Cadmium spectrum

Figure 2. Computer plot of a cadmium spectrum spiked with enriched...

In contrast to the low-pressure lamps (1—130 Pa) which primarily emit at the resonance line at A = 254nm, high-pressure lamps (lO —10 Pa) also produce numerous bands in the UV and VIS regions (Fig. 16). Table 3 lists the emission lines and the relative spectral energies of the most important mercury lamps (see also [44]). The addition of cadmium to a mercury vapor lamp increases the numbei of emission lines particularly in the visible region of the spectrum [45] so that it i. also possible to work at A = 326, 468, 480, 509 and 644 nm [46]. 

Light detectors fall into two categories photoconductors and photodetectors. Photoconductors are devices whose resistance decreases upon exposure to light. Cadmium sulfide (CdS) and cadmium selenide (CdSe) are the most commonly used photoconductor materials in the visible spectrum. They are still mostly produced by sputtering but CVD is used increasingly (see Ch. 12, Sec. 4.0). 

A prime contender for leading thin film technology as applied to solar cells is cadmium telluride (CdTe). Its bandgap is almost ideal for use as a solar cell for energy conversion from the Sun s spectrum. Here, CdTe and cadmium sulfide (CdS) are used to produce a low cost thin film solar cell... 

Figure 1. FT-IRRAS double modulation spectrum of six monolayers of Cadmium Arachidate on silver. The inset shows the background water vapor and carbon dioxide present in the measurement. IBM Instruments IR/90 series.

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... 

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. 

Figure 6. Association reaction of Cd with benzene. Cd was formed by laser desorption/ionization from a cadmium-contaminated stainless steel surface and allowed to react with benzene at a pressure of about 5 x 10" torr. The spectrum shown is for a 6 s reaction time, after which the ions were excited by impulse excitation and detected by FTICR. The multiplets show the cadmium isotope pattern.

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. " ... 

Both compounds crystallize with the cadmium diiodide structure (space group P3ml) as previously reported on polycrystalline samples.3 For platinum disulfide, ao = 3.542(1) A and c0 = 5.043(1) A, and for platinum ditelluride, a0 = 4.023(1) A and c0 = 5.220(3) A. Direct chemical analysis for the component elements was not carried out. Instead, precision density and unit-cell determinations were performed to characterize the samples. The densities of both compounds as determined by a hydrostatic technique with heptadecafluorodeca-hydro-l-(trifluoromethyl)naphthalene as the density fluid4 indicated that they are slightly deficient in platinum. For platinum disulfide, = 7.86 g/cm3 and Pmeas = 7.7(1) gm/cm3, and for platinum ditelluride, p = 10.2 gm/cm3 and Pmeas = 9.8(1) gm/cm3. In a typical experiment an emission spectrum of the platinum disulfide showed that phosphorus was present in less than 5 ppm. A mass spectroscopic examination of the platinum ditelluride revealed a small doping by sulfur (less than 0.4%) and traces of chlorine and phosphorus (less than 100 ppm). 

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. 

Still another method used to produce PV cells is provided by thin-film technologies. Thin films are made by depositing semiconductor materials on a solid substrate such as glass or metal sheet. Among the wide variety of thin-film materials under development are amorphous silicon, polycrystalline silicon, copper indium diselenide, and cadmium telluride. Additionally, development of multijunction thin-film PV cells is being explored. These cells use multiple layers of thin-film silicon alloys or other semiconductors tailored to respond to specific portions of the light spectrum. 

Fig. 1.5 IR spectrum, obtained in ATR mode, of a sample of yellow pigment from Jose Benlli-ure s palette (1937). (BM) IR bands ascribed to the drying oil used as a binding medium. Interestingly, and (CS) appearing at 1569cm is associated with carboxylate groups from cadmium soaps formed as a consequence of natural aging...

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. 

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