Stimulated Raman scattering technique

The technique of stimulated Raman scattering (SRS) has been demonstrated as a practical method for the simultaneous measurement of diameter, number density and constituent material of micrometer-sized droplets. 709 The SRS method is applicable to all Raman active materials and to droplets larger than 8 pm in diameter. Experimental studies were conducted for water and ethanol mono-disperse droplets in the diameter range of 40-90 pm. Results with a single laser pulse and multiple pulses showed that the SRS method can be used to diagnose droplets of mixed liquids and ensembles of polydisperse droplets. 

A major breakthrough in the measurement of VER occurred in 1972. Laubereau et al. (32) used picosecond laser pulses to pump molecular vibrations via stimulated Raman scattering (SRS) and time-delayed incoherent anti-Stokes probing to study VER of C-H groups in ethanol and methanol ( " -3000 cm-1). Alfano and Shapiro (33) used the same technique to monitor both the decay of the initially excited (parent) C-H stretch excitation and the appearance and subsequent decay of a daughter vibration,... 

Coherent Raman spectroscopy Coherent Raman spectroscopy is a term that refers to a series of closely related nonlinear Raman techniques in which the scattered Raman radiation emerges from the sample as a coherent beam -coherent meaning that the photons are all in phase with one another. The coherent techniques include Stimulated Raman Scattering (SRS), Coherent anti-Stokes Raman Spectroscopy (CARS), Coharent Stokes Raman Spectroscopy (CSRS), and Stimulated Raman Gain Spectroscopy (SRGS). Although most of the nonlinear Raman techniques are also coherent techniques, there is one incoherent nonlinear Raman process called Hyper Raman. 

Nonlinear Raman spectroscopy The nonlinear techniques include stimulated Raman scattering (SRS), hyper Raman, stimulated Raman gain (SRG), inverse... 

Photoacoustic Raman spectroscopy (PARS) Photoacoustic Raman spectroscopy (PARS) is again a nonlinear spectroscopic technique. In this technique, selective population of a given energy state of a system (transitions must involve change in polarizability) is amplified using coherent Raman amplification (also known as stimulated Raman scattering). In this process, it is also important that the frequency difference of the two incident laser beams must be adjusted to equal the frequency of Raman-active transition. 

Besides the various types of tunable lasers discussed in the foregoing sections, sources of tunable coherent radiation have been developed that are based on the nonlinear interaction of intense radiation with atoms or molecules in crystals or in liquid and gaseous phases. Second-harmonic generation, sum- or difference-frequency generation, parametric processes, or stimulated Raman scattering are examples of such nonlinear optical mixing techniques. These techniques cover the whole spectral range from the vacuum ultraviolet (VUV)... 

Other Raman-based forms of nonlinear spectroscopy include stimulated Raman gain (SRG) or stimulated Raman scattering, stimulated Raman loss (SRL) or inverse Raman spectroscopy, and Raman induced Kerr effect spectroscopy (RIKES). Some information on these techniques are provided in Table 1. Many of these other forms do not produce light at wavelengths that are different from the input lasers, do not involve phase matching, and may be susceptible to multiple effects that may interfere with the measurement. Consequently, these techniques have not been as widely used as CARS. 

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]. 

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