Diffuse reflectance spectrum

Fig. 5. Total reflectance spectra (diffuse and specular components) for three commercial zinc oxide ceramic powders.

The ratio r = r sample)/(ro , reference) is determined, and the K-M function is calculated from this value. For the sake of convenience, r itself or the reflection-absorbance (-logr ,) is often used for the ordinate of a diffuse-reflection spectrum. Diffuse-reflection spectra in the mid-IR region are mostly plotted with the K-M function, whereas near-IR spectra often use the (- log r ). 

As a contradistinction to the relatively simple case of AI2O3 Cr(III) where the color is due to a metal-centred electronic transition, we mention now on one hand the fact that the Cr(III) ion colors many transition-metal oxides brown (e.g. rutile Ti02 or the perovskite SrTi03 [15]), and on the other hand the fact that the color of blue sapphire (AI2O3 Fe, Ti [16]) is not simply due to a metal-centred transition. By way of illustration Fig. 1 shows the diffuse reflection spectrum of SrTiOj and SrTi03 Cr(III) [17], and Fig. 2 the absorption spectrum of Al203 Ti(III) and Al203 Ti(III), Fe(III) [18]. It has been shown that these colors are due to MMCT transitions and cannot simply be described by metal-centred transitions [19],... 

Fig. 1. Diffuse reflection spectrum of SrTiOs (fl) and SrTiOj LaCrOj (0.5 mole %) (h). The extra absorption in (b) is due to Cr (III) Ti(IV) MMCT...

Fig. 6. Diffuse reflection spectrum of KMn04 (a) and AgMn04 (6) (modified from Ref. [45])...

These latter authors though also reported a diffuse reflectance spectrum for their compound, which showed absorption peaks at 23.5, 29.7, and 38.2 kK. (See Fig. 1) the first two bands were assigned as the 4A 2g - 4T2g and 4A 2g 4T g (F) d—d transitions respectively, as shown... 

The diffuse reflectance spectrum of CsReFg was studied between 10 and 40 kK. by Brown et al. (32), but no clearly defined peaks could be recognised above a high background absorption. Since Laporte-allowed charge-transfer bands usually yield well marked intense absorptions it is reasonable therefore to conclude that these are absent below 40 kK. (See also Section 5). 

Figure 12 A room-temperature diffuse reflectance spectrum of compound [C6H5NH (CH3)2]2Te2Iw...

We conclude that when log f(R3.) is plotted against the detection wavelength, the resulting diffuse reflectance spectrum will be identical to the transmission spectrum of the compound. The only difference will be a displacement in the ordinate by the magnitude of -log s. 

The reaction of a primary amine with lactose is accompanied by a browning of the solids, and the path of such reactions is easily following by means of diffuse reflectance spectroscopy. For instance, the reaction of isonicotinic acid hydrazide (Isoniazid) with lactose could be followed through changes in the reflectance spectrum [31]. As may be seen in Fig. 4, a steady decrease in reflectance was noted as the sample was heated for increasing amounts of time. The spectral data were used to deduce the rate constants for the browning reaction at various heating temperatures, and these rates could be correlated with those... 

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... 

These paramagnetic, air-stable complexes could equally well be formulated as Os " arylnitrenes. Both crystal-field and charge-transfer bands have been observed in the diffuse reflectance spectrum of [OSF5] . Calculation of the spin-orbit coupling constant and crystal-field parameters indicated that the degree of covalency is high for a fluorocomplex, and comparable with that found for quadrivalent [MFg] species of the first transition series. ... 

Fig. 1 a. Diffuse reflection spectrum of BaTh03-Pr + at room temperature (Pr + in six-coordination)... 

Direct Synthesis reaction of, 6 395 fluoride, 21 235, 237, 239, 249 homopolyatomic cations, 17 82 ion, stereochemistry, 2 40-41, 44-45 isocyanates, preparation, 9 158 properties, 9 157 isothiocyanates, properties, 9 177 mixed valence compounds of, 10 375-381 crystal structure of, 10 376 diffuse reflectance spectrum of, 10 380 structure of Pb," ion, 10 381 nuclear magnetic shielding, 22 224 organometallic compounds, 2 82, 88, 89 oxide, neutron diffraction studies on, 8 231-233... 

