Stroboscope technique

Merely as a result of software changes, the stroboscope method with our experimental setup afforded a time resolution of a few microseconds (Souvignier and Gerwert, 1992). 

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Use of stroboscopic techniques and carefully designed apparatus can make this sort of method very accurate. 

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

Peterson and Bridenbaugh (92) observed a distinct rise and decay in fluorescence from the 5D4 state of terbium in the compound KLa0.9g-Tbo.o2W208 (Fig. 21). Data were collected at room temperature using a stroboscopic technique. Excitation in the form of 5 /usee bursts was applied simultaneously to the 57)3 state and to a few higher levels. The conclusion again was that there was slow internal conversion from the 5Z)3 to the... 

The sensitization of terbium by gadolinium and a study of the kinetics of the exchange process have been reported by Pearson and Peterson (98, 100). Experiments were conducted at room temperature in a doped Calibo glass using a stroboscopic technique. 

Lead by some earlier work on terbium (54), Peterson and Bridenbaugh (109) examined the effects of rare-earth-ion substitution upon the neodymium -state mean life. The series of compounds studied was Na0.5Gdo.46Ndo.02 rare earth0 02WO4. All data were collected at room temperature using a stroboscopic technique. Figure 28 shows the result of this analysis. 

The study by Bhaumik et al. (63) of the time-resolved spectra of europium emissions from the same compound is quite interesting. Their data were taken on microcrystals at 77°K, again using a stroboscopic technique. Figure 41 shows the result in which excitation was again applied to the organic part of the molecule. 

Glasses and Liquids. Gallagher and co-workers (152) examined in some detail the absorption, fluorescence spectra, and fluorescent lifetimes of trivalent europium in a variety of borate glasses. All data were taken at room temperature, and attempts were made to correlate the emission characteristics with various europium-glass interactions. The fluorescentlifetime measurements were made using a stroboscopic technique. 

Another aspect of pulse radiolysis which has been improved is the pulse duration. For most experiments of interest to the physical organic chemist the common machines with pulse durations of 10 7-10-5 s are quite satisfactory, though for certain reactions, such as those involving protonation, examination on a shorter time scale can be of value. Several accelerators which supply nanosecond pulses are currently in use, but they are employed mostly with microsecond detection systems. Work in the 10-12-10-1° s region has recently become possible by the stroboscopic technique utilizing the fine structure pulses from a linear accelerator (Bronskill et al., 1970). More recently, a system which produces a single pulse of 40 picoseconds has been constructed (Ramler et al., 1975) and utilized for the observation of hydrated electrons at very short times (Jonah et al., 1973). 

A final requirement arises if the stroboscopic technique is to be used, where the experiment is repeated many times and the resulting data summed to provide enough photon statistics. In this case the detector must be capable of being synchronised to the experiment. 

The chapter begins with the fundamental measurements of resistance, capacitance, charge, and particle force. We proceed with flow measurements with various probes followed by a listing of some commercial electrostatic instruments. Nonelectrosatic measurements in multiphase flow such as the laser-Doppler anemometer, radioactive tracers, and stroboscopic techniques (Polaskowski, et. al, 1995 Soo, 1982) have not been discussed unless in relation to an electrostatic effect. 

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