Coherent Raman techniques

An alternate approach is to perform coherent Raman spectroscopy in the time domain rather than in the frequency domain. In this case, a single laser that produces short pulses with sufficient bandwidth to excite all of the Raman modes of interest is employed. One pulse or one pair of time-coincident pulses is used to initiate coherent motion of the intermolecular modes. The time dependence of this coherence is then monitored by another laser pulse, whose timing can be varied to map out the Raman free-induction decay (FID). It should be stressed at this point that the information contained in the Raman FID is identical to that in a low-frequency Raman spectrum and that the two types of data can be interconverted by a straightforward Fourier-transform procedure (12-14). Thus, whether a frequency-domain or a time-domain coherent Raman technique should be employed to study a particular system depends only on practical experimental considerations. 

From an experimental point of view, it is quite evident that for the four nonlinear coherent Raman techniques discussed until now, one either measures the radiation generated at anti-Stokes frequency (CARS, ll)as = 2ui-cvs) or at Stokes frequency (CSRS, 2cJs - or one determines the change AS in the laser beam power (o/ z, IRS uJs -SRGS). In order to get full Raman information of the medium, it is necessary to tune the frequency difference ojl-ujs, then, successively all Raman-active vibrations (or rotations, or rotation-vibrations) will be excited and a complete nonlinear Raman spectrum is then obtained. 

The advantages of the two coherent Raman techniques, stimulated Raman gain (SRGS) and inverse Raman spectroscopy (IRS), have been described in detail in Secs. 3.6.1.3 and 3.6.2.3. Here, we present an instructive example for each technique emphasizing the high resolution capability of the.se methods. 

Natural ROA offers the interesting prospect of measuring optical activity in pure rotational transitions of gas phase chiral molecules. Although such observations have not yet been reported, the detailed theory of rotational ROA in chiral symmetric tops has been published lS7, and the experiment should be feasible using existing technology such as optical multichannel detection. It is also possible that one of the coherent Raman techniques discussed below could be advantageous. 

Laser techniques have a great potential for studies of microscopic as well as macroscopic combustion in flames and engines. Combustion diagnostics with lasers was discussed in several reviews [10.8-13]. We will here give examples of measurements of concentrations and temperature (flame kinetics) using fluorescence, Raman and coherent Raman techniques. In practical combustion systems turbulence is extremely important and we will also briefly discuss laser techniques for flow and turbulence measurements. 

Gomez J S 1996 Coherent Raman spectroscopy Modern Techniques in Raman Spectroscopy ed J J Laserna (New York Wiley) pp 305-42... 

Chang T-C and DIott D D 1988 Picosecond vibrational cooling in mixed molecular crystals studied with a new coherent Raman scattering technique Chem. Phys. Lett. 147 18-24... 

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

In impulsive multidimensional (1VD) Raman spectroscopy a sample is excited by a train of N pairs of optical pulses, which prepare a wavepacket of quantum states. This wavepacket is probed by the scattering of the probe pulse. The electronically off-resonant pulses interact with the electronic polarizability, which depends parametrically on the vibrational coordinates (19), and the signal is related to the 2N + I order nonlinear response (18). Seventh-order three-dimensional (3D) coherent Raman scattering, technique has been proposed by Loring and Mukamel (20) and reported in Refs. 12 and 21. Fifth-order two-dimensional (2D) Raman spectroscopy, proposed later by Tanimura and Mukamel (22), had triggered extensive experimental (23-28) and theoretical (13,25,29-38) activity. Raman techniques have been reviewed recently (12,13) and will not be discussed here. 

If coherent radiation with a very high intensity is applied continuously or as pulse, non-linear effects can be observed which produce coherent Raman radiation. This is due to the quadratic and cubic terms of Eq. 2.4-14, which describe the dipole moment of a molecule induced by an electric field. Non-linear Raman spectroscopy and its application are described in separate chapters (Secs. 3.6 and 6.1), since this technique is quite different from that of the classical Raman effect and it differs considerably in its scope. 

Figure 3.6-4 Schematic diagram for a few techniques in nonlinear (coherent) Raman spectroscopy (CSRS Coherent Stokes Raman Spectroscopy SRGS Stimulated Raman Gain Spectroscopy IRS Inverse Raman Spectroscopy (= SRLS Stimulated Raman Loss Spectroscopy) CARS Coherent anti-Stokes Raman Spectroscopy PARS Photoacoustic Raman Spectroscopy).

Nonlinear vibrational spectroscopy provides accessibility to a range of vibrational information that is hardly obtainable from conventional linear spectroscopy. Recent progress in the pulsed laser technology has made the nonlinear Raman effect a widely applicable analytical method. In this chapter, two types of nonlinear Raman techniques, hyper-Raman scattering (HRS) spectroscopy and time-frequency two-dimensional broadband coherent anti-Stokes Raman scattering (2D-CARS) spectroscopy, are applied for characterizing carbon nanomaterials. The former is used as an alternative for IR spectroscopy. The latter is useful for studying dynamics of nanomaterials. 

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