Diffused reflectance spectrum of the 7.9 % V205/AIP04-5 sample after calcination at 550°C was drastically different from that of the uncalcined sample (Fig. 6). The sample before calcination showed a broad charge-transfer band at 400-550 nm. However, the sample after calcination showed a charge-transfer band at 270 nm. In a previous study on surface phases of VjOg supported catalysts [23,24], ions in a distorted... 

Support of this work was provided by the Amoco Oil Co. We thank Dr. S. M. Kuznicki for providing a sample of ETS-10 and Dr. R. J. Davis for recording the UV-VIS diffuse reflectance spectrum of TS-1 samples. 

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. 

A = solution absorption spectrum B = diffuse reflectance spectrum C = crystal transmission spectrum D = transmission spectrum as KC1 pellet E = absorption spectrum of melt at 400 °C maximum extinction coefficient in parentheses following the band maxima. 

All infrared spectrometers generate data that are contained in the infrared spectrum (see Fig. 10.1). The spectrum represents the ratio of transmitted intensities with and without sample at each wavelength. This intensity ratio is called transmittance (7 ) can be replaced by percent transmission (%7 ) or by absorbance A = log(l/T). If the experiment is conducted using reflected or diffuse light, pseudo-absorbance units are used (cf. 10.10.2). Finally, it is common to report wavelengths in terms of wave number v (cm-1 or kaysers) knowing that ... 

Dead time, 7 Dead volume, 13 DEAE cellulose, 88 Deconvolution (spectrum), 215 Deflection equation, 297 Degrees of freedom, 168 Derivative spectrometry 215 Deshielding, 139 Deuterium arc lamp, 199 Diffuse peak, 324 Diffuse reflection, 180 Diffusion coefficient, 5, 102 Diffusion current, 362 Diode array, 200 Distribution isotherm, 9 Double beam, 169 DTGS, 175... 

The 2-carboxylic acid derivative had normal magnetic behaviour (ju = 1.81 BM at 295 K). A diffuse reflectance spectrum was characteristic of square planar complexes and an IR spectrum showed the presence of covalently bound carboxylate groups. 

It is noteworthy that the most active sample, MCB anatase, shows a diffuse reflection spectrum which is closer tc the spectrum of rutile than to that of anatase.13) This result may indicate the inclusion of rutile Ti02 in the MCB sample, although XRD of the MCB sample shows the anatase spectrum.l3) Ti02 particles consisting of both rutile and anatase are sometimes claimed to show hign photocatalytic activity, but the reason for this is not understood yet. 

The known Fe(OAlk)2 are green polymeric substances. The diffuse reflectance spectrum of Fe(OMe)2 indicates the octahedral coordination of iron atoms [6], According to the X-ray powder data its structure is isomorphous to those of other divalent metals, — i.e., resembles that of Fe(OH)2 (Mg(OH)2 type) [1656], The solubility is characteristic only of the derivatives of rather branched radicals — CH2Ph, CPh3 [114,941] — or of the phenoxides solvated by such bulky ligand as Dipy [941], The octahedral coordination is observed not only for the methoxide but quite unexpectedly for the derivatives 3,5-sus-... 

Routine measurements of d-d spectra are performed on solutions. If a suitable solvent cannot be found for a solid sample, a diffuse reflectance spectrum of a powdered sample can be taken. This is actually an absorption spectrum of the surface layers of the sample and is subject to a number of anomalies and artefacts. It is much better to study microscopic single crystals, preferably at low temperatures. Large crystals (if they can be grown) tend to absorb too strongly around band maxima small, thin (c. 0.01mm) plates are best. It is usually necessary to condense the incident beam by means of a lens in order to obtain detectable intensities of transmitted radiation. Thus the technique is more difficult and time-consuming than the familiar, routine solution measurement but it can provide much more information. 

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