Specular reflectance spectra

Specular reflectance spectra Specular reflectance accessory (SRA) ... 

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. 

Specular reflectance is the reflectance spectrum obtained from a flat, clean surface (e.g., a mirror). 

Similar cases with high absorptivities can be found in the UV/VIS region, in which fundamental electronic absorptions exist. When particle size increases, the proportion caused by specular reflectance may become stronger and readily evident in the diffuse reflectance spectrum. A discussion of previous UV/VIS work is given by Wendlandt and Hecht. ... 

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... 

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. 

The reflectance spectra differ from those recorded in transmission, they appear as "derivative-like" bands. These spectra can be converted into absorption one by using of Kramers-Kronig transformation (K-K transformation) that is available in most spectrometer software package. Figure 10 depicts specular reflectance spectrum of oil on surface of machined steel cylinder. 

Fig. 10. Specular reflectance spectrum of oil on surface of machined sfeel cylinder.

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°). 

The specular reflectance spectra of the surfaces of the commercial film 2 are seen in Fig. 1.8 curve 1 show the specular reflectance spectrum of the polypropylene side of the film and curve 2 show the specular reflection spectrum of the A1 side of the film. While the polypropylene surface has the spectrum of polypropylene, the A1 coated surface reflected the infiared rays. The absorWce values close to zero indicated that the light was not absorbed but reflected by the A1 surface. 

There were six refiaction fringes observed in specular reflection spectrum of commercial Film 2 in Fig. 1.5. Using Equation 1 and inserting values of six fringes between 1600 cm to 2650 cm and the film thickness was calculated as 19 pm. 

Specular reflectance Pure specular reflection spectra may be recorded directly from the surfaces of flat, nonscattering, optically thick (opaque) samples from which the absorption index spectrum may be extracted by application of the Kramers-Kronig algorithm. This is sometimes a useful approach for generically fingerprinting intractable or heavily filled polymer samples. An example is shown in Figure 9. 

The majority of samples for mid-infrared investigations are presented as finely divided powders dispersed in an excess of a dry powdered nonabsorbing matrix, commonly KCl. This ensures that superimposed interferences from specular (front-surface) reflections are minimized in the recorded diffuse reflection spectrum. A useful approach for sampling intractable, composite, or gross objects is to abrade a fine powder from the article s surface with some SiC... 

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.

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 ... 

For continuous solids with a shiny surface a specular reflection spectrum is obtained. In principle, the absorption spectrum can be derived from this by the Kramers-Kronig transformation. Examples of this are shown for both PP (Figure 4.4(a)) and polyester (Figure 4.4(b)) samples. Equally good results are obtained from both unfilled and carbon-filled samples. The presence of inorganic fillers does not alter the nature of the spectra and any contribution from the filler appears specular. The presence of glass as a filler has little effect on the spectra. The spectra obtained are adequate for qualitative identification, but there are limitations. Band shapes often appear nnsymmetrical and baselines are uneven. When the surface is not shiny the spectra are weaker and may contain a diffuse component. When there are surface species the reflection spectrum may be unrepresentative of the bulk material. No spectra were obtainable from those carbon-filled samples that did not have shiny surfaces. 

External reflection. This is not as well developed a technique as internal reflection the physics of reflection of light from surfaces is less accommodating to the infrared spectroscopist. Smooth or shiny surfaces are particular problems. Specular reflection from the surface itself is governed by Fresnel s equations—the reflectance depends on a complicated combination of refractive index, sample absorbance and polarisation. Consequently, samples where the reflectance is mainly from the surface give rise to spectra which bear little relation to conventional transmission spectra. A transformation known as the Kramers-Kronig transformation does exist which attempts to convert a specular reflectance spectrum into a conventional-looking one. It is not 100% successful, and also very computer-intensive. For these reasons, specular reflectance is not commonly used by the analyst. 

NIR reflectance spectra were collected using a Laser Precision PCM 4000 Fourier transform near-infrared (FT-NIR) spectrometer, equipped with CaF beam splitters and a thermoelectrically cooled PbSe detector. An Axiom difftise/specular reflectance attachment, set at 15" C, was used to collect the reflectance spectrum from each sample coupon. Each sample spectrum was the result of a 5 scan... 

Eckhardt, C.J. and Nichols, L.F., 1972, Observation of excitons in a molecular liquid specular reflection spectrum of a-methylnaphta-lene, Phys. Rev. Lett., 29 1221. 

Alexander has described a method for obtaining spectra of thin films of polymer which are fi ee of interference fringes. The method is based on measuring a transmission/reflection spectrum of the polymer using a specular reflectance accessory on the infrared instrument. The film is placed on a reflectance accessory with a mirror above it. The radiation transmitted through the film is returned by the mirror and so is measured together with the radiation reflected by the film. 

On the other hand, the maximum absorption index of bands in the spectra of most organic molecules (including polymers) rarely exceeds 0.3 above 1000cm In these cases, ( 2 — 1) > and the reflection spectrum looks more like the n spectrum (see Figure Ab) than the k spectrum. The specular reflection spectrum of a polycarbonate polymer is shown in Figure 13.2. Few experienced spectroscopists would immediately recognize this spectrum as a polycarbonate. To convert this spectrum to the optical constant (n and k) spectra, the Kramers-Kronig... 

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. 

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