Lasers Raman experiments

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]. 

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. 

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. 

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. 

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. 

The first paper on the laser Raman spectrum of a polymer appeared in January 1967 and described experiments on isotactic polypropylene pellets (9). The sample was illuminated from above with a... 

Raman spectra are naturally closely related to the photodissociation cross sections for vibrationally excited parent molecules. The latter contain, without any doubt, more details about the potential energy surfaces in the lower as well as the upper states, but on the other hand, they are more difficult to measure. Compared to the experiments described in Chapter 13, which requires three lasers, Raman spectra are rather cheap to obtain. 

Figure 4,14 (a) Experimental Raman spectra of PNA in several solvents of increasing polarity. The wavelength of the laser used in the Raman experiment is also reported. The uppermost spectrum is a simulation from B3LYP/6 — 311 + +G calculation (frequency scaled by 0.98). (b) a detailed plot of the NQ2 stretching doublet. 

Eckbreth, A. C. "Laser Raman Thermometry Experiments in Simulated Combustor Environments" AIAA Paper No. 76-27, 1976. 

The hydroxyl concentration profile for a stoichiometric CH -air flame is presented in Figure 8. Here the maximum mole fraction observed and the predicted mole fraction are equal to better than 10% accuracy. The abscissas of the theoretical and the experimental results were matched by setting the theoretically predicted temperature equal to the measured hydroxyl rotational temperature. At all positions in the flame the hydroxyl 2j[(v,=o) state exhibited a Boltzmann distribution of rotational states. This rotational temperature is equal to the N2 vibrational temperature to within the +100 K precision of the laser induced fluorescence and laser Raman scattering experiments. An example of this comparison is given in Figure 9. 

Schreiber (6i) pointed out the usefulness of single-pulse CARS for combustion work. The apparatus described above can be easily adapted to perform a number of coherent Raman experiments with single 20 ns (FWHM) laser pulses. Following are examples of the application of single-pulse RIKES and IRS, first to static solutions, then to Xenon-lamp irradiated solutions. 

---