Attenuated total penetration depth

A special O-ring cell design is needed for in situ infrared (IR) vibrational characterization of an electrochemical interface. The absorption of one monolayer (i.e. <1015 cm 2 vibrators) can be measured if the silicon electrode is shaped as an attenuated total reflection (ATR) prism, which allows for working in a multiple-in-ternal-reflection geometry. A set-up as shown in Fig. 1.9 enhances the vibrational signal proportional to the number of reflections and restricts the equivalent path in the electrolyte to a value close to the product of the number of reflections by the penetration depth of the IR radiation in the electrolyte, which is typically a tenth of the wavelength. The best compromise in terms of sensitivity often leads to about ten reflections [Oz2]. 

Attenuated total reflectance infrared (ATR-IR) is used to study films, coatings, threads, powders, interfaces, and solutions. (It also serves as the basis for much of the communication systems based on fiber optics.) ATR occurs when radiation enters from a more-dense material (i.e., a material with a higher refractive index) into a material that is less dense (i.e., with a lower refractive index). The fraction of the incident radiation reflected increases when the angle of incidence increases. The incident radiation is reflected at the interface when the angle of incidence is greater than the critical angle. The radiation penetrates a short depth into the interface before complete reflection occurs. This penetration is called the evanescent wave. Its intensity is reduced by the sample which absorbs. 

Figure 10.20—Devices allowing the study of samples by reflection, a) Diffuse reflection device b) attenuated total reflection (ATR) device c) comparison of the spectra of benzoic acid obtained by transmission (KBr disc) and by diffuse reflection using the Kubelka Munk correction. The depth of penetration of the IR beam depends on the wavelength. The absorbance for longer wavelengths would be overestimated if no correction was applied.

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. 

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

Another approach, internal reflection spectroscopy (IRS) or attenuated total reflectance (ATR) spectroscopy, takes advantage of the total reflection observed when a light beam is directed to the back of the OTE at an angle greater than the critical angle. Since the reflection takes place at the electrode-solution interface, the light beam is attenuated only by molecules close to the electrode surface. The penetration depth is about 1000 A in a typical experiment. Because of this very small effective cell length, the electrode is most frequently used in a multiple reflection mode [Fig. 44(b)]. 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

The internal reflectance technique is usually called attenuated total reflection (ATR) spectroscopy. It is especially useful for studying strongly absorbing media, for example, aqueous solutions. When the infrared radiation is absorbed in the test medium, one obtains a spectrum similar to that from a transmission experiment. However, there are distortions in the ATR spectrum, especially in the region of intense bands. One reason for distortion is the fact that the depth of penetration varies with wavelength. The other effect is due to the change of the refractive index of the solution in the region of the intense band. ATR spectra should be corrected for these effects so that they may be compared to normal transmission spectra. 

Upon the Ag and In deposition the totally symmetric modes are initially strongly enhanced. Subsequently the signal is attenuated exponentially with an exponent that is much smaller than the penetration depth of the incident radiation in a smooth closed metal film, reflecting a high level of roughness of the metal overlayer. On the other hand, the intensity of the normally infrared active modes relative to the Raman modes provides information on the metal diffusion depth in the organic films. 

The complexity of the FT-IR spectra collected in the Attenuated Total Reflectance (ATR) mode which can include contributions from (a) the adsorbed protein layer or (b) the soluble proteins in the liquid layer adjacent to the ATR surface but within the "depth of penetration" of the evanescent field of the IR beam (this so-called "bulk" effect is especially significant when near-physiological protein concentrations are studied) ... 

Next to the critical penetration depth it is also useful to define a critical thickness below which absorption and enhancement effects can be neglected. For analysis of such thin-fihn samples, the calibration relations of Eq. (8) and (9) are valid and matrix effect corrections need not be applied. By convention, dthi corresponds to the situation where the total attenuation in the sample is equal to 1 %. Table 11.7 lists typical dthm values for various fluorescent line energies in two matrices. 

In attenuated total reflection (ATR) measurements, the depth of penetration, dp, of the IR beam for a non-absorbing medium is a function of the wavelength A ... 

The combination of infrared spectroscopy with the theories of reflection has made advances in surface analysis possible. Attenuated Total Reflectance (ATR) spectroscopy is an innovative technique for proving chemical information of a sample surface and the ability to quantify newly formed species, based upon Pick s second law. The fundamentals of attenuated total reflection (ATR) spectroscopy date back to the initial work of Jacques Fahrenfort and N.J. Harrick, both of whom independently devised the theories of ATR spectroscopy and suggested a wide range of applications. The schematic showing ATR-FTIR configuration is illustrated in Fig. 1 (KS. Kwan, 1998). The penetration depth, d, can be estimated as ... 

The majority of reported mid-IR fibre probes rely on the well-established attenuated total reflection (ATR) technique, revealing many advantages in the general applicability over e.g. absorption measurements in short pathway flow cells. In the ATR method only a thin film (a few micrometers) at the proximity of the ATR element is subject to the measurement The thickness of the analyzed film is defined by the penetration depth of the evanescent field... 

An alternative reflection setup makes use of single (or multiple) internal reflection within an OTE (Figure 4). However, at each internal reflection, a small portion of the intensity leaks out (it is correctly known as an evanescent wave) into the thin film electrode layer and beyond into the solution and can be used to detect any absorbing species. The method is also known as attenuated total reflection (ATR). The evanescent wave intensity decays exponentially as exp( — bix) with distance x from the interface. The penetration depth, 5, depends on the wavelength and the optical properties of the substrate, electrode film, and solution d = /l/(4/i Im... 

For opaque samples, such as fibers or paint chips, attenuated total reflectance-infrared (ATR-IR) spectroscopy is more commonly used. The sample is positioned over a crystal, and pressure is applied to ensure good contact between the sample and crystal. Infiared radiation is passed through the crystal, and because of the close contact, the radiation penetrates a small depth into the sample. Certain energies are absorbed depending on the chemical bonds within the sample, resulting in the characteristic spectrum of the sample. 

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