Diffuse reflectance electronic spectra

The diffuse reflectance electronic spectra of the title LB films are given in Figure 2 and Table 2. 

Diffuse reflectance electronic spectra of the metals have also been studied in polymeric complexes (Allum et al., 1973). 

Fig. 28.28 (a) Diffuse reflectance electronic spectra of compounds 1-5 in the solid phase (b) electronic absorption spectra of the same compounds in solution of CH2CI2. (From Ref. 234.)... 

O-Donor ligands. The pyroxene NaTiSi206, synthesized at high pressure (65 kbar) and temperature (1550 °C), is obtained as light-green crystals when quenched to room temperature. A-Ray studies showed that the crystals are of the NaM SijOg structural type, and the diffuse reflectance electronic spectrum confirmed that this is one of the few oxides known to contain Ti . ... 

Complete back electron transfer upon charge-transfer excitation has also been observed in crystalline EDA complexes. For example, laser excitation of the charge-transfer crystals of ferrocenyl donor and polyoxomolybdate acceptor results in short-lived (ps) transients the diffuse-reflectance absorption spectrum of which is shown in Figure 6 [137]. This transient spectrum can be deconvoluted as the sum of the absorption spectra of the oxidized ferrocenyl donor (ferrocenium) and the re-... 

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],... 

Potassium hexafluororhodate(III), K3RI1F6, was obtained by Peacock (22) as a buff solid by fusion of K3Rh(N02)6 with KHF2. It was found to be diamagnetic (19), thus implying the presence of a low-spin lA g (tig) ground state. The electronic spectrum was studied by diffuse reflectance by Schmidtke (23) over the range 15—45 kK., as shown in Fig. 3, and the bands observed are listed in Table 3. 

The hexafluoroargentate(III) anion was first obtained as the CS2K salt by Hoppe and Homann [26), who fluorinated a 2 1 1 mixture of CsCl, KC1, and AgNC>3 at 300 °C. A moment of 2.6 B.M. was reported for the product, and the electronic spectrum was studied by Allen and Warren (9), using the diffuse reflectance technique. 

The electronic transitions of silicalite and TS-1 in the UV-visible spectrum have provided significant information about the structure of TS-1. The diffuse reflectance spectra of the two materials (Fig. 11) show a strong transition at 48,000 cm-1 that is present in the spectrum of TS-1 and absent from that of silicalite. This transition must be associated with a charge-transfer process localized on Tiiv. The frequency of this transition is modified by the presence of H20 (Fig. 12). As the H20 partial pressure increases, the peak at 48,000 cm- is progressively eroded with formation of a lower-frequency absorption, which reaches a new stable maximum value at 42,000 cm. These frequencies come very close to those that can be calculated by the Jorgensen equation for Tiiv tetrahedrally and octahedrally coordinated to oxygen, respectively. Furthermore,... 

Maximum (nm) of the electronic band obtained from the diffuse reflectance spectrum after 10% dispersion in silica (27). 

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 ... 

Here v is the frequency under consideration, Vij the absorption frequency and fij the oscillator strength of the electronic transition between the states i and j. From the diffuse reflection spectrum discussed above it is clear that a(OH ) at the frequency of the vibronic transition involved will be large, since i, / - is relatively small. 

As an illustration of the current state of the art for electronic spectroscopy of transition metal ions in zeolites, refer to the recent review by Schoonheydt of Cu2+ in different zeolites [56]. Schoonheydt shows that experimental measurement of diffuse reflectance spectra (and in the case of Cu2 + EPR spectra) must be combined with theoretical calculations if a complete interpretation is to be made. The exact frequencies of the d-d transitions in the electronic spectrum of Cu2+ are independent of the zeolite structure type, the Si Al ratio, and the co-exchanged cations, but depend solely on the local coordination environment. Figure 20 shows the diffuse reflectance spectrum of dehydrated Cu-chabazite the expanded portion reveals the three d-d transitions in the region around 15000 cm l. 

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. ... 

Fig. 3-1. Band assignment for the diffuse reflectance spectrum of a goethite. Note that one band eould not be assigned. EPT electron pair transition OMCT oxygen-metal charge transfer. From Seheinost et al., 1999 with permission.

