Quantum efficiency of excitation

The fluorescence quantum efficiency of excited lanthanides in most liquids is very low. To reduce fluorescence quenching due to interactions with high-frequency vibrations in liquids, solvent molecules should have no tightly bonded atoms of low atomic mass (16). Solvents containing hydrogen or other light atoms are therefore undesirable. Aprotic liquid laser materials consist of solutions of a rare-earth salt and an inorganic aprotic solvent. 

Photo DSC experiments conducted in a ciear coating at L 400 nm - where only the substituted thioxanthone Z (Ri - Rg R3 H R4 = C00(CH2CH20)8 H) absorbs -demonstrates the increase of the poiymerization efficiency [25]. Under exposure, the mixture of Z + fi leads to a considerabie polymerization enthalpy whereas, in the presence of Z or S alone, the exothermic signal remains very small. The same is true when using a laser light at X, - 440 nm [26] for the excitation of a mixture S+fi- No polymerization occurs in the presence of or . The relative reactivity of fi at 363 nm and -t- at 440 nm shows a 35 1 ratio, thus defining a low quantum efficiency of excitation transfer. 

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

Nonradiative Decay. To have technical importance, a luminescent material should have a high efficiency for conversion of the excitation to visible light. Photoluminescent phosphors for use in fluorescent lamps usually have a quantum efficiency of greater than 0.75. AH the exciting quanta would be reemitted as visible light if there were no nonradiative losses. 

Producing electron-hole pairs by light excitation in the small particles (d < dg ) electrons and holes can easily be transferred to an electron acceptor and donor, respectively, provided that the energetic requirements are fulfilled. The quantum efficiency of the reaction depends on the transfer rate at the interface, on the recombination rate within the particle and on the transit time, the latter being given by ... 

The choice of new complexes was guided by some simple considerations. The overall eel efficiency of any compound is the product of the photoluminescence quantum yield and the efficiency of excited state formation. This latter parameter is difficult to evaluate. It may be very small depending on many factors. An irreversible decomposition of the primary redox pair can compete with back electron transfer. This back electron transfer could favor the formation of ground state products even if excited state formation is energy sufficient (13,14,38,39). Taking into account these possibilities we selected complexes which show an intense photoluminescence (0 > 0.01) in order to increase the probability for detection of eel. In addition, the choice of suitable complexes was also based on the expectation that reduction and oxidation would occur in an appropriate potential range. 

Griseofulvin exhibits both fluorescence and luminescence. A report by Neely et al., (7) gives corrected fluorescence excitation (max. 295 nm) and emission (max. 420 nm) spectra, values for quantum efficiency of fluorescence (0.108) calculated fluorescence lifetime (0.663 nsec) and phosphorescence decay time (0.11 sec.). The fluorescence excitation and emission spectra are given in Figure 7. 

An optical microcavity produced by the latter process has been applied to tune the emission from erbium-doped PS [Zh6], Erbium compounds like Er203 are known to exhibit a narrow emission band at 1.54 pm, which is useful for optical telecommunications. Several methods have been used to incorporate erbium in PS. A simple and economical way is cathodic electrochemical doping. External quantum efficiencies of up to 0.01% have been shown from erbium-doped PS films under electrical excitation [Lo2]. The emission band, however, is much broader than observed for Er203. This drawback can be circumvented by the use of an optical cavity formed by PS multilayers. In this case the band is narrowed and the intensity is increased because emission is only allowed into optical cavity modes [Lo3]. 

It is clear from Equation (1.15) that the emitted intensity is linearly dependent on the incident intensity and is proportional to both the quantum efficiency and the optical density (this only for low optical densities). A quantum efficiency of < 1 indicates that a fraction of the absorbed energy is lost by nonradiative processes. Normally, these processes (which are discussed in Chapters 5 and 6) lead to sample heating. The proportionality to OD, which only holds for low optical densities, indicates that the excitation spectra only reproduce the shape of absorption spectra for samples with low concentrations. 

EXAMPLE 1.5 The sensitivity of luminescence. Consider a photoluminescence experiment in which the excitation source provides a power of 100 ptW at a wavelength of400 nm. The phosphor sample can absorb light at this wavelength and emit light with a quantum efficiency of r] = O.I. Assuming that kg = 10 fii.e., only one-thousandth of the emitted light reaches the detector) and a minimum detectable intensity of l(f photons per second, determine the minimum optical density that can be detected by luminescence. 

Table E7.5 The lifetimes and quantum efficiencies of three excited states of trivalent rare earth ions in lithium niobate...

Franck-Rabinowitch hypothesis physchem The hypothesis that the decreased quantum efficiencies of certain photochemical reactions observed in the dissolved or liquid state are due to the formation of a cage of solvent molecules around the molecule which has been excited by absorption of a photon. frat)k ro bin-o.wich hT,path-3-s3s ... 

Table 7 Quantum Efficiencies of Ring Opening by Direct Excitation and by Sensitization...

Eigure 5.41 summarizes the temperature behavior of decay time and quantum efficiency of red benitoite luminescence at 660 nm in the forms ln(r) and ln(q) as a functions of 1/T. In such case the luminescence may be explained using simple scheme of two levels, namely excited and ground ones. The relative quantum yield (q) and decay time (r) of the red emission may be described by simple Arhenius equations ... 

Another important feature of the ZnS Mn/AA nanocrystal is that PAA is excited by the photon, which simultaneously can excite ZnS with the same energy. This eases energy transfer from PAA to ZnS Mn. At the same time, coordination of AA or PAA to ZnS Mn increases the local electron density around the COO group due to the abstraction of some S2- ions at the moment of oxidation to S6+ as we observed from IR and XPS spectra. The electrons concentrated near COO- might contribute to enhance the quantum efficiency of the energy transfer from the s-p exciton of ZnS to the d band of Mn(II). All these hybrid effects involved in the ZnS.Mn/AA enhance the PL intensity as a whole. 

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