Time-resolved stroboscopic

A novel data analysis procedure is described, based on a variational solution of the Schrddinger equation, that can be used to analyze gas electron diffraction (GED) data obtained from molecular ensembles in nonequilibrium (non-Boltzmann) vibrational distributions. The method replaces the conventional expression used in GED studies, which is restricted to molecules with small-amplitude vibrations in equilibrium distributions, and is important in time-resolved (stroboscopic) GED, a new tool developed to study the nuclear dynamics of laser-excited molecules. As an example, the new formalism has been used to investigate the structural and vibrational kinetics of C=S, using stroboscopic GED data recorded during the first 120 ns following the 193 nm photodissociation of CS2. Temporal changes of vibrational population are observed, which can... 

In most early reports of time-resolved stroboscopic FT-IR spectrometry, data were not acquired with a clock. Instead, the ADC was triggered at each zero crossing after the one at which the reaction was initiated. In this case, the time resolution was equal to (/i,) seconds. Thus, if the frequency of the laser interferogram was... 

Wulff M, Schotte F, Naylor G, Bourgeois D, Moffat K, Mourou G (1997) Time-resolved stmctures of macromolecules at the ESRF single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis. Nucl Instr Meth Phys Res A 398 69-84... 

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. 

B. Lerner and M. Daun, Time Resolved Spectroscopy Stroboscopic and Step Scan Methods, Nicolet FTIR Technical Note TN-9252, Nicolet Instrument, Madison, 1992. 

If any change occurs in a crystal, the diffraction data may be changed. It is possible to detect the time-resolved diffraction data by the stroboscopic method. However, any strucmral change cannot be obtained from the diffraction data using Fourier transform technique before the change extends to all the unit cells and the periodic lattice structure is reconstructed. There is an essential difference between... 

An alternative technique for time-resolved infrared measurements on a rapidscanning FT-IR spectrometer that not only overcomes the limitations of stroboscopic spectroscopy described in Section 19.3 but under certain circumstances appears to have better time resolution than measurements made on a step-scan interferometer has been developed by Masutani et al. [22-25]. In this technique, the sample perturbation is not timed to coincide with the scanning and data acquisition of the spectrometer, (i.e., the two are asynchronous). The basic instrument used for asynchronous time-resolved FT-IR spectrometry is a standard rapid-scanning FT-IR spectrometer to which is added a boxcar integrator and some timing circuitry. The instrumental setup is shown in Figure 19.8. 

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. 

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