Spontaneous Raman scattering measurements

Corresponding lineshape data of the isotropic component of spontaneous Raman scattering are presented in Fig. 8b. Knowing T2 within 30 fs (Fig. 5), the measured lineshape (experimental points) is readily plotted versus normalized frequency units Aco x T2. Comparison with the computed lineshapes [Fourier transformation of Equation (9)] leads to 350 100 fs for rc, in good agreement with the time-domain results. Combining the measurements, one arrives at rc = 330 70 fs (45). 

Kreutner et al., 1983, 1987 Stephenson and Blint, 1979). A review of recent progress in this field has been provided by Eckbreth (1988) and Laurendeau (1988). Spontaneous Raman scattering lends itself to the measurement of concentrations of species above a few percent and to the determination of temperature. It can therefore also be used to investigate mixing processes (Leipertz, 1981b, 1981c). 

Since in densely populated industrial areas air and water pollution has become a serious problem, the study of pollutants and their reactions with natural components of our environment is urgently needed [15.75]. Various techniques of laser spectroscopy have been successfully employed in atmospheric and environmental research direct absorption measurements, laser-induced fluorescence techniques, photoacoustic detection, spontaneous Raman scattering and CARS (Chap. 8), resonant two-photon ionization, and many more of the sensitive detection techniques discussed in Chap. 6 can be applied to various environmental problems. This section illustrates the potential of laser spectroscopy in this field by some examples. 

Very short vibrational relaxation times of molecules in liquids or in gases at high pressures can be studied with picosecond techniques (see Chap. 11). The excitation of vibrational levels may be performed by stimulated Raman scattering (see Sect.9.3) and the time evolution of the excited state can be monitored by spontaneous Raman scattering. This method allows direct measurements of vibrational relaxation times of large molecules, such as long-chain alcohols [12.21]. 

Hydrogen is the most abundant chemical element in the universe, and in its various atomic and molecular forms furnishes a sensitive test of all of experimental, theoretical and computational methods. Vibration-rotational spectra of dihydrogen in six isotopic variants constituting all binary combinations of H, D and T have nevertheless been recorded in Raman scattering, in either spontaneous or coherent processes, and spectra of HD have been recorded in absorption. Despite the widely variable precision of these measurements, the quality of some data for small values of vibrational quantum number is still superior to that of data from electronic spectra [106], almost necessarily measured in the ultraviolet region with its concomitant large widths of spectral lines. After collecting 420... 

Similar difference measurements can be made using resonance-enhanced or spontaneous Raman spectroscopy. In Raman spectroscopy, in general, the signal from an aqueous solvent is not large (water is a very weak Raman scatterer). 

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