Cadmium absorption spectra

The luminescence of macrocrystalline cadmium and zinc sulfides has been studied very thoroughly The colloidal solutions of these compounds also fluoresce, the intensity and wavelengths of emission depending on how the colloids were prepared. We will divide the description of the fluorescence phenomena into two parts. In this section we will discuss the fluorescence of larger colloidal particles, i.e. of CdS particles which are yellow as the macrocrystalline material, and of ZnS particles whose absorption spectrum also resembles that of the macrocrystals. These colloids are obtained by precipitating CdS or ZnS in the presence of the silicon dioxide stabilizer mentioned in Sect. 3.2, or in the presence of 10 M sodium polyphosphate , or surfactants such as sodium dodecyl sulfate and cetyldimethylbenzyl-ammonium... 

Vanadium(n) Complexes.—Dehydration of VSO. THjO has been shown to proceed via the formation of VS04,mH20 (where n = 6, 4, or 1) and V(OH)-(SO4), which were characterized by X-ray studies. The polarographic behaviour and the oxidation potential of the V -l,2-cyclohexanediamine-tetra-acetic acid complex, at pH 6—12, have been determined.Formation constants and electronic spectra have been reported for the [Vlphen),] " and [V20(phen)] complexes. The absorption spectrum of V ions doped in cadmium telluride has been presented and interpreted on a crystal-field model. The unpaired spin density in fluorine 2pit-orbitals of [VF ] , arising from covalent transfer and overlap with vanadium orbitals, has been determined by ENDOR spectroscopy and interpreted using a covalent model. " ... 

Cadmium sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect (28-37) is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For the CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change (28,33,34). This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon occurs as the cristallite size is comparable or below the excitonic diameter of 50-60 A (34). In a first approximation, a simple electron hole in a box model can quantify this blue shift with the size variation (28,34,37). Thus the absorption threshold is directly related to the average size of the particles in solution. 

In the presence of an excess of sulfide ions, [Cd2+]/[S2 ] = 5, a strong change in the absorption spectra at low water content is observed compared to that obtained for a ratio of [Cd2+]/[S2-] equal to 2. By increasing the water content, the sharp peak disappears and a similar behavior as in the case of excess of cadmium is observed, i.e., a red shift in the absorption spectrum. The sharp peak observed at low water content increases with the relative amount of sulfide ions (45). This peak is attributed to sulfide clusters (55) formed on the CdS particles because of the high local concentration of sulfide ions. The disappearance of this peak when increasing the water content could be explained by the fact that sulfide clusters, with negative charges, are repelled to the center of the droplets and redissolve themselves inside the water pool. 

When solutions of CdS colloids containing no additional electron and hole acceptor in the solution, are exposed to a high intensity laser flash, a rather large absorption of an intermediate is observed around 700 nm, similarto that described for the laser excitation of Ti02 in the previous section. The absorption spectrum of the intermediate is given in Fig. 9.17 [52]. It is not due to trapped electrons and holes but it is identical with to the well-known spectrum of hydrated electrons as proved by radiolysis experiments [52]. The half-life of the hydrated electrons is a few microseconds. In the presence of typical hydrated electron scavengers, such as oxygen, acetone or cadmium ions, the decay of the intermediate became much faster. 

The absorption spectrum of cadmium LADH differs markedly from that of the zinc or cobalt enzyme and perhaps bears upon the nature of the metal-binding ligands (Figure 19). An intense band centered at 245 m/i with a molar absorptivity per cadmium atom of 10,200 is shown by the difference spectrum at the bottom of the figure zinc LADH is employed as the reference. Notably, the molar absorptivity of this band is nearly 14,000, close to that reported for the cadmium mercaptide chromophores of metallothionein (23). This is consistent with the hypothesis that sulfhydryl groups may serve as metal ligands in LADH. 

Figure 12. Absorption spectrum of a I.O IO" M silver sol containing 1.9i0 M cadmium perchlorate after deposition of different amounts of cadmium. The m-values indicate the number of atomic Cd layers deposited. ...

The species 4 (Fig. 3.15), for example, is a selective and nontoxic fluorescent molecular sensor proposed to probe cadmium content in living cells in case of heavy metal contamination. [10] The variation in the absorption spectrum of the ligand during the titration with increasing amounts of Cd " (Fig. 3.15) allowed to evaluate the efficacy of the complexation process calculating a log Kass 6.0 with a 1 1 stoichiometry. 

Since the photophoretic force depends on the electromagnetic absorption efficiency Q y , which is sensitive to wavelength, photophoretic force measurements can be used as a tool to study absorption spectroscopy. This was first recognized by Pope et al. (1979), who showed that the spectrum of the photophoretic force on a 10 foa diameter perylene crystallite agrees with the optical spectrum. This was accomplished by suspending a perylene particle in a Millikan chamber with electro-optic feedback control and measuring the photophoretic force as a function of the wavelength of the laser illumination. Improvements on the technique and additional data were obtained by Arnold and Amani (1980), and Arnold et al. (1980) provided further details of their photophoretic spectrometer. A photophoretic spectrum of a crystallite of cadmium sulfide reported by Arnold and Amani is presented in Fig. 11. 

In his early work Pedersen investigated crown ether complexation by UV spectroscopy. He reported that complexation caused a shift in the absorption maximum of dibenzo[18]crown-6 of about 6 nm to a longer wavelength (B-78MI52101). The test was not totally reliable as cadmium caused no change in the spectrum yet gave a crystalline complex. In general, however, UV-visible spectroscopy is of limited use in the study of macrocyclic complexes. 

If E. coli is grown in a cadmium-containing, zinc-deficient medium, the enzyme is found to be active, but to contain six Cd2+ per molecule. The presence of cysteinyl ligands is confirmed by the observation of the characteristic charge-transfer bands. The binding of substrate perturbs the absorption and CD spectra of the zinc and cadmium enzymes, and the d-d spectrum of the nickel enzyme, showing that "the conformation of the R subunit is affected by the binding of substrate to the C subunit.530,531... 

The absorption due to cysteine residues in proteins is usually more or less obscured by the aromatic residues. With thionein, a cadmium- and zinc-binding protein from equine renal cortex, this is not the case. Its spectrum in both a metal-containing and metal-free form is shown in Fig. 

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