Fast Kinetic of W(CO)5(Xe). TRS measurement on nanosecond (ns) time scales has also been successfully conducted on a research-grade Nicolet FT-IR spectrometer. As illustrated in Figure 4.7, W(CO)5(Xe) was generated in situ from W(CO)6 by a 308nm laser pulse in a supercritical Xe and CO solution at room temperature. The lifetime of this reaction is ~100ns, thus the spectra can be measured by FT-IR only in the step-scan mode. The difference spectrum is a 10ns time slice of the TRS spectra from a 4-scan data collection, clearly indicating a decrease and increase in concentration of the reactant and product, respectively.

4.4.3 Liquid Crystal Dynamics
Liquid crystal ($$lc) dynamics can be studied by using either the sample modulation technique where an alternating electric field is the source of perturbation, or TRS where molecular responses to the change of electric field can be monitored over time. In the sample modulation $$lc experiment, ideally an instantly responding reference sample should be used to set the experimental phase. In TRS $$lc experiment, such reference is not required as the sequence of events is recorded precisely as a function of time.

As its name implies, $$lc’s possess some of the characteristics of both liquids (mobility) and crystals (molecular orientations). A typical molecular structure of a $$lc contains a rigid part and a long flexible tail such as 4-cyabo-4-pentyl-biphenyl (5CB) shown below:

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$$lc molecules align themselves along the appropriately treated electrode surfaces. A typical $$lc cell structure is shown in Figure 4.8. The sample preparation follows these procedure: 1) Apply 0.1% aqueous PVA solution to the Ge windows; 2) Rub residual PVA in a uniaxial direction; 3) Place and align Ge windows face to face; and 4) Inject $$lc sample from the top gap between the two windows spaced by the Teflon.

The orientation of the $$lc changes when an electric field I applied across the material. This is the basis for $$lc displays used in devices such as watches, cars, and notebook computers, and flat screen TVs.

Studies of the reorientation dynamics of LCs were among the first step-scan TRS applications because of the ability to monitor the reorientation dynamics of different parts of the molecule. For most nematic LCs, the molecular orientation change (switching) typically occurs on millisecond (ms) time scales. The actual dynamics depends on the experimental conditions such as the strength of the electric field, sample thickness, and temperature. For some ferroelectric LCs, the dynamics can occur on the microsecond time scale. As stated earlier, these relatively slow experiments that occur on ms to μs time scales can be conducted with the internal digitizer of the spectrometer. The time sequence, experimental setup, and some TRS data for 5-CB are presented in Figure 4.9.

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4.4.4 Polymer Stretching
Polymer stretching experiments are normally conducted with the sample modulation as discussed in Section 3.4 of Chapter3, where a sinusoidal modulation is applied on the sample. Polymer stretching can also be studied by S2TRS where a short mechanical pulse, a sinusoidal or square wave can be applied to the sample. Since in the S2TRS mode, the spectral changes are measured as an explicit function of time, detailed spectral information of a polymeric sample under mechanical strain can be obtained to understand the chemistry responsible for the rheological properties of the sample.

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