Stroboscopic measurements

Peterson and Bridenbaugh (122) made measurements on calcium tungstate Nd and charge compensated with sodium. The crystals were provided by Nassau and were of laser quality. Using a stroboscopic-measuring technique at room temperature, they found exactly the same lifetime for both the 4 3/2 >4 9/2 an[Pg.258]

One of the most interesting applications of Femtochemistry is the stroboscopic measuring of observables related to molecular motion, for instance the vibrational periods or the breaking of a bond [1], Because femtosecond laser fields are broadband, a wave packet is created by the coherent excitation of many vibrational states, which subsequently evolves in the electronic potential following mostly a classical trajectory. This behavior is to be contrasted to narrow band selective excitation, where perhaps only two (the initial and the final) states participate in the superposition, following typically a very non-classical evolution. In this case one usually is not interested in the evolution of other observables than the populations. 

In step-scan mode, the moving mirror of the interferometer is stopped at each data acquisition point and held for some time (seconds to minutes) during which data are acquired. In step-scan mode the collected interferograms contain the same information as in continuous-scan mode, only the time required for the complete experiment is much longer. Under stroboscopic measuring conditions, a time resolution of 100 ns can be achieved. This technique can be applied to processes which can repeatedly be started under highly reproducible conditions. The step-scan technique can also be applied for the acquisition of voluminous data. This... 

This is commonly done by a stroboscopic measurement setup comprising a microscope-mounted CCD camera with a short exposure time and a flashlight which are synchronized to the rotational motion (Fig. 15). Regarding simulation, the majority of commercially available computational fluid dynamic (CFD) packages is able to cope with rotating channels. 

Rapid reversible processes can be studied by FT-IR spectrometry in at least four ways, two using rapid-scan interferometers and two using step-scan interferometers. Three of these approaches, asynchronous sampling and stroboscopic measurements with a rapid-scan interferometer and time-resolved spectroscopy with a step-scan interferometer, were described in Sections 19.2 and 19.3. The fourth approach involves the use of a step-scan interferometer and some type of sample modulation. We have seen one application in the earlier part of this chapter, and two other applications will now be described. The reorientation of liquid crystals induced by rapid switching of the electric field to which they are being subjected has been studied by at least three of these approaches. Results have been summarized in an excellent article by Czamecki [17]. In this section we discuss the application of sample-modulation FT-IR spectrometry to this problem. 

The dynamics of reactions are either reversible or irreversible. Sufficient counting statistics can be obtained in the reversible processes through stroboscopic measurement where repeated measurements using short radiation time sUces are performed. In the irreversible experiments the processes have to be followed by rapid consecutive exposures. 

Fig. 9.2.6. Stroboscopic measurements of core precessional frequency for cyclone illustrated...

The first experimental measurements of the time dependence of the hydrated electron yield were due to Wolff et al. (1973) and Hunt et al. (1973). They used the stroboscopic pulse radiolysis (SPR) technique, which allowed them to interpret the yield during the interval (30-350 ps) between fine structures of the microwave pulse envelope (1-10 ns). These observations were quickly supported by the work of Jonah et al. (1973), who used the subharmonic pre-buncher technique to generate very short pulses of 50-ps duration. Allowing... 

The results in this discussion will include those from our laboratory and experiments on electron solvation from other laboratories. The experiments that were done at Argonne made use of the stroboscopic pulse radiolysis technique, which will be discussed below. Experiments from other laboratories have made use of pulse radiolysis and laser photolyis techniques for the measurement of electron solvation. 

The stroboscopic pulse radiolysis with the single bunch electron pulse instead of pulse trains started in Argonne National Laboratory in 1975 [54]. The research fields have been extended by the stroboscopic pulse radiolysis with the picosecond single electron bunch, although most of researches had been limited to hydrated and solvated electrons in the aqueous and alcoholic solutions. This system was unable to study the kinetics of the geminate ion recombination in liquid hydrocarbons until the modification of the Argonne linac in 1983, which made possible the quality measurements of the weak absorption. 

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. 

