Luminescence, observing

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

Cathodoluminescence is the luminescence observed upon excitation by high-energy electrons. 

The excitation spectrum demonstrates that for an effective luminescence not only the presence of an emitting level is important, but also the presence of the upper levels with a sufficiently intensive absorption. The excitation spectra enable us to choose the most effective wavelength for luminescence observation. The combination of excitation and optical spectroscopies enable us to determine the full pattern of the center s excited levels, which may be crucial for luminescence center interpretation, energy migration investigation and so on. The main excitation bands and fines of luminescence in minerals are presented in Table 2.2. 

Similar to the results observed in previous cases, the structural differences affect the optical properties of the complexes. While in the former, a single emission is observed in the solid state, both at room temperature (440 nm, exc. 390 nm) and at 77 K (460 nm, exc. 360 nm) the latter shows one band (510 nm, exc. 450 nm) and one shoulder (560 nm) at room temperature and two independent emissions (510 nm, exc. 370 nm 550 nm, exc. 480 nm) at 77 K. Furthermore, the C6C15 derivative is also luminescent in solution, displaying a band at 530 nm (exc. 345 nm), which is not present in the precursor complexes or in the pentafluorophenyl derivative. This fact indicates, as in the previous case, that the Tl-Tl interaction present in the solid state remains even in solution and is the responsible for the luminescence observed in this state. 

However, the presence of lines at 700 cm"1 in the Gd2 sample which are independent on scattering geometry and the existence of the same line at 696 cm 1 in the Gdl spectra both evidence an impurity origin of these excitations. We propose that Co oxides like CoO and Co203 are present at inter crystallite boundaries and on the surface of the none annealed sample Gdl. Such phases could also be the reason for the luminescence observed in the polycrystalline sample G2 seen in Fig.5 as a linear increase of the scattering background. 

The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. 

The lunar transient events could be excited by protons in the solar wind but experiments with silicate minerals in proton beams show that the process is inefficient, quantum efficiencies from lxl0 4 to 1x10 , and given the concentration of protons in the solar wind the mechanism cannot account for the intensity of the observed luminescence (33). Another possibility is that neutral particles in the background solar wind or associated with disturbances on the sunfs surface provide the excitation source (34). This would be a process very similar if not identical to the candoluminescence and radical recombination luminescence observed in the laboratory. 

Probably the first modern reference to meteorite thermoluminescence is that of Herschel (1), although it is possible that the luminescence observed by Howard after passing an electric charge "from 34 square feet of glass" across the Siena meteorite could, in part, have been TL (2). Herschel reported naked eye observations of the TL produced by dust from the Middlesbrough meteorite as it was sprinkled on a hotplate, and he suggested, correctly, that the luminescent component was feldspar. 

Figure 6.18 Transient absorption spectra, uncorrected for luminescence, observed after 532 nm excitation of a [Ru(deebpy)(bpy)2]2+-modified Ti02 film in neat CH3CN under argon. The apparent negative absorption change observed beyond 570 nm is due to emission. This part of the spectrum is included to illustrate the correspondence between absorption and luminescence kinetics. Reprinted with permission from C. A. Kelly, F. Farzad, D. W. Thompson and G. J. Meyer, Langmuir, 15, 731 (1999). Copyright (1999) American Chemical Society...

There is strong evidence for making the assumption that the increase in luminescence observed is caused by induction of a metabolite for most of the compounds tested. First, outliers in QSAR regressions can be used to determine the limits of applicability of a QSAR (Lipnick, 1991). If the biosensor response to all compounds other than naphthalene was a non-specific response with no relationship to biotransformation, then it would be expected that the value for naphthalene would be a clear outlier. Instead, the value for naphthalene is close to the predicted value, as shown in Figure 17.3. A dose-response behavior is indicative of a specific mechanism. 

In the case of the Cr(CO)5 or Fe(CO)4 fragments a Jahn-Teller induced surface intersection between the Sj and S0 states explains the rapid transition to the S state following excitation of the parent. However for the Ni(CO)3 fragment the ground state is the 1A state and this has a planar structure (D3A). The first excited state Sj is the degenerate lE" state which splits by planar distortion producing 1Bl and A, states but not an Al state at the C2v limit. Consequently such a distortion does not provide a deactivation route to the S0 surface of Ni(CO)3. The Ni(CO)3 fragment remains in its Sl state which explains the luminescence observed in this system. 

The luminescence observed was associated with the photodegradation products rather than with the photoinduced ana form.54 The luminescence spectrum coincided with the luminescence spectrum of ll-hydroxy-5,12-naphthacenequinone.50... 

Piezoluminescence Luminescence observed when certain solids are subjected to a change in pressure. 

Such differences in multiphonon relaxation rates are believed to be largely responsible for the strongly enhanced visible luminescence observed in low-... 

Effects of CO2 laser radiation on CH4-SFg mixtures. 596 Visible luminescence observed characteristic of fluorine-supported flames... 

Fig. 3. Flash luminescence observed before cell shrinkage by Luminoview.

Other studies of the TMPD system have been reported.78- 79 One of these reports claims the observation of a new emission at 430 nm attributed to a solute-solvent charge-transfer state.78 The luminescence observations do tend to indicate that impurities may be responsible for the emission, but the authors seem to have eliminated the possibility with some degree of certainty. 

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