Raman experiment

The key for optimally extracting infonnation from these higher order Raman experiments is to use two time dimensions. This is completely analogous to standard two-dimensional NMR [136] or two-dimensional 4WM echoes. As in NMR, tire extra dimension gives infonnation on coherence transfer and the coupling between Raman modes (as opposed to spins in NMR). 

Schematic diagrams of modem experimental apparatus used for IR pump-probe by Payer and co-workers [50] and for IR-Raman experiments by Dlott and co-workers [39] are shown in figure C3.5.3. Ultrafast mid-IR pulse generation by optical parametric amplification (OPA) [71] will not discussed here. Single-colour IR pump-probe or vibrational echo experiments have been perfonned with OP As or free-electron lasers. Free-electron lasers use...

Por IR-Raman experiments, a mid-IR pump pulse from an OPA and a visible Raman probe pulse are used. The Raman probe is generated either by frequency doubling a solid-state laser which pumps the OPA [16], or by a two-colour OPA [39]. Transient anti-Stokes emission is detected with a monocliromator and photomultiplier [39], or a spectrograph and optical multichannel analyser [40]. 

With broad-band pulses, pumping and probing processes become more complicated. With a broad-bandwidth pulse it is easy to drive fundamental and overtone transitions simultaneously, generating a complicated population distribution which depends on details of pulse stmcture [75], Broad-band probe pulses may be unable to distinguish between fundamental and overtone transitions. For example in IR-Raman experiments with broad-band probe pulses, excitation of the first overtone of a transition appears as a fundamental excitation with twice the intensity, and excitation of a combination band Q -t or appears as excitation of the two fundamentals 1761. 

By using different catalysts and growth temperatures for the synthesis of ropes of SWCNTs, it is possible to obtain a different diameter distribution for SWCNT samples. At present, it is possible to vary the peak in the diameter distribution between 0.9 and 2.0 nm [7,27,29]. By carrying out Raman experiments on CNT samples with different diameter distributions, changes in the characteristics of the Raman spectra can be investigated. 

Figure 9-31. Raman speclra in m-LPPP waveguides obtained at dillerenl laser wavelengths as depicted in the figure. The bottom spectrum shows the result of a conventional cw Raman experiment -from Ref. 1147 (.

In-sltu Raman experiments were performed on a Spex 1401 double monochrometer Raman spectrometer, using a Spectra-Physlcs Model 165 argon Ion laser with an exciting wavelength of 5145 A. The In-sltu Raman cell consists of a quartz tube situated In a temperature controlled heating block. The Raman spectra were collected In the 180° backscatterlng mode. 

Raman experiments are confirmed by XPS and secondary ion mass spectrometry (SIMS) measurements performed by Thiine et al. [38] on a surface... 

It has been found that various material properties are thickness-dependent. Raman experiments show a dependence on the type of substrate (glass, c-Si, stainless steel, ITO on glass) and on the thickness (up to 1 /nm) of the films [392,393]. Recent transmission electron microscopy (TEM) results also show this [394]. This is in contrast to other results, where these effects are negligible for thicknesses larger than 10 nm [395, 396], as is also confirmed by ellipsometry [397] and IR absorption [398] studies. 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

The behavior of D2 in the Raman experiments is strongly correlated with the Q4 chemical shift, 6, in the NMR spectra. 6 equals about -110 to -111 ppm when D2 is absent or when it exhibits low relative intensities comparable to those in conventional vitreous silica, for example the 50 and 1050°C sample spectra and the rehydrated 600°C sample spectrum. From the regression equation cited above -110 to -111 ppm corresponds to - 147 to 149°, values quite close to the average in conventional v-Si02, 151° (4 ). The average 64 is shifted downfield to about -107 ppm in the 600°C sample in which D2 is observed to be quite intense. Deconvolution of this peak reveals two Q4 resonances at -110 and -105 ppm. -105 ppm corresponds to - 138°, which is very near the equilibrium 4> calculated for the isolated cyclic trisiloxane molecule, HgSi303, ( = 136.7°) (46). The positions of the Q2 and Q3 resonances, however, appear to be totally unaffected by the presence or absence of D2 (as shown in the 600°C CP MASS sample spectrum). 

Time-resolved IR spectra of similar peptides following a laser-excited temperature jump showed two relaxation times, unfolding 160 ns and faster components <10 ns (Williams et al., 1996). These times are very sensitive to the length, sequence, and environment of these peptides, but do show that the fundamental helix unfolding process is quite fast. These fast IR data have been contrasted with Raman and fluorescence-based T-jump experiments (Thompson et al., 1997). Raman experiments at various temperatures have suggested a folding in 1 /xs, based on an equilibrium analysis (Lednev et al., 2001). But all agree that the mechanism of helix formation is very fast. 

Finally, as in macro-Raman experiments, orientation-insensitive spectra can also be calculated for spectromicroscopy. A method has been developed recently for uniaxially oriented systems and successfully tested on high-density PE rods stretched to a draw ratio of 13 and on Bombyx mori cocoon silk fibers [65]. This method has been theoretically expanded to biaxial samples using the K2 Raman invariant and has proved to be useful to determine the molecular conformation in various polymer thin films [58]. 

A stress-induced alignment can also be detected in Raman experiments. The sensitivity of a vibration to the polarization of the incident and scattered light in a Raman experiment is determined by the polarizability tensor for the vibration. Even in the absence of polarization information, IR absorption or Raman measurements made in the presence of stress can be used to detect a preferential alignment of a defect by the effect the alignment has on the relative intensities of the stress-split-components of a vibrational band. 

In the Raman experiments with stress (Herrero and Stutzmann, 1988b) the suggested displacements under stress from the trigonal axis are not slight. Large displacements from the BC site are not in agreement with the picture of the B—H complex that most workers now accept. 

The IR and Raman experiments are sufficiently different to prevent comparison of data taken under identical conditions. The Raman experiments were performed at 100 K where the B—H center can reorient during the measurements while the IR experiments were done near 15 K where the complex is static. Much higher stresses were used in the Raman experiments. Also, the Raman experiments were performed under injection conditions (because of the incident laser light) whereas the IR experiments were not. To resolve the differences between the experiments, it would be helpful if both could be done under conditions that are as similar as possible, preferably at a temperature low enough to freeze in the orientation of the B—H complex to simplify the analysis. 

Raman scattering are generally different so information from Raman experiments in general supplements information that is obtainable from infrared absorption. Using lasers as light sources can greatly reduce experimental difficulties in this technique. At present it has become one of the standard tools for polymer analysis. 

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