Temperature emissions

J. Lautikko and N-O. Nyland, Regulated and Unregulated Emissionsfrom Catalyst Vehicles atEow Ambient Temperatures, SAE 930946, Society of Automotive Engineers, Warrendale, Pa., 1993. Good reference for low ambient temperature emissions. 

A fictive sky temperature, dependent on ambient temperature, emissivity, and cloudiness, is introduced to account for the long-wave radiative heat exchange between the building envelope and the sky. 

The emissivity of a material is defined as the ratio of the radiation per unit area emitted from a real or from a grey surface (one for which the emissitivity is independent of wavelength) to that emitted by a black body at the same temperature. Emissivities of real materials are always less than unity and they depend on the type, condition and roughness of the material, and possibly on the wavelength and direction of the emitted radiation as well. For diffuse surfaces where emissivities are independent of direction, the emissivity, which represents an average over all directions, is known as the hemispherical emissivity. For a particular wavelength X this is given by ... 

Figure 5.3 Room-temperature emission spectrum of [Au (PPh3)3] in acetonitrile. Reproduced with permission from [14]. Copyright (1992) American Chemical Society.

Figure 5.19 Room temperature emission spectra ofiAuiP P)] (CIO4) in the presence of PPh3 in degassed acetonitrile solution. Molar ratio of PPh3 [Au(P P)] =X 1. Inset a plot ofthe emission intensityofiAuiP P)(PPh3)] vs.X l (concentrationofiAuiP P)] (CIO4) = 10 mol dm ). Reproduced with permission from [12b]. Copyright (1998) Royal Society of Chemistry.

At elevated temperatures emission of ions can also occur but this normally requires an even higher temperature level than does electron emission. 

Rh(bpyL3+ is an example of a complex that exhibits an almost pure n-n phosphorescence and demonstrates one of the limitations of nearly pure ligand localized emissions. At 77K, the complex is highly emissive with a beautifully structured blue ligand phosphorescence (Amax = 446 nmfor the first peak) having at in the tens of msec,(17) but it has no detectable room temperature emission. It is this very long radiative lifetime that causes the absence of room temperature emission. The radiative decay is so slow that it cannot compete effectively against inter- and intramolecular radiationless decay at room temperature. 

ThermoWood is not resistant to exterior weathering and the colour will gradually change to the grey that is characteristic of outdoor exposed wood. In addition, exposure in exterior conditions results in the formation of small cracks on the surface of uncoated wood. Unpigmented or low-build stain coatings do not protect the surface of the wood, but solvent-borne alkyds and water-borne acrylic paints have been found to exhibit better performance than on unmodified wood. VOC emissions from the heat-treated wood are lower compared to unmodified wood and the compositions of the emissions differ. The level of emissions is lower when the wood is treated at a higher temperature. Emissions of terpenes are reduced to very low levels, and the VOC content is dominated by furfural, hexanal and acetic acid (treated at 180 °C), and by acetic acid (treated at 230 °C). ThermoWood passes ecotoxicity tests. 

EXAMPLE 5.6 The low-temperature emission lifetime of a very low concentration ofCr ions in ruby (AfOs Cr) is about 4 ms. For a 1%... 

Figure 6.3 The low-temperature emission spectrum of Eu + ions in LiNbOs. Part of the Dieke diagram for the Eu + ion is included for explanation (reproduced with permission from Munoz et at., 1995).

Yttrium aluminum borate, YAlj (603)4 (abbreviated to YAB), is a nonlinear crystal that is very attractive for laser applications when doped with rare earth ions (Jaque et al, 2003). Figure 7.9 shows the low-temperature emission spectrum of Sm + ions in this crystal. The use of the Dieke diagram (see Figure 6.1) allows to assign this spectrum to the " Gs/2 Hg/2 transitions. The polarization character of these emission bands, which can be clearly appreciated in Figure 7.9, is related to the D3 local symmetry of the Y + lattice ions, in which the Sm + ions are incorporated. The purpose of this example is to use group theory in order to determine the Stark energy-level structure responsible for this spectrum. 

The behaviour in room-temperature fluid solutions of excited Rhodium(III)-polypyridyl complexes remains unclear. These compounds are weak emitters, and perhaps because of this, contradictory reports on the room temperature emissions of Rh(bpy)3 and Rh(phen)3" have been published. Indelli et al. [129] detected the emission at 588 nm (dd ) and 455 nm nn ) for Rh(phen)3 while Nishizawa et al. [127] observed only the nn emission at 455 nm. The tris-polypyridyl Rhodium(III) complexes photodissociate, giving rise to the loss of a ligand [130], as is expected when the MC state can be populated. 