Characterization of catalysts The zeolite structure was checked by X-ray diffraction patterns recorded on a CGR Theta 60 instrument using Cu Ka, filtered radiation. The chemical composition of the catalysts was determined by atomic absorption analysis after dissolution of the sample (SCA-CNRS, Solaize, France). Micropore volumes were measured by N2 adsorption at 77 K using a Micromeritics ASAP 2000 apparatus and by adsorption of cyclohexane (at P/Po=0.15) using a microbalance apparatus SET ARAM SF 85. Incorporation of tetrahedral cobalt (II) in the framework of Co-Al-BEA and Co-B-BEA was confirmed by electronic spectroscopy [18] using a Perkin Elmer Lambda 14 UV-visible diffuse reflectance spectrophotometer. Acidity measurements were performed by Fourier transform infrared spectroscopy (FT-IR, Nicolet FTIR 320) after pyridine adsorption. Self-supported wafer of pure zeolite (20 mg/cm ) was outgassed at 673 K for 6 hours at a pressure of lO Pa. After cooling at 423 K, the zeolite was saturated with pyridine vapour (30 kPa) for 5 min, evacuated at this temperature for 30 min and the IR spectrum was recorded. 

Powder x-ray diffraction (XRD), emission spectrum analysis, electron microscopy (EM) with micro-diffraction, BET, UV-visible diffuse reflectance spectroscopy, temperature-programmed desorption of O2 (TPD), temperature-programmed reduction with... 

Garbowski and Mirodatos have recently shown that two charge transfer transitions are present in the u.v. diffuse reflectance spectrum of many zeolites. That at 240 nm, present whatever the zeolite and whatever the chemical or thermal treatment, is related to Al-0 units belonging to the zeolite framework, which are inert towards catalysis. The band at 320 nm, more stable towards dealumination and dehydroxylation, is specifically detected or significantly enhanced for catalytically active samples. The authors relate it to extra-lattice structures, like (AlO)" cations inside the zeolite matrix, in which A1 is highly electron deficient. 

Over the last few years severd spectroscopic investigations of the NiFe system have been reported. As early as 1965 Westtand, Hoppe, and Kaseno 106) recorded, without discussion, a rough spectrum of KaNiFe, whilst in 1968 Bougon 107) prepared several salts of the NiFe anion, and reported the vz and V4 (ti ) vibrational frequencies, and noted the presence of a band at about 18.5 K. in the electronic spectrum. In 1969 two studies of the NiF spectrum were published, the first by Reisfeld, Asprey, and Penneman 31), who used a mull technique, and the other by Allen and Warren 108), who made measurements by diffuse reflectance, using samples sealed in silica cells. 

Diffuse reflection spectroscopy is particularly useful for measuring the spectra of adsorbed molecules, chemically modified surfaces, and catalysts. AU spectral ranges from the UV to the infrared are used. Different information can be obtained in different spectral ranges. Adsorption under the influence of Van der Waals forces generally changes the electronic spectrum of the substrate only slightly. 

The local structure of iron sites in Fe-mazzite and Fe-ZSM-5, in which iron was incorporated during zeolite synthesis, was studied by X- and Q-band ESR, electron spin echo detected ESR (ED-ESR), electron spin echo envelope modulation (ESEEM), and diffuse reflectance UV-vis [94G1]. The X-band ESR spectra of Fe-MAZ (100 Fe/(Fe + A1 + Si) = 1.20) render three signals at g = 4.3, g = 2.3, and g = 2.0 - Table 14a. The Q-band spectra testifies only the signal atg = 2.0. The linewidths of the g = 2.0 signals are smaller in the Q-band spectra - Table 14. This narrowing indicates that the linewidth is at least partially due to the second-order broadening of the -l/2> l/2> transition. The X-band spectrum of Fe-MAZ with 100 Fe/(Fe + A1 + Si) = 0.07 exhibits the... 

Considering its electrical and magnetic properties, ScSe like ScS and ScTe may be regarded as a monovalent metal in which the conduction electron density n corresponds to one electron per formula unit. Theoretically, n = 2.54x 10 cm" for nominal stoichiometric ScSe, Zhuze et al. [3]. The carrier density n = 1 x 10 cm" was calculated from the minimum In the diffuse reflection spectrum (at 2.26 eV) of ScSe obtained by the metal hydride method. Similarly, the effective free carrier mass m = 1.00 mo, the mobility of free carriers = 2.5 cm V" s" and the relaxation time t = 5.3x 10" s were obtained for this sample, Obolonchik et al. [5]. The reflection spectrum for a Bridgman-single crystal gives t = 2.2 x 10" s calculated with use of the Drude theory. The optical effective mass was estimated to be m = 2.9 mo [3]. 

The diffuse reflection spectrum at 300 to -850 nm was measured at room temperature on powder samples (pure or mixed with LiF). It shows two minima at -730 and 800 nm which are connected with electron excitation into the conduction band from energetic levels corre-... 

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