Normal emission, quantum efficiencies

Table I. Ratio of Quantum Efficiencies of Excimer and Normal Emission in Acenaphthylene Homopolymer and Equimolar Copolymers Comonomer IdZIM—...

In ethanol, the 31,850 cm maximum (A) is reduced to a shoulder near 33,000 cm. Whereas in heptane a normal "Weller shift" of fluorescence indicating Intramolecular proton transfer is observed, in alcohols we note a fluorescence band at higher frequency with a maximum at 23,800 cm. The latter is due to the anion (D) and corresponds to the absorption maximum of D at 29,400 cm The intensity of this emission increases with increasing pH and remains constant above pH = 9 (in ethanol). The anilide of 2-methoxy benzoic acid shows a ultraviolet fluorescence that was too weak to be recorded (21). Low quantum efficiency may, therefore, be the reason that the fluorescence of C has not been detected. The only evidence of the presence of C is the close similarity of the absorption spectra of SAN and the methoxy derivative in ethanol (21). 

In summary, the two main advantages of fluorescence analysis are that it is capable of measuring much lower concentrations than spectrophotometric analysis (high sensitivity), and that it is potentially more selective because both the excitation and emission wavelengths can be varied. At its best, fluorometric analysis is sensitive to 10 to 10 Af, depending on the intensity of the source and the quantum efficiency and molar absorptivity of the sample. Where the molar absorptivity or quantum efficiency are small, the source or monochromator can be adjusted to make analysis possible. Such an adjustment is normally not done in absorption spectrophotometry. 

Chemical interference effects in atomic fluorescence are similar to those observed in atomic absorption spectroscopy. In addition, any process that affects the quantum efficiency of the fluorescence or disrupts normal emission of the energy of the excited state also can be considered as a chemical interference. 

Lab-on-a-Chip Devices for Chemical Analysis, Fig. 12 (a) Microchip layout. The inlets are 400 pm wide, 800 pm deep, and 10 mm long. The mixing channel is 800 pm wide, 800 pm deep, and 520 mm long. The active area of the photodiode used for chemiluminescence detection is 1 x 1 mm. The photodiode is located at a position 10 mm downstream from the point of confluence of the two inlet streams, (b) Quantum efficiency spectrum of the organic photodiode and the normalized emission spectra for the two chemiluminescent dyes used in the work (cyalume blue and cyalume green). The emission spectra of both dyes overlap the spectral response of the photodiode (Reprinted with permission from [20])... 

Ta = life time of luminescent state of S q = quantum efficiency Zja = distance between centres S and A n = dielectric constant of the host lattice f,(E) = normalized emission band /a(E) = normalized absorption band Oa = integrated absorption of A E = photon energy Z = overlap integral r, = position coordinate of an electron a = lattice spacing ti, = average hopping time c.t. = charge transfer Ar = expansion of luminescent center... 

The operating principle of this type of laser is rather simple and is illustrated in Fig. 1.1. The incoming radiation is produced by an infrared laser, usually a CO2 laser, and is absorbed on vibrational-rotational transitions in a molecular vapour. The vapour is thereby excited to states normally almost totally empty. The resulting population inversions provide several possibilities for laser emission, and as shown in Fig. 1.1 these can be either in the upper or lower vibrational level. It may be reasoned that, since stimulated absorption and emission have equal probabilities, the maximum quantum efficiency for the optically pumped laser is 0.5 in practice the value is seldom as high as 0.1. In terms of energy, the efficiency is lower, of course, by the ratio of the far-inffared to the infrared wavelengths, not to mention the efficiency of the infrared pump laser. Nevertheless the cleanness and simplicity of the system make it very attractive. 

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