Specular-reflection spectra measurements

The principle problem with diffuse reflectance is that the specular component of the reflected radiation, that which does not penetrate the sample, is measured along with the diffuse reflected light which penetrates the sample. Generally, the change in specular reflection with frequency is small except in regions of strong absorption bands where the anomalous dispersion leads to Reststrahlen bands in the specular reflection spectrum. When the Reststrahlen bands are observed, the absorption bands can appear inverted at their center. This effect makes quantitative measurements on samples with strong absorptivity very difficult. 

Figure 9 Specular reflection FT-IR and application of Kramers-Kronig algorithm (A) schematic showing external (front-surface, specular) mid-infrared reflection measurement from an optically thick sample (B) specular reflectance spectrum recorded from a 0.6-mm thick polymer molding (C and D) refractive index and absorption index spectra derived by applying Kramers-Kronig algorithm to the recorded specular reflection spectrum (B), respectively.

As described in detail in Chapter 8, calculations based on the Kramers-Kronig relations give the real and imaginary parts of a complex refractive index (n(v) and k(v) see Section 1.2.4) from a reflection spectrum measured by the method of specular reflection from a... 

As normal incidence is assumed for deriving Equation (8.3), the angle of incidence in actual measurements should ideally be <15. If both R and 8 can be obtained from the reflection spectrum measured under this condition, the k(y) spectrum corresponding to a absorbance spectrum would be calculated by using Equation (8.4b). Although <5(v) cannot be directly obtained in a specular-reflection measurement, it is possible to calculate it from R v) by using the Kramers-Kronig (KK) relations [4]. 

The above-mentioned bands are found at the same positions in the transmission spectrum of polyethylene, indicating that the recorded specular-reflection spectrum can indeed be converted, by using the KK relations, into a spectrum that is essentially the same as the absorbance spectrum derived from a transmission measurement. However, several bands observed in the region of 2000-1700 cm in the (v) spectmm of Figure 8.5 are not expected for polyethylene. A more careful examination is needed to clarify their origin. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Specular reflectance infrared involves a mirrorlike reflection producing reflection measurements of a reflective material or a reflection-absorption spectrum of a film on a reflective surface. This technique is used to look at thin (from nanometers to micrometers thick) films. 

Compared to the PAS spectra of cotton cloth, in the Specular Reflectance spectra, the signal-to-noiae is higher and any noise is low enough to show clearly the spectral features described here. Even though there are differences due to the change in sampling conditions, the spectra can be compared for a one-to-one comparison and identification of absorption bands. Mote however that in the PAS spectrum relative intensities are displayed and that the Specular Reflectance spectra show an absolute response and measurement which leads to a better possibility for quantification and a more rigorous treatment of the data. 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

One important practical problem in DR measurement is the need to cancel the specular reflectance from the front surface of the sample, which generates negative bands in the DRIFT spectrum, so causing apparent shifts in the true absorption bands. This is achieved by using appropriate cell designs in commercial... 

In the past, extensive investigations were made to obtain better insight into the limitations of the diffuse reflectance measurement technique. Studies demonstrated that sample properties such as particle size and packing affect, in addition to the optical constants, the diffuse reflectance spectrum. The characteristics of the diffuse and specular components were studied for different particle sizes and dilution within a non-absorbing inert matrix. It was found that specularly and diffusely reflected radiation coexist in the measured diffuse reflectance spectrum, even in KCl-diluted samples. In addition, the specular component, which is certainly sample-dependent, is not necessarily the same as from the front-surface reflection.To prove this, a top layer of pure KCl powder was... 

Different types of reflectance spectroscopy depend upon the reflecting behavior of the radiation on the solid. Fig. 8 illustrates various categories used to distinguish techniques for reflecting radiation off solids. Specular reflection spectroscopy is used to measure the reflectance spectrum of a smooth, glossy surface. In reflection-absorption spectroscopy, the radiation passes through a thin surface film on a reflective... 

