Temperature quenching luminescence

Temperature-dependent luminescence measurements in the range from 77 to 300 K show quenching of the peak luminescence by a factor of about 15. Similar behavior is observed in the lifetime quenching [665, 666], As the band gap of the PECVD a-Si H is about 1.6 eV, nonradiative deexcitation of Er may occur at elevated temperatures. The amount of quenching lies in between that of c-Si and LPCVD a-Si H, just like the bandgap. 

Influence of Charge-transfer and 4/ " 5ti States on the Temperature Quenching of Lanthanide Luminescence... 

During recent years it has become clear that the temperature quenching of lanthanide luminescence is determined by the properties of the c.t. or 4f 5d state. Let us first mention shortly the mechanisms that have been proposed for temperature quenching of lanthanide ion emission. 

In this section we will discuss first the influence of the c.t. state of the Eu3+ ion on the temperature quenching of its luminescence, because it has been studied in detail. Secondly we will consider temperature quenching of some other lanthanides. 

Finally it may be mentioned that other c. t. states can also play a role in the temperature quenching. The absence of Tb + emission in YVO4 has been ascribed to the presence of a low-lying metal-metal c.t. state in which one of the Tb + electrons is transferred to the vanadate group (formally written as Tb4+-1-V4+). (30, 83). Assuming that this c.t. state has a large Franck-Condon shift it is easy to explain the absence of Tb + luminescence. Because one of the 4/ electrons of Pr + is also easily excitable, similar phenomena are expected for Pr +. In fact Pr + in YVO4 luminesces only very weakly. 

The authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

Measuring the variations of the intensity ratio /inp/Zyb under pressure, Takarabe (1996) was able to determine the energy difference bt and found a pressure-induced shift of 70 meV/GPa which is close to the shift of the band-edge related luminescence due to the bound e-h pairs. Furthermore, under pressure it was possible to completely recover the thermally quenched luminescence of the Yb3+ ion at temperatures of 220 K and 260 K (Takarabe et al., 1994) as well as at room temperature (Takarabe, 1996). The minimum pressure at which the luminescence could be observed again was shown to increase with increasing temperature. All these facts fitted well to the proposed back-transfer model, which was thus strongly supported by the pressure experiments. 

The temperature quenching of the luminescence of the complexes can be related to other properties. This situation asks for quantitative calculations as have been performed for other centres (uranates with charge-transfer transitions )). 

These phenomena can be explained by the occurrence of host-sensitized energy migration down to very low temperatures. The energy is trapped by uranate groups near defects (Fig. 13). When the temperature is raised, the uranate traps get emptied one by one, as follows from the temperature dependence of the zero-phonon lines in the emission spectrum. Now trapping at killer sites becomes also important this results in the temperature quenching of the luminescence. 

The method developped by Struck and Fonger" offers the possibility for a quantitative description of the temperature quenching of broad band and narrow line emissions. The parameters which are used in this method to calculate the non-radiative rate can be obtained from the band shapes and the peak positions of the features observed in the luminescence spectra. The influence of small variations of the... 

The recent investigations have shown that the temperature quenching of the uranate luminescence can be described in a satisfactory way in terms of single configurational coordinate diagrams, although we realize that this is quite a crude approximation. Especially the temperature dependence of the electronic factor which is constant in the Mott-Seitz and Struck and Fonger methods should probably be taken into account. 

Group IV Donors. Luminescence spectra, decay times, and quantum yields of A [Ir-(CN)g] (A = K, Cd or In ) have been measured at various temperatures. The luminescence, assigned to a Tij Aig transition, is at 23.4 kK for the salt and is quenched thermally well below room temperature. 

Analytical chemistry makes use chiefly of the native luminescence lanthanides(III), U(VI), mercury-like ions and Cr(III) compounds. They are distinguished by temperature-quenching of luminescence. Therefore, the determination (espcially of Cr(III) and mercury-like ions) is performed at low temperatures (usually 77 K). A very simple sample cell compartment unit (Fig. 5) permits the luminescence of small analyte volumes to be measured366). Also, solvents which do not solidify to clear glasses at low temperature can be used. This is of practical importance. 

Suzuki et al. [33] have reported on the ultraviolet and y-ray excited luminescence of Gd2SiOs Ce. At 11 K they were able to find luminescence from two different Ce ions, one with an emission maximum at about 425 nm, the other with an emission maximum at about 500 nm. The respective lowest excitation bands have their maximum at 345 and 380 nm, and the respective decay times are 27 and 43 ns. The former luminescence is hardly quenched at room temperature, the intensity of the latter decreases above 200 K, and at room temperature only 20% is left. Under y-ray excitation at room temperature the luminescence is dominated by the 425 nm emission, since the other is quenched for the greater part. Peculiarly enough, the decay shows under these conditions a long component (t 600 ns) which is not observed for YiSiOs Ce and LuiSiOs Ce ". ... 

Acetonitrile, room temperature From luminescence quenching of the zinc porphyrin unit... 

The fluorescence intensity of naphthalene in aqueous solution decreases upon aeration. However, the quenching of naphthalene by aeration is totally suppressed in the presence of a water-soluble sulfopropylated fi-CD [70]. Similarly, the quenching of halonaphthalene phosphorescence in water by NaN02 can be substantially inhibited by j8-CD. The rate of inhibition depends on the bond-tightness between the analyte and the CD. Retinal, which is normally insoluble in water and is not fluorescent in solution at room temperature, emits luminescence in the region of 450 nm and permits fluorescence detection when incorporated by j8- or y-CD even in air-saturated aqueous solution [71]. In the luminescence detection of volatile compounds, CDs can be used as solid matrices which efficiently trap the volatile compounds for obtaining room temperature... 

The first indication that the c.t. state of Eu " plays a role in the luminescence quenching process was the fact that there is a relation between the spectral position of the first c.t. band of Eu and the quenching temperature and room-temperature quantum-efficiency of the luminescence under excitation into the c.t. band (Blasse, 1966). A similar relation exists also for some other luminescent groups, e.g. the niobate octahedron [NbOsf" (Blasse, 1968a) and the uranate octahedron [UOef" (Blasse, 1968b). Bril and coworkers (1968) showed that at room temperature the luminescence quantum efficiency for Eu in YAI3B4O12 amounts to 35% for excitation into the c.t. band and to 100% for excitation into the narrow 4f levels. It is a simple task to show that in a simple... 

Oxygen quenches phosphorescence of aromatic hydrocarbons in plastics at r.t. but not at liquid nitrogen temperature [547]. Luminescence spectroscopy (phosphorescence at 77 K and fluorescence at r.t.) may be used to evaluate oxidation processes in plastic materials, e.g. in LDPE films [548]. Recently, Allen et al. [549] have reported that prolonged melt oxidation of PET results in extensive discoloration and the formation of highly fluorescent hydroxy-lated terephthalate units which exist in equilibrium with highly phosphorescent stilbenequinone units. Phosphorescence characteristics of some common polymers are available [505]. 

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