Efficiency quantum

We have already stated that proper choice of host and activator is essential to obtain an efficient phosphor. Now consider the case where one-hundred (100) photons are incident upon the phosphor. From our discussion given above, we know that a few are reflected, some are transmitted, and, if the phosphor is an efflclent combination of host and activator, most of the quanta are absorbed. But not all the absorbed quanta will result in an activated center, and, once these centers become activated, not aU will emit a subsequent photon. Some become deactivated via relaxation processes. To determine just how efficient a phosphor may be, we measure what is termed the quantum efficiency , i.e.- QE. The quantum efficiency is defined as  

To obtain specific values, we measure total energy of emission and the total energy absorbed. It is easier to measure intensity of photons emitted as a function of wavelength. This gives us  

We have already briefly discussed luminescence decay times. Reiterating, the decay time of a phosphor has been defined as the time for the steady state luminescence intensity to decay to 1/e, or 0.368, of its original Intensity. It has been found that the intensity of photon emission builds up in the order of microseconds, i.e.- 10 sec. to a specific value, i.e.- the excitation process takes only a few microseconds. Since the intensity also decays in microseconds (if the excitation source is removed), there is an equilibrium value attained in the presence of the excitation source, which is a combination of both excitation time and decay time. This so-called steady state is called Iq, and is promulgated by the population of emitting 

Two types of decay are normally encountered. We have already mentioned exponential decay. The other is a logarithmic decay. It is essential to realize that both types of decay arise from a Gaussian array of emitters. That is, the energy states of the excited centers and the energy spectrum of the emission band are the consequence of a random process of phonon perturbations of the excited state of the activator center. Hence a random distribution of excited states results. The distribution is Gaussian and the decay times of this Gaussian array of potential emitters can be described via the binomial derivation, as shown In 5.5.15. on the next page  

This equation presents the change in numbers of excited states with time in relation to the decay time, t, and is a Gaussian expression for a 

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Sandborg, M. and G. Alm-Carlsson, Influence of x-ray energy spectrum, contrasting detail and detector on the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) in projection radiography. Phys. Med. Biol., 1992. 37(6) p. 1245-1263. 

The intensity of fluorescence therefore, increases with an increase in quantum efficiency, incident power of the excitation source, and the molar absorptivity and concentration of the fluorescing species. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

The fluorescent emission for quinine at 450 nm can be induced using an excitation frequency of either 250 nm or 350 nm. The fluorescent quantum efficiency is known to be the same for either excitation wavelength, and the UV absorption spectrum shows that 250 is greater than 350- Nevertheless, fluorescent emission intensity is greater when using 350 nm as the excitation wavelength. Speculate on why this is the case. 

Quantum efficiencies Quantum efficiency Quantum electronics Quantum fluids Quantum mechanics Quantum size effect Quantumwell... 

Efficiency. Efficiency of a device can be reported in terms of an internal quantum efficiency (photons generated/electrons injected). The external quantum efficiency often reported is lower, since this counts only those photons that escape the device. Typically only a fraction of photons escape, due to refraction and waveguiding of light at the glass interface (65). The external efficiency can be increased through the use of shaped substrates (60). 

The internal quantum efficiency of a LED is governed by the relative radiative and nonradiative recombination rates. The total recombination rate,... 

Thushigh internal quantum efficiency requires short radiative and long nonradiative lifetimes. Nonradiative lifetimes are generally a function of the semiconductor material quaUty and are typically on the order of microseconds to tens of nanoseconds for high quahty material. The radiative recombination rate, n/r, is given by equation 4 ... 

This confinement yields a higher carrier density of elections and holes in the active layer and fast ladiative lecombination. Thus LEDs used in switching apphcations tend to possess thin DH active layers. The increased carrier density also may result in more efficient recombination because many nonradiative processes tend to saturate. The increased carrier confinement and injection efficiency faciUtated by heterojunctions yields increasing internal quantum efficiencies for SH and DH active layers. Similar to a SH, the DH also faciUtates the employment of a window layer to minimise absorption. In a stmcture grown on an absorbing substrate, the lower transparent window layer may be made thick (>100 /tm), and the absorbing substrate subsequendy removed to yield a transparent substrate device. 

Typical light output versus current (L—I) and efficiency curves for double heterostmcture TS AlGaAs LEDs lamps are shown in Eigure 8. The ir LED (Eig. 8a) is typically used for wireless communications appHcations. As a result, the light output is measured in radiometric units (mW) and the efficiencies of interest are the external quantum efficiency (rj y. = C y., photons out/electrons in) and power efficiency. As a result of the direct band gap... 

The first detailed investigation of the reaction kinetics was reported in 1984 (68). The reaction of bis(pentachlorophenyl) oxalate [1173-75-7] (PCPO) and hydrogen peroxide cataly2ed by sodium saUcylate in chlorobenzene produced chemiluminescence from diphenylamine (DPA) as a simple time—intensity profile from which a chemiluminescence decay rate constant could be determined. These studies demonstrated a first-order dependence for both PCPO and hydrogen peroxide and a zero-order dependence on the fluorescer in accord with an earher study (9). Furthermore, the chemiluminescence quantum efficiencies Qc) are dependent on the ease of oxidation of the fluorescer, an unstable, short-hved intermediate (r = 0.5 /is) serves as the chemical activator, and such a short-hved species "is not consistent with attempts to identify a relatively stable dioxetane as the intermediate" (68). 

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. 

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