Time-Resolved Scattering Experiments

The above discussion represents a necessarily brief simnnary of the aspects of chemical reaction dynamics. The theoretical focus of tliis field is concerned with the development of accurate potential energy surfaces and the calculation of scattering dynamics on these surfaces. Experimentally, much effort has been devoted to developing complementary asymptotic techniques for product characterization and frequency- and time-resolved teclmiques to study transition-state spectroscopy and dynamics. It is instructive to see what can be accomplished with all of these capabilities. Of all the benclunark reactions mentioned in section A3.7.2. the reaction F + H2 —> HE + H represents the best example of how theory and experiment can converge to yield a fairly complete picture of the dynamics of a chemical reaction. Thus, the remainder of this chapter focuses on this reaction as a case study in reaction dynamics. 

With the advent of short pulsed lasers, investigators were able to perfonn time resolved coherent Raman scattering. In contrast to using femtosecond pulses whose spectral widtii provides the two colours needed to produce Raman coherences, discussed above, here we consider pulses having two distinct centre frequencies whose difference drives the coherence. Since the 1970s, picosecond lasers have been employed for this purpose [113. 114], and since the late 1980s femtosecond pulses have also been used [115]. Flere we shall briefly focus on the two-colour femtosecond pulsed experiments since they and the picosecond experiments are very similar in concept. 

Limits of Time-Resolved and Simultaneous Measurements. Structure evolution studies are based on the ability to carry out time-resolved scattering experiments. The power of this scattering technique is a function of the minimum cycle time during which a scattering pattern with sufficient signal-to-noise ratio can be recorded. As cycle times for anisotropic 2D SAXS patterns have fallen below a value... 

Automated Extraction of Interference Functions. For the classical synthetic polymer materials it is, in general, possible to strip the interference function from the scattering data by an algorithm that does not require user intervention. Quantitative information on the non-topological parameters is lost (Stribeck [26,153]). The method is particularly useful if extensive data sets from time-resolved experiments of nanostructure evolution must be processed. Background ideas and references are presented in the sequel. 

It is clear that the unmistakable resonance fingerprint provided by a narrow Lorentzian peak in the integral cross section (ICS) will be rare for reactive resonances in a collision experiment. However, a fully resolved scattering experiment provides a wealth of data concerning the reaction dynamics. We expect that the state-to-state differential cross sections (DCS) as functions of energy can be analyzed, using various methods, to reveal the presence of reactive resonances. In the following subsections, we discuss how various collision observables are influenced by existence of a complex intermediate. Many of the resonance detection schemes that have been proposed, such as the use of collision time delay, are purely theoretical in that the observations required are not currently feasible in the laboratory. Nevertheless, these ideas are also discussed since it is useful to have method available... 

We illustrate the use of multichannel single-ionization scattering states for the interpretation of time-resolved experiments in the case of the attosecond-XUV-pump IR-probe attosecond interferometric spectroscopy of the doubly... 

Because of the short measurement times of stopped flow, or the small sample volumes associated with continuous flow, a synchrotron source is required for time-resolved SAXS studies of RNA folding. To date studies have been carried out at the APS, at CHESS, and at SSRL. The optics vary by beamline, but all involve intensity enhancement by an insertion device. Monochromatic undulator beam was employed for stopped flow experiments at the 12-ID station at APS (Seifert etal, 2000). Multilayer beam was employed at SSRL beamline 4-2 (Tsuruta et al, 1998), and focused multilayer beam was employed at the CHESS G1 station (Kazimirov et al., 2006). For the continuous flow cell, pink or 3% bandwidth undulator beam was employed at the 8-ID beamline at APS (Sandy et al., 1999). All experiments employed a CCD detector to record a 2D image of the scattering, as illustrated in Fig. 12.1. 

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