Resonance CARS spectroscopy

Materny, A. Kiefer, W. Absorption, luminescence, resonance Raman, and resonance CARS spectroscopy on FBS diacetylene single crystals with color zones. Macromolecules 1992, 25, 5074-5080. 

Figure 13 Resonance CARS spectra of a substituted diacetylene single crystal (FBS-DA) at 10 K. The pump wavelength Ip used is labelled for each spectrum. (A) and (B) show CARS spectra of the P-colour zone, and (C)-(L) those for the Y-colour zone. Spectra on the left side correspond to the C=C stretching region, and those on the right side to the C=C stretching region. For further details, see text. Reproduced by permission of John Wiley Sons from Materny A and Kiefer W (1992) Resonance CARS spectroscopy on diacetylene single crystals. Journal of Raman Spectroscopy 2Z 99-106.

Materny A and Kiefer W (1992) Resonance CARS spectroscopy on diacetylene single crystals. Journal of Raman Spectroscopy 23 99-106. 

Nuclear magnetic resonance (NMR) spectroscopy, which tells us about the car bon skeleton and the environments of the hydrogens attached to it Infrared (IR) spectroscopy, which reveals the presence or absence of key func tional groups... 

CARS spectroscopy utilizes three incident fields including a pump field (coi), a Stokes field (CO2 C02nonlinear polarization at cOcars = 2c0i — CO2. When coi — CO2 coincides with one of the molecular-vibration frequencies of a given sample, the anti-Stokes Raman signal is resonantly generated [22, 23]. We induce the CARS polarization by the tip-enhanced field at the metallic tip end of the nanometric scale. 

The nonresonant background in CARS spectroscopy originates from instantaneous four-mixing processes, while the resonant contribution involves real vibrational states. This provides a basis for possible discrimination against the nonresonant background. To do so, one has to come up with a pair of pulses that excite the vibrational state, and the third, time-delayed pulse will only contribute to the resonant part of the CARS signal. However, to make this scheme work efficiently, one has to overcome certain obstacles. To achieve high spectral resolution, the bandwidth of the third pulse should... 

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. 

A typical CARS device for condensed phase spectroscopy which has been developed at the University of Wurzburg (Materny et ah, 1992b) is shown in Fig. 3