Electron lifetime measurements

An important quantity in the application of electron pulses is the electron lifetime, Tgi, which is a measure of the impurity concentration present in the liquid. 

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The UV-visible absorption and emission spectra and excited state lifetimes of polymers are sensitive to chemical structure, polymer conformation and molecular environment and thus information concerning these properties is accessible by electronic spectroscopy measurements (4-6). One example of the application of such measurements is given in Figure 3 which illustrates the possible energy dissipation pathways which can occur in a polymer containing aromatic side groups following absorption of radiation. 

The subsequent development of laser diode sources at low cost, and improved electronic detection, coupled with new probe fabrication techniques have now opened up this field to higher-temperature measurement. This has resulted in an alexandrite fluorescence lifetime based fiber optic thermometer system,(38) with a visible laser diode as the excitation source which has achieved a measurement repeatability of l°C over the region from room temperature to 700°C, using the lifetime measurement technique. 

In a third method for lifetime measurements, the levels to be investigated are excited by a short pulse and subsequently the number of fluorescence quanta is counted as a function of the delay time (e.g. with a multichannel analyzer and a time-to-pulse height converter). The experimental procedure is described by Bennet 2 and has been perfected by several authors using light pulses 22) or electron pulses 23) for excitation. 

In his article mainly mode-locked tunable dye lasers are discussed. Giant pulse ruby lasers (3 nsec pulse halfwidth) have been successfully used to probe electron densities as a function of time in a rapidly expanding plasma 22). The electron lifetime in the conduction band can be determined with nanosecond semiconductor lasers. By absorption of the laser pulse the electrons in the semiconductor probe are excited into the conduction band, resulting in a definite conductivity. The mean lifetime is obtained by measuring the decrease of conductivity with time 26). 

Figure 12. Simple, purely electronic, fluorescent-lifetime-measuring apparatus.

In another study, Kondrat eva (103) made a determination of the luminescent quantum yield of the 5D4 state of the terbium ion in aqueous solution. The method used was based upon fluorescent-lifetime measurements and had previously been used by Rinck (96) and Geisler and Hellwege (96) to determine the quantum yield of rare earths in crystals. Kondrat eva made his studies on chloride and sulfate solutions, using the electronic shutter technique of Steinhaus et al. (66). 

Electronic Absorption and Luminescence (Volume 12) Absorption and Luminescence Probes Fluorescence Imaging Microscopy Fluorescence Lifetime Measurements, Applications of Indirect Detection Methods in Capillary Electrophoresis Surface Measurements using Absorption/Luminescence... 

Fluorescence resonance energy transfer (FRET) is a spectroscopic means of obtaining distance information over a range up to 80 A in solution. It is based on the dipolar coupling between the electronic transition moments of a donor and acceptor fluorophore attached at known positions on the RNA species of interest. It can be applied in ensembles of molecules, either by steady-state fluorescence or by lifetime measurements, but it is also very appropriate for single-molecule studies. In addition to the provision of distance information, recent studies have emphasized the orientation dependence of energy transfer. 

Staab and coworkers have prepared stacked Q-P-Q triad 28 [80-83]. An X-ray structure determination of the molecule shows that the quinones are situated directly above and below the plane of the porphyrin, with their planes parallel to the porphyrin plane and 3.4 A from it. Steady state fluorescence measurements demonstrate strong quenching of the porphyrin first excited singlet state, which in turn suggests rapid electron transfer. Additional data were obtained from fluorescence lifetime measurements [83]. In dichloromethane, for instance, the lifetime of an analog of 28 in which the quinones were replaced with redox inactive dimethoxyphenyl substituents was 9.0 ns. In 28, this lifetime was reduced to 2 ) ps. 

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

For supramolecular assemblies, intramolecular processes may quench the emission of A to a degree which depends on the relative efficiency of the process when compared with emission. It is often useful to compare the photophysical and chemical behavior of the supramolecular species, e.g. A -L-B, with an appropriate model compound, for instance, AH, which contains the photochemically active component, A, in the absence of any units capable of interacting with A. For example, from luminescence lifetime measurements, the rate of electron transfer may be estimated by comparing the excited-state lifetime of the mononuclear model complex, tModei, with that of the supramolecular species, rsupra, by using the following equation ... 

Precise measurements on g factors of electrons bound in atomic Hydrogen and the Helium ion 4He+ were carried out by Robinson and coworkers. The accuracies of 3 x 10-8 for the Hydrogen atom [5] and of 6 x 10-7 for the Helium ion [6] were sensitive to relativistic effects. Other measurements of the magnetic moment of the electron in Hydrogen-like ions were performed at GSI by Seelig et al. for Lead (207Pb81+) [7] and by Winter et al. for Bismuth (209Bi82+) [8] with precisions of about 10-3 via lifetime measurements of hyperfine transitions. These measurements were also only sensitive to the relativistic contributions. 

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