Measurement of quantum efficiency

The previous formula indicates that the radiative lifetime tq (and hence the radiative rate A) can be determined from luminescence decay-time measurements if the quantum efficiency rj is measured by an independent experiment. Methods devoted to the measurement of quantum efficiencies are given in Section 5.7. 

Emission characteristics of a molecular system can be expressed by three types of measurements (1) observation of emission and excitation spectra, (2) measurement of quantum efficiencies, and (3) determination of decay constants or radiative lifetimes. 

Okamoto. S. Tanaka. K. Izumi. Y. Adachi. H. Yamaji. T. Suzuki. T. (2001). Simple Measurement of Quantum Efficiency in Organic Electroluminescent Devices. Japanese Journal of Applied Physics, vol. 40, no. 7B, L783-4. 

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% al a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excilons. 

Between 1923 and 1927, the concepts of quantum efficiency (number of photons emitted divided by number of photons absorbed by a sample) and quantum yield (fraction of excited molecules that emit) had been defined and values determined for many compounds by Vavilov (34). The quantum yield indicates the extent that other energy loss mechanisms compete with emission in an excited molecule. Although the quantum yield is influenced by the molecular environment of the emitter, for a given environment it depends on the nature of the emitting compound and is independent of concentration and excitation wavelength, at least at low concentrations (35). Tlius, it serves as another measurable parameter that can be used to identify the compounds in a sample and also, because of its sensitivity to the surroundings of the luminophore, to probe the environment of the emitter. 

Understand the importance of quantum yield as a measure of the efficiency of a photoreaction. 

Quantification of fluorescence usually involves studies of the quantum yield (Q) and/or the fluorescence lifetime (t). In simple terms, the quantum yield is a measure of fluorescence efficiency and this is defined hy the ratio of the number of emitted photons to the numher ahsorhed. In the terminology oiO Figure 5-1, this is given hy ... 

The noise in the experimental data can be estimated using the measured detector quantum efficiency (DQE) of the detector... 

Measurement of Quantum Yield, Quantum Requirement, and Energetic Efficiency of the 02-Evolving System of Photosynthesis by a Simple Dye Reaction 127... 

Two useful fluorescence parameters are the quantum yield and the lifetime. Quantum yield is a property relevant to most photophvsical and photochemical processes, and it is defined for fluorescence as in (1.101. More generally it is a measure of the efficiency with which absorbed radiation causes the molecule to undergo a specified change. So for a photochemical reaction it is the number of product molecules formed for each quantum of light absorbed ... 

QUANTUM EFFICIENCY. A measure of the efficiency of conversion or utilization of light or other energy, being in general the ratio of the number of distinct events produced in a radiation sensitized process to the number of quanta absorbed (the intensity-distribution of the radiation in frequency or wavelength should be specified). In the photoelectric and photoconductive effects, the quantum efficiency is the number of electronic charges released for each photon absorbed. For a phototube, the quantum... 

Time of reaction. A measure of the efficiency of a photochemical synthesis is given by the quantum yield ([Pg.114]

The quantum yield (9) is a measure of the efficiency of the photochemical excitation process, which may result in herbicide degradation and indicates the number of herbicide molecules degraded per photon absorbed. A value of 0 indicates that no chemical reaction occurred, while a value of 1 indicates that all molecules excited due to photon absorption were converted to products. Chain reactions, which can lead to quantum values greater than unity, are unlikely at the very low concentrations found in the aquatic environment. 

In this laboratory we use an SLM-Aminco 8100, equipped with Glan-Thompson polarizers. The electronics have been updated by the ISS Phoenix system. Measurements of FRET efficiency are performed under photon counting conditions, with the polarizers crossed at the magic angle (54.7°) to remove polarization artifacts. Fluctuation of lamp intensity is corrected using a concentrated rhodamine B solution as a quantum counter. 

Quantum yield is a measure of how efficiently absorbed light is converted to emitted fluorescence by a fluorochrome. It is the ratio of the number of photons emitted to the number of photons absorbed. 

The quantum yield of a photochemical process is an absolute measure of the efficiency of this process (cf. Chapter 3-13). Tab. 6-4 summarizes several quantum yields of primary photo-induced processes that are related to AOPs. The data were collected from original literature and from review articles. The stated values refer either to the degradation of a substrate [ (-)] or to the formation of a specified product [ (+)]. 

By making the PL and EL measurements with the same device, the EL and PL data are compared from the same film and with the same electrodes. Using the device configuration also simplifies the determination of the ratio of quantum efficiencies. Ignoring (initially) possible differences in recombination zones for EL and PL, identical ratios are expected for the internal and external QE(el/ QE(pl), because of cancellation of the same loss factors in both EL and PL both are subjected to quenching by proximity to the metal electrode and to losses from internal reflection (and waveguiding), scattering and absorption). 

The importance of acid formation requires a determination of the efficiency of this process. We have adapted an indicator dye method (see Scheme 4) to the measurements of quantum yields for acid generation. The results obtained using several sensitizers and I(i) are summarized in Fig. 1. 

FIGURE 4.9. Electroluminescence spectra (along the cavity axis) from three representative cavity devices from the same substrate but with a terraced filler (as in Fig. 4.2). The anode is a 55-nm-thick ITO layer. The three spectra have major peaks at (a) 625 run, (b) 512 nm, and (c) 545 nm. The measured external quantum efficiencies (in units of photons/electron) are also indicated in each case. 

FIGURE 4.12. Electroluminescence spectra from a noncavity LED with an emissive layer consisting of Alq doped with 0.5% pyrromethene. Also shown is the EL spectrum of a LED with an Alq emissive layer. The spectra have been scaled so that the areas are proportional to the measured external quantum efficiencies. 

The objective of this exercise is to design meaningful experiments to investigate the potential use of HQS as a fluorimetric reagent for trace metal ions. Changes in luminescence properties that you can measure include quantum efficiency, wavelength maximum (Amax), and/or band shape of the emission. Keep in mind that many factors can affect the luminescence intensity of a sample. In a well-designed experiment, the effect that a particular variable (metal ion, temperature, pH, etc.) has on the HQS fluorescence intensity needs to be separated from other effects. 

Quantum yields are often useful quantitative measures of the efficiency of photochemical processes. Among the more traditional methods of quantum yield determination is ferrioxalate actinometry by UV-vis spectrophotometry. Since many organometallic complexes contain strong infrared chromophores, e.g. CEO, CENK, ve describe herein a method for determining quantum yields in Ca solution IK cells using a novel and simple actinometer the photochemical disappearance of tta lCO)in neat CCl to give Mn(CO)5CI (1). 

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