Coupling in PDA Crystals. The polarized specular reflection spectra for the most highly reflective principal directions are shown in Figure 1. For the first three members of the series, the characteristic LT spectrum is observed. Backreflection was a severe problem because of the thinness of the crystals and, in the case of the less reflective principal direction, structure was altered sufficiently to render the spectra unuseable. Thus, direct measurement of exciton splitting, if present, was not possible. In the spectra reported, the backreflection in the intensely reflecting principal directions was corrected using a curve fitting procedure described elsewhere (21). 

Optical Characterization. The relative reflectivity measurements were taken at 300K by use of a diffuse reflectance attachment on a FT-IR instrument with the sample in the form of a sintered pellet. In this configuration, it was determined that the reflectance in the investigated energy range is dominated by the specular component of the sample s reflectivity. Well ground powder samples were also investigated to insure the specular nature of the reflectance spectrum observed with the pellet samples, and to help differentiate particle size effects from intrinsic absorption effects. No particle size artifacts were identified. 

In absorption-reflection measurement, one fraction of the radiation is reflected on the upper interface and contributes towards the spectrum via specular reflection. Another part of the radiation penetrates the surface film and is reflected by the reflective surface, thus, the light... 

LSM, SEKZ and LSCF powders were characterized by XRD using a Shimadzu XDR-7000 diffractometer and scanning electron microscopy (SEM-SSX 550, Shimadzu). Infrared spectra were also recorded with FTIR (IR Prestige-21, Shimadzu) in the 400 - 4600 cm"i spectral range. Specific surface area measurements were performed only for the LSM powders. An infrared reflectance spectrum of a LSM pellet prepared from a powder calcined at 900 °C was recorded with a Fourier-transform spectrometer (Bomem DA 8-02) equipped with a fixed-angle specular reflectance accessory (external incidence angle of 11.5°). 

Specular reflection spectroscopy enables us to study the redox behaviour of adsorbed species at a mirror electrode when either or both oxidized and reduced forms of the adsorbed species exhibit strong absorption bands in the UV-Vis region. In this case, the mode of measurement is, in fact, the absorption spectra of the adsorbed species at the electrode surface [55]. The potential modulated reflectance spectrum of the adsorbed molecules in the vicinity of its formal potential is given by the following equation ... 

Sloane and co-workers [129] described a specular reflectance system for the infrared analysis of micro-sized samples. They compare the advantages and limitations of this technique with other micro infrared techniques. Samples are mounted on small metal mirrors (Figure 4.20), which reflect the light beam back through the sample. A transmission spectrum is obtained but the effective path length is twice that of the actual sample thickness and a given absorption band consequently has twice the absorbance obtained by conventional transmission measurements. This system was applied successfully to gas chromatographic fractions, and is particularly useful for the examination of non-volatile liquids such as, for example, dioctyl phthalate. Crystalline solids are easily deposited and... 

A brief overview [4] is given next of one theory of DRS for particles that absorb IR radiation. Incident radiation from the spectrometer is focused onto the surface of the sample and reflected energy collected. The reflected energy can either be classified as specular or diffuse reflectance. Specular reflectance arises from energy that is reflected by the particles but is not absorbed. The energy that penetrates one or more particles and is collected is called diffuse reflectance. With these experimental parameters optimised, a high-quality spectrum is obtained. For some samples, the specular component can be large and difficult to avoid. This can lead to problems in the measurement of the... 

There are surface-oriented sampling techniques such as internal reflectance (ATR) and external (specular) reflectance, both providing data that are influenced in one way or another by the sample s refractive index (at the measurement wavelength), the refractive index of the sampling medium (in particular, ATR), and the polarization of the infrared beam (in particular, external reflectance). For ideal cases, the mathematical relationship between the recorded spectrum and an idealized absorption (or transmission) spectrum is understood. In these situations, a correction algorithm can be... 

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