Chromium 3 3 + luminescence

In some systems it has been shown to be possible to measure strains in oxide films by laser Raman spectroscopy and, in alumina films, by photostimulated chromium luminescence spectroscopy. ... 

Chemical data presented in Table 4.20 demonstrate that the alexandrite samples which exhibit only one type of chromium luminescence in its lattice, namely Cr " m, are characterized by low concentrations of chromium impurity. Figure 5.41a presents time-resolved luminescence spectra at 300 K after very long delay of 5 ms of alexandrite from Siberia which is characterized by the highest chromium. It is structurally complicated emission composed of two narrow lines peaking at 690.3 and 695.9 nm superimposed by the broad emission band. All those narrow lines and structured broad band combination remains the same under different excitations. 

Some oxide-type minerals have been found to luminesce when irradiated. A simple example is ruby (aluminium oxide with chromium activator), which emits bright-red light. The phosphors are incorporated into colour television screens to emit the colours blue (silver-activated zinc sulphide), green (manganese-activated zinc orthosilicate), and red (europium-activated yttrium vanadate). 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

For our purposes, it is interesting to note that the perovskite ErAlo.9Cro.1O3 emits the continuous spectrum of Cr(III) without the red and green Er(III) bands in spite of the fact that the aqueous solution has e of erbium(III) at 5230 A equal to 3.2 to be compared with a-tenth of e, 1.3 of chromium(III) at 5750 A. The absorption spectrum of the nitrate solution is even more striking in a spectroscope, because the transitions of Er(III) are so much narrower. Quite generally, Cr(III) seems to be very effective to prevent narrow band luminescence of the lanthanides. 

For chromium containing lilac chlorite two types of luminescence were observed at 15 K phosphorescence at 14,518 and 1,547 cm with a decay time of 60 ps and a fluorescence band at about 13,850-13,500 cm with a decay time of several microseconds. For green chlorite weak fluorescence at 13,900 cm and phosphorescence at 14,320 and 14,665 cm were observed at room tern-... 

Chromium activated ruby was the first laser material and its luminescence properties are carefully studied. It is a classical example of Cr + in octahedral crystal field. Here Cr + substitutes the AP ions, while such a possibihty can be rationalized by an excellent chemical fit of Cr in place of Al. Ruby is a high crystal field material and thus the T2g state Hes above the E2g level. Pumping is accomplished by a spin-allowed transition into the state, while emission occurs from the level without vibrational broadening and almost all excited... 

Combination of broad emission band and narrow line is typical for elements with d electronic configuration, such as Cr +, Mn and Manganese participation is supported by chemical analyses of benitoite, where chromium was never mentioned as micro-impurity, while Mn is known with concentrations changing from 0.03 to 0.11% (Laurs et al. 1997). Such concentrations are quite enough for luminescence generation. Substitution in Mn +form substituting... 

As was already mentioned, the narrow lines at 692 and 710 nm in the luminescence spectrum of zoisite have been connected with element emission, while Cr and were considered as the possible candidates (Koziarsca et al. 1994). Laser-induced time-resolved luminescence spectra of zoisite reveal the same lines (Fig. 4.59). We are inclined to connect these lines with for the reason that vanadium concentration in our sample is much higher than the chromium concentration. 

Luminescence of Chromium(III) Complexes in Rigid Glass Solutions... 

Chromium is unusual in that it forms a stable hexakis O-bonded urea complex. The complex was first prepared as the chloride salt by Pfeiffer804 and a crystal structure of the complex salt [Cr OC(NH2)2 6][Cr(CN)6]-2DMSO>2EtOH has recently been reported.805 Coordination at chromium(IH) is octahedral r(Cr—O) is in the range 1.96-1.98 A. The reduction of the perchlorate salt of this complex to a chromium(II) species has been studied polarographically.806 Detailed studies of the luminescence spectra of several salts of the chromium urea complex have been reported.807,808... 

Luminescence spectrocopy is potentially a powerful technique for studying chromium(III) complexes and a series of fluoro and aqua complexes have been studied.1066 The luminescence correlates well with ligand field strength and, at. liquid air temperatures, the lifetime of the doublet state from which phosphorescence originates is 2 x 1CT7 s 1. 

Spectroscopy has not proven to be very conclusive in solving this problem. Similarities between the visible spectrum of the calcined catalyst and that of bulk dichromates have been noted (5,12-14). In the end, however, there is always doubt about the interpretation of spectra because no adequate reference data exist for these surface bound species (76). Krauss and coworkers have carefully studied the luminescence of Cr/silica and concluded that at least a portion of the chromium is present as chromate (75). 

Chromium (Cr+6) TT luminescent bacteria are exposed to assess the influence of pH speciation of chromium on toxicity response. B (Villaescusa et ah, 1997)... 

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

Based on our research laboratory s observations of MEF from metals other than silver, MEC from chromium, coppo, nickel and zinc was also studied. Figure 1. 12 shows the enhancement factors for green chemiluminescence from diese metals with various thicknesses. A typical enhancement factor of 2-3-fold is observed from all metal surfaces, vdiich implies that chemically excited states can couple to these plasmon resonant metal particles. It is interesting to note that the chemiluminescence emission is dependent on the amount of reactants in the solution and diminishes once one of the reactants is used up. To test viiether the remainder of the inactive chemiluminescent dye can be excited with an external light source and still emit luminescence, additional experiments were undertaken where the chemiluminescence solution was excited with a laso at 473 nm. Interestingly, the inactive chemiluminescence dye can be optically excited and still emit luminescence with enhancement factors similar to the chemically excited conditions being observed. A detailed investigation of MEC from different metals is currently underway and will be reported in due course. 

There is hardly a better example to illustrate this than the case of ruby (AI2O3 Cr +). Ruby is a beautiful gemstone whose color varies from pale pink to deep red, depending on the chromium concentration. Artificial crystals are presently available. The cold fire of ruby—its deep-red luminescence—increases the attraction these crystals have for many people, not only scientists. 

As soon as the quartet-quartet luminescence of chromium(III) compounds was discovered by Porter and Schlafer the criterion for it being feasible (it may need low temperature and small chromium concentration, as discussed in 9.d, and it may still be undetectable for obscure reasons) was recognized to be a short-lived T2 being trapped... 

The excited-state redox reaction, equation (8.12), is thermodynamically favorable (E° > 0) while ground-state reaction, equation (8.13) is not (E° < 0). Therefore, a mixture of [Cr(phen)3]3+ and such a substrate will only undergo a redox reaction after the chromium complex has been excited. This is the process of photo-induced electron transfer light initiates an electron-transfer reaction. This experiment will explore how substrates such as DNA may be oxidized by the excited-state [Cr(phen)3]3+ complex. Because the electron-transfer reaction competes kinetically with luminescence, the presence of such a suitable substrate leads to a decrease in the intensity of luminescence. For this reason, the substrate is termed a quencher. 

For each solution in the Cr + G series, Table 8.2 (1) measure the UV-vis spectrum to obtain the absorbance A at Aex, and (2) measure an emission spectrum and obtain the integrated emission intensity, I. Also, prepare a solution having [G] the same as the last in your series (Table 8.2), but with no chromium complex. This will serve as a control sample. Save your sample solutions for time-resolved luminescence quenching if you are conducting these experiments (see below). Save all UV-vis and luminescence spectra files that you acquire—they may be useful for data analysis. 

Results Summary for the Luminescence Studies of the Tris(l,10-phenanthroline)chromium(III) Ion, Cr(phen)3]3+... 

---