Nonlinear Raman techniques

There exists a family of nonlinear Raman techniques, of which... 

The Application oi Single-Pulse Nonlinear Raman Techniques to a Liquid Photolytic Reaction... 

Planned experiments on shock-induced chemical reactions and detonations of explosives will be carried out with the nonlinear Raman techniques. Heterodyne detection of the transient products of such rapid reactions seems the most promising. 

Such single-mode lasers, often pulse amplified by dye laser amplifiers pumped by injection-locked Nd YAG lasers, are used in nonlinear Raman techniques by which an instrumental resolution better than 0.001 cm is achieved (Esherick and Owyoung (1982), Schrotter et al. (1988a)). 

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. 

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. 

The advantages of these nonlinear Raman techniques are the greatly increased signal-to-noise ratio and thus the enhanced sensitivity, the higher spectral and spatial resolution, and in the case of the hyper-Raman spectroscopy, the possibility of measuring higher-order contributions of molecules in the gaseous, liquid, or solid state to the susceptibility. 

Certainly, another way to achieve lower detection levels of contaminants in water is to turn to the numerous variants of RS that offer higher sensitivity, namely RRS, SERS, and the various nonlinear Raman techniques. At present, there seems to be more promise and certainly more attention devoted to SERS for this application. With recent improvements in the stability and reproducibility of substrates, it appears that SERS may soon become more much widely used as an analytical technique. However, we continue to believe that there are, for many environmental applications, considerable advantages to NRS over other Raman techniques. (See Section II for a more detailed discussion.) Therefore, we see a pressing need for further improving the sensitivity of NRS for the analysis of contaminants in water. 

The nonlinear Raman techniques discussed above are special cases of a general four-wave mixing process, which is schematically illustrated in Figure 6. Here, three independent fields with angular frequencies (0, 0)2 and 6 3 may be incident upon the matter. A fourth field, which is phase coherent relative to the input fields, is then generated at angular frequency 0)=(d -(D2 + o)y When the angular frequency... 

Mainly, the Q-branches of simple molecules, like di-, tri-, and four-atomic as well as spherical XY4 top molecules have been studied. As an example Figure 10 shows the Q-branch of methane. The complicated rotational structure seen there has been resolved by applying this powerful nonlinear Raman technique. This very high resolution of the order of 10 cm i allows us to study in detail colli-sional effects, which is of particular importance as a basis for the determination of temperatures and pressures. 

The fact that the resolution of the nonlinear Raman techniques is limited only by the laser line widths gives the stimulated Raman techniques particular appeal under conditions where interference from background luminescence is problematic or in situations where very high resolution is required. The main disadvantage of these techniques, however, is that they are quite sensitive to laser noise. The latter requires high stability in laser power. 

Nonlinear spectra are produced by monitoring the intensity of the output beam while varying some parameters, such as the frequency o) of one or more of the laser beams. When the difference in frequency between two laser beams matches the frequency of a Raman-active mode, the resulting resonance enhances the nonlinear optical effect, causing a change in the intensity of the output beam. The result is a peak in the nonlinear Raman spectrum. Some nonlinear Raman techniques that use this approach are given in Table 1. 

Table 1 Comparison of some nonlinear Raman techniques ...

In addition to CARS, other closely related but less commonly used nonlinear Raman techniques have been developed. The energy level diagram for coherent Stokes Raman spectroscopy (CSRS) is shown in Figure 1. Unlike CARS, CSRS is nonparametric the final state is not the same as the initial state. Therefore, CSRS spectra may exhibit extra peaks due to coherence dephasing. Furthermore, the CSRS output beam is generated to the Stokes (lower frequency) side of 0)3. CSRS is therefore more susceptible to spectral interference from fluorescence and Rayleigh scattering of the input beams. 

Most nonlinear Raman techniques correspond to the - sign above — o)p- (Raman-like reso-... 

The non-resonant part (3)nr j e l and includes the contribution of every species present. It will usually show a weak dispersion, being approximately constant for extended frequency ranges. In this case, the permutation of the frequency arguments alone does not alter much the numerical value of the susceptibility. This is the Kleinman s symmetry conjecture which, combined with the strict intrinsic permutation symmetry allows to freely permute the Cartesian subscripts in jj(3)NR This property, although not exact, is of great relevance for the polarization nonlinear Raman techniques. 

We are at last able to give a plane wave signal expression for the different nonlinear Raman techniques, collected in Table 1. 

This article describes the elements of Raman spectrometers for routine analyses which are available commercially. Instruments designed only for special research are not covered. Only spectrometers for classical (linear) Raman scattering are mentioned, not those for observing resonance Raman scattering (RRS), surface-enhanced Raman scattering (SERS) and all nonlinear Raman techniques they are described elsewhere in this Encyclopedia. 

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