Applications of Laser Raman Spectroscopy

Both linear and nonlinear Raman spectroscopy can be combined with time-resolved detection techniques when pumping with short laser pulses [8.781. Since Raman spectroscopy allows the determination of molecular parameters from measurements of frequencies and populations of vibrational and rotational energy levels, time-resolved techniques give information on energy transfer between vibrational levels or on structural changes of short-lived intermediate species in chemical reactions. One example is the vibrational excitation of molecules in liquids and the collisional energy transfer from the excited vibrational modes into other levels or into translational energy of the collision partners. These processes proceed on picosecond to femtosecond time scales [8.77,8.79]. 

Time-resolved Raman spectroscopy has proved to be a very useful tool to elucidate fast processes in biological molecules, for instance, to follow the fast structural changes during the visual process where, after photoexcitation of rhodopsin molecules, a sequence of energy transfer processes involving isomerization and proton transfer takes place. This subject is treated in more detail in Chap. 11 in comparison with other time-resolved techniques. 

The primary object of Raman spectroscopy is the determination of molecular energy levels and transition probabilities connected with molecular transitions that are not accessible to infrared spectroscopy. Linear laser Raman spectroscopy, CARS, and hyper-Raman scattering have very successfully collected many spectroscopic data that could not have been obtained with other techniques. Besides these basic applications to molecular spectroscopy there are, however, a number of scientific and technical applications of Raman spectroscopy to other fields, which have become feasible with the new methods discussed in the previous sections. We can give only a few examples. 

Since the intensity of spontaneous Raman lines is proportional to the density N(vi, Ji) of molecules in the initial state (u/, 7/), Raman spectroscopy can provide information on the population distribution N vi, Ji), its local variation, and on concentrations of molecular constituents in samples. This allows one, for instance, to probe the temperature in flames or hot gases from the rotational Raman spectra [8.80-8.83] and to detect deviations from thermal equilibrium. 

With CARS the spatial resolution is greatly increased, in particular if BOX CARS is used. The focal volume from which the signal radiation is generated can be made smaller than 0.1 mm [8.53]. The local density profiles of reaction products formed in flames or discharges can therefore be accurately probed without disturbing the sample conditions. The intensity of the stimu- 

One definite advantage of CARS is the fact that the detector can be far away from the sample (remote Raman-spectroscopy) because the intensity of the signal does not decrease as 1/r as for spontaneous Raman spectroscopy or fluorescence 

A further example of the scientific application of CARS is the investigation of cluster formation in supersonic beams (Sect. 4.3), where the decrease in the rotational and vibrational temperatures during the adiabatic expansion (Sect. 4.2) and the degree of cluster formation in dependence on the distance from the nozzle can be determined [379], 

CARS has been successfully used for the spectroscopy of chemical reactions (Sect. 8.4). The BOX CARS technique with pulsed lasers offers spectral, spatial, and time-resolved investigations of collision processes and reactions, not only in laboratory experiments but also in the tougher surroundings of factories, in the reaction zone of car engines, and in atmospheric research (Sect. 10.2 and [380, 381]). 

Nondestructive analysis of various materials, such as rocks, composite materials, phases and inclusion in solids, can be performed with a laser molecular microprobe [8.70], which is based on a combination of an optical microscope with a Raman spectrometer. The laser beam is focused into the sample and the Raman spectrum, emitted from the smal focal spot, is mon- 

CARS has been successfully used for the spectroscopy of chemical reactions (Sect. 13.4). The BOX CARS technique with pulsed lasers offers spectral, spatial and time-resolved investigations of collision processes and 

Using CARS the spatial resolution is greatly increased and the focal volume from which the signal radiation is generated can be made smaller than 

1 mm [9.15]. The local density profiles of reaction products formed in 

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Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. 

S.K. Freeman, Applications of Laser Raman Spectroscopy, Wiley, New York, 1974. 

Freeman RD, Hammaker RM, Meloan CE, Fateley WG (1988) Appl Spectrosc 42 456 Freeman SK (1974) Applications of Laser Raman Spectroscopy, Wiley-Intersc, John Wiley Sons, New York London Sydney Toronto Frei K, Giinlhard HH (1963) J Opt Soc Am 51 83 French MJ, Long DA (1975) J Raman Spectrosc 3 391 Freund SM, Maier II WB, Holland RF, Beattie WH (1978) Anal Chem 50 1260 Friedel G (1922) Ann Phys (Paris) 18 273 Friedl B, Thomson C, Cardona M (1990) Phys Rev Lett 65 915 Friedrich HB, Jung-Pin Yu (1987) Appl Spectrosc 41 227... 

This paper reports the application of laser Raman spectroscopy to the characterisations of a series of ZSM-5 materials containing occluded organic cations. 

Although Raman spectroscopy does not employ absorption of infrared radiation as its fundamental principle of operation, it is combined with other infrared spectroscopies into a joint section. Results obtained with various Raman spectroscopies as described below cover vibrational properties of molecules at interfaces complementing infrared spectroscopy in many cases. A general overview of applications of laser Raman spectroscopy (LRS) as applied to electrochemical interfaces has been provided [342]. Spatially offset Raman spectroscopy (SORS) enables spatially resolved Raman spectroscopic investigations of multilayered systems based on the collection of scattered light from spatial regions of the samples offset from the point of illumination [343]. So far this technique has only been applied in various fields outside electrochemistry [344]. Fourth-order coherent Raman spectroscopy has been developed and applied to solid/liquid interfaces [345] applications in electrochemical systems have not been reported so far. 

H. A. Szymanski, Interpreted Infrared Spectra, Vol. 1. Plenum, New York, 1964. S. K. Freeman, Applications of Laser Raman Spectroscopy. Wiley, New York,... 

Application of Laser Raman Spectroscopy to Biological Problems... 

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