, the intensity of an absorption... 

Depending on the absorptivity of the sample, multiple reflection elements (Figure 7) may be necessary, rather them the single reflection as described here. 

Figure 6.4-3 Attenuated total reflection (ATR) multiple reflections element.

Figure 7. Reflection elements for single (upper part) or multiple (lower part) internal reflection. (A), (D) fixed angle, (B), (E) variable angle, (C) achromatic prism (19), (F) liquid prism for improved optical contact.

A ideal Knudsen cell holds the source in an isothermal box with a relatively small exit orfice. This can be implemented by placing the crucible and filament inside a radiation shield without making contact between the three elements. The filament heats the crucible radiatively (there is no gas or direct mechanical connection to conduct heat), and this multiple reflection and conduction inside the crucible homogenizes the temperature along the crucible length. This configuration also increases the temperature that can be achieved for a given power input, since less heat is lost to the environment via radiation. The maximum temperature which can be achieved by a K-cell is limited by the metal elements used to fabricate the cell and the thermocouples, and is typically 1200-1400°C. 

The problem of reflections is most critical if highly reflective surfaces are involved. Optical elements to be considered are interference filters, reflective ND filters, and photomultiplier cathodes. Problems can occur by backreflection of excitation light into the sample, and by multiple reflection in the detection path. 

Internal reflection approaches rely upon the total reflection of the infrared beam one or more times, (multiple internal reflection, MIR), at the internal surface of an infrared transparent crystal or internal reflection element (IRE) such as Ge, Si, GaAs, or ZnSe, Medium 1 in Fig. l(a)(i and ii). If the IRE is a semiconductor such as Ge, Si, or GaAs, it can serve as the working electrode, which vras the case in the work of Mark and Pons, who employed a Ge IRE. More recently, Chazalviel and coworkers have pubhshed extensive and detailed studies on the GaAs, Ge, and Si electrolyte interfaces,... 

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 ... 

The concept of ATR at the interface of two media is described in 1.4.10° and Section 1.8.3. In situ ATR measurements of ultrathin films started in the mid-1960s with studies of the adsorption of a stearic acid monolayer from D2O onto Ge [448], and chemical [449] and electrochemical [450] oxidation of Ge, where a Ge multiple internal reflection element (MIRE) acts as both the substrate and the electrode. Later, coated ATR [60, 451-454] and MO ATR with the SEIRA effect [455] were introduced in in situ experiments. The principal advantage of the ATR geometry is that the corresponding in situ cells are free from diffusion effects (the volume of solution phase in contact with the IRE is arbitrary), which is useful when studying time-dependent phenomena (Section 4.9.1). 

More recent multiple internal reflection accessories, such as the FI-ATR (see the section Liquids ), have enabled mid-infrared measurements to be made routinely and practically on a wide range of pastelike and latex-like samples and soft waxes. With dispersions, though, there may be a tendency for the solid phase to migrate preferentially to the surface of the internal reflection element. In some circumstances a capillary layer of the material between a pair of transparent infrared windows may suffice. The solute phase of a latex sample is most frequently presented for study as a thin film deposit on a transparent infrared support after elimination (evaporation) of the solvent (see the section Films cast from solution ). 

Figure 51. Schematic representation of total internal reflection with A) Single reflection B) Multiple reflection IRE (internal reflection element)...

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