Raman nonresonance

RIKES Raman-induced birefringence Modulation of 0)2, is CW Induced change in 0) polarization for 0) -0)2 = ft>Raman Nonresonant background can be suppressed, no phase matching Limited sensitivity, susceptible to turbulence and birefringence from windows, optics, sample... 

The nomesonant background prevalent in CARS experiments (discussed above), although much weaker than the signals due to strong Raman modes, can often obscure weaker modes. Another teclmique which can suppress the nonresonant background signal is Raman induced Kerr-efifect spectroscopy or RIKES [96, 97]. 

As already mentioned, electronically resonant, two-pulse impulsive Raman scattering (RISRS) has recently been perfonned on a number of dyes [124]. The main difference between resonant and nom-esonant ISRS is that the beats occur in the absorption of tlie probe rather than the spectral redistribution of the probe pulse energy [124]. These beats are out of phase with respect to the beats that occur in nonresonant ISRS (cosinelike rather tlian sinelike). RISRS has also been shown to have the phase of oscillation depend on the detuning from electronic resonance and it has been shown to be sensitive to the vibrational dynamics in both the ground and excited electronic states [122. 124]. 

Walsh A M and Loring R F 1989 Theory of resonant and nonresonant impulsive stimulated Raman scattering Chem. Phys. Lett. 160 299-304... 

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

The key requirements for ISRS excitation are the existence of Raman active phonons in the crystal, and the pulse duration shorter than the phonon period loq1 [19]. The resulting nuclear oscillation follows a sine function of time (i.e., minimum amplitude at t=0), as shown in Fig. 2.2e. ISRS occurs both under nonresonant and resonant excitations. As the Raman scattering cross section is enhanced under resonant excitation, so is the amplitude of the ISRS-generated coherent phonons. 

Nonresonance Raman study of the order-disorder transition of the hydrocarbon chains at high temperatures... 

Nonresonance Raman spectra of the alternating LB films were measured by a total reflection method shown in Figure 23. The films were deposited on quartz prisms. The s-polarized beam of 647.1 nm from a Kr laser was incident upon the interface between the quartz and film at an angle of 45° from the quarz side, and totally reflected. Raman line scattered from the film in the direction of 45° from the surface was measured through a Spex Triplemate by a Photometries PM512 CCD detector with 512x512 pixels operated at -125 °C. The spectral resolution was about 5 cm 1. 

Figure 24. Nonresonance Raman spectra of the CH stretching region of DOPC-Ba in S(PS)n-Bafflm[23]. 

In resonant Raman spectroscopy, the frequency of the incident beam is resonant with the energy difference between two real electronic levels and so the efficiency can be enhanced by a factor of 10 . However, to observe resonant Raman scattering it is necessary to prevent the possible overlap with the more efficient emission spectra. Thus, Raman experiments are usually realized under nonresonant illumination, so that the Raman spectrum cannot be masked by fluorescence. 

The spectral line shape in CARS spectroscopy is described by Equation (6.14). In order to investigate an unknown sample, one needs to extract the imaginary part of to be able to compare it with the known spontaneous Raman spectrum. To do so, one has to determine the phase of the resonant contribution with respect to the nonreso-nant one. This is a well-known problem of phase retrieval, which has been discussed in detail elsewhere (Lucarini et al. 2005). The basic idea is to use the whole CARS spectrum and the fact that the nonresonant background is approximately constant. The latter assumption is justihed if there are no two-photon resonances in the molecular system (Akhmanov and Koroteev 1981). There are several approaches to retrieve the unknown phase (Lucarini et al. 2005), but the majority of those techniques are based on an iterative procedure, which often converges only for simple spectra and negligible noise. When dealing with real experimental data, such iterative procedures often fail to reproduce the spectroscopic data obtained by some other means. 

Uzunbajakava, N., Lenferink, A., Kraan, Y, Volokhina, E., Vrensen, G., Greve, J., and Otto, C. 2003. Nonresonant confocal Raman imaging of DNA and protein distribution in apop-totic cells. Biophys. J. 84 3968-81. 

Kamga, F. M., and Sceats, M. G. 1980. Pulse-sequenced coherent anti-Stokes Raman scattering spectroscopy A method for suppression of the nonresonant background. Opt. Lett. 5(3) 126-28. 

Marks, D. L., Vinegoni, C., Bredfeldt, J. S., and Boppart, S. A. 2004. Interferometric differentiation between resonant coherent anti-Stokes Raman scattering and nonresonant four-wave-mixing processes. AppZ. Phys. Lett. 85(23) 5787-89. 

The nonresonant contributions pertain to electron cloud oscillations that oscillate at the anti-Stokes frequency but do not couple to the nuclear eigenfrequencies. These oscillatory motions follow the driving fields without retardation at all frequencies. The material response can, therefore, be described by a susceptibility that is purely real and does not depend on the frequencies of the driving fields. The resonant contributions, on the other hand, are induced by electron cloud oscillations that are enhanced by the presence of Raman active nuclear modes. The presence of nuclear oscillatory motion introduces retardation effects relative to the driving fields i.e., there is phase shift between the driving fields and the material oscillatory response. 

The amplitude and phase of Xr are plotted in Figure 9.6a, whereas in Figure 9.6b the same function is depicted in terms of real and imaginary parts. It is clear from Figure 9.6a that the phase of the material s resonant oscillatory response undergoes a r phase shift relative to the nonresonant response in the vicinity of the spectral resonance. This is a direct manifestation of the retardation observed when driving the oscillators near their Raman resonances. In nonlinear interferometry, the Xr and... 

FIGURE 9.12 (a) Calculated FE-CARS radiation profile when a HGOl excitation field overlaps with a lateral interface between a resonant and a nonresonant material. Note that the intensity along the optical axis is no longer zero due to partial lifting of the phase step by the interface. The inset shows the excitation field relative to the orientation of the interface, (b) Comparison of the calculated spectral dependence of CARS in a bulk material with a weak resonance and FE-CARS measured at an interface similar to the one considered in (a). Note the Raman-like spectral dependence of the FE-CARS signal. 

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