The emission from molecular halogens in the 3n0u+ state resulting from shock heating could, in principle, arise from both inverse predissociation and direct recombination on account of the large thermal populations of the excited atoms at these high temperatures. Emission from this state has been observed hitherto at high temperatures from iodine, bromine, and chlorine... 

A Room-Temperature Emission Lifetime Experiment for the Physical Chemistry Laboratory 186... 

These materials were luminescent in the solid state, exhibiting white luminescence that became brilliant at 77 K. For example, the room temperature emission spectrum of the methyl derivative revealed, as expected from the previous comments, one emission band at 422 nm, but at 77 three emission bands were detected at 415,456 and 560 nm, with the first two being much more intense. The time-resolved measure-... 

Collision-induced absorption is a well developed science. It is also ubiquitous, a common spectroscopy of neutral, dense matter. It is of a supermolecular nature. Near the low-density limit, molecular pairs determine the processes that lead to the collision-induced interactions of electromagnetic radiation with matter. Collision-induced absorption by non-polar fluids is particularly striking, but induced absorption is to be expected universally, regardless of the nature of the interacting atoms or molecules. With increasing density, ternary absorption components exist which are important especially at the higher temperatures. Emission and stimulated emission by binary and higher complexes have also... 

Room temperature emission has been observed for a number of transition metal complexes. Examples include Rh111 ammines,53 [Pt(CN)4]2-,54 and some Cu1 phosphine complexes.55 An important class is that of the polypyridine complexes of Ru11 and related species.56 This last emission, probably from a 3CT state, is quite strong and its occurrence has made possible a number of detailed studies of electron transfer quenching reactions. 

Figure 1. Relative emission intensity monitored at 600 nm vs. temperature in 1M OH /lM S2 electrolyte of CdS Te (100 ppm) excited at open circuit with 514.5 and 501.7 nm (O) light in identical geometries. The excitation intensity at 501.7 nm is 17X that at 514.5 nm in order to match approximately room temperature emission intensities.

No information was located on the amount of BCME released to air, water or soil. Because BCME is readily volatile at room temperature, emissions into the atmosphere could occur, but OSHA regulations require that processes involving BCME be contained (OSHA 1974). Releases into water could occur but would be of little significance, due to the rapid hydrolysis of BCME in water. 

Gliemann et al. also studied the luminescence of systems with vibrational structure [31], viz. the Se4+ ion in Cs2SeCl6 and Rb2SeCl6. Figure 10 presents the emission spectrum of Cs2SeOe as a function of temperature and magnetic field. The low-temperature emission intensity increases with temperature as well... 

In recent years, however, it has become clear that some of these complexes show vibrational structure. The very first report was the low-temperature emission spectrum of K2Cr207 [54], This is one of the rare chromates which show luminescence. The emission spectrum shows a progression in 361 cm-1, which is clearly a bending vibration of the chromate complex. 

A small value of AEst facilitates intersystem crossing. We expect singlet state to be fast depleted along this pathway if the lowest excited state is of (n, n ) type. This pathway is further promoted due to the fact that Tnjt > by a factor of ten, due to the forbidden character of n -> n transition. Fluorescence with decreased rate constant for emission cannot compete efficiently with intersystem crossing. This explains the absence of room temperature emission in heterocyclics like benzophenone, acetophenone, quinoline, acridines, etc. They phosphoresce at low temperatures only. 

In complexes of the quinoline based ligands we saw room-temperature emission which was weak or absent and could be related to unfavorable steric factors. Most likely [Ru(dpt)2]2+ is non-luminescent for the same reason. Kirchhoff et al.258) have argued that steric repulsions may cause a 3MC state to lie at lower energy than the 3CT state so that no CT emission occurs. A metal centered state should be more photoactive and [Ru(dpt)2]2+ does indeed undergo photolysis in the presence of nucleophiles. The photolysis product has been formulated as containing a bidentate dpt ligand. 

Fig. 4 Structure on the low-temperature emission spectra of [MC>2(pic)2]BPh4. Left spectrum is for M=Re, right for M=Tc...

The room temperature emission characteristics of the molecular polygons varied. The emission maximum of rectangle A (R = - (CFbhiCHj) was located near 600 nm and its intensity was dependent on the solvent. 

The first example of such a complex was reported by Che in 1994, who attributed the broad, structureless, room temperature emission band of Pt(phen)(-C=C-Ph)2 to a Pt(d)-7r (phen) 3MLCT excited state (Aem = 578 nm, r = 2.1 xs in CH2CI2) [25]. Eisenberg et al. later examined this complex and the 4-tolyl-acetylide and 4-huorophenyl-acetylide analogues as part of a study of the concentration-dependent self-quenching of square planar Pt(II) complexes [26]. They noted that, in CH2CI2, the self-quenching of... 

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