As far as the controls are concerned, we here consider time-continuous modulation of the system Hamiltonian, which allows for vastly more freedom compared to control that is restricted to stroboscopic pulses as in DD [42, 55, 91]. We do not rely on rapidly changing control fields that are required to approximate stroboscopic a -pulses. These features allow efficient optimization under energy constraint. On the other hand, the generation of a sequence of well-defined pulses may be preferable experimentally. We may choose the pulse timings and/or areas as continuous control parameters and optimize them with respect to a given bath spectrum. Hence, our approach encompasses both pulsed and continuous modulation as special cases. The same approach can also be applied to map out the bath spectrum by measuring the coherence decay rate for a narrow-band modulation centered at different frequencies [117]. 

Converse flexoelectric effects (i.e. voltage-generated curving) have been demonstrated in uranyl-acetate-stabilized phosphatidylserine BLMs by real-time stroboscopic interferometric measurements the obtained satisfactory agreement between the converse and the direct (i.e. curvature-generated voltage) flexoelectric coefficients have been in accord with the Maxwell relationship [8]. 

Bennett (64) was able to increase greatly the time resolution of a stroboscopic instrument. With his apparatus he was successful in measuring lifetimes of the order of a few millimicroseconds. The application of such high temporal resolution equipment to rare earths has not been made as yet. There is good reason to believe, however, that certain nonradiative processes in rare earths are in this range. 

Steinhaus and colleagues (66) have described another version of a stroboscopic instrument that is useful for routine lifetime measurements. They point out that switching a photomultiplier tube on and off by removal and reapplication of the high voltage may at times yield troublesome transient problems. 

A simplified instrument for the measurement of fluorescent lifetimes using the stroboscopic method has been described by Brown (67). The major virtue of this system is that it makes use of a Tektronix oscilloscope to obtain all the necessary trigger pulses, including a trigger of continuously variable delay. Since most laboratories are equipped with a good oscilloscope, the need to purchase expensive trigger-delay apparatus is thus eliminated. 

Rieke and Allison (97) studied the fluorescent lifetime of terbium in the chelate terbium trianthranilate (TbAn3). They concluded that the spectra and fluorescent lifetime of the chelate differ markedly from unchelated terbium compounds. Fluorescent-lifetime measurements were made at 25°C, 0°C, and 77°K using a stroboscopic light source with a decay time of 20 /xsec and a comparison was made with TbCl3 4H20. At all... 

Bhaumik (148) measured the rise time of the red fluorescence in some europium chelates. His data were collected with a stroboscopic instrument having a time-resolution capability of around 0.2 /xsec. Figure 40 shows the fluorescence-rise curve of the sDq- >1F2 transition in the piperidine adduct of the four-ligand europium dibenzoylmethide chelate. Data were collected at 77°K. 

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. 

A technique for distinguishing between phase-locked and quasi-periodic responses, and which is particularly useful when m and n are large numbers, is that of the stroboscopic map. This is essentially a special case of the Poincare map discussed in the appendix of chapter 5. Instead of taking the whole time series 0p(r), for all t, we ask only for the value of this concentration at the end of each forcing period. Thus at times t = 2kn/a>, with k = 1, 2, etc., we measure the surface concentrations of one of our species. If the system is phase locked on to a closed path with a>/a>0 = m/n, then the stroboscopic map will show the measured values moving in a sequence between m points, as in Fig. 13.12(a). If the system is quasi-periodic, the iterates of 0p will never repeat and, eventually, will draw out a closed cycle (Fig. 13.12(b)) in the... 

The flame is not, however, a discontinuity. There are definite gradients of temperature and composition because of conduction of heat and diffusion of reaction products into the fresh gas. The temperature gradients have been studied by three techniques direct measurement with very fine thermocouples (24, 25, 43) refraction of a narrow slit of light (11, 16) and tracing the path of a stroboscopically lighted dust particle and computing temperature from its direction and velocity (l, 27). 

Power measurements require the measurement of the torque and the rotational speed. The torque can be measured using a torsion shaft with strain gauges, electrically with eddy-current torque transducers, or mechanically with a swiveling motor. The speed of rotation can be measured using mechanical, electrical (photocell), or optical (stroboscope) instruments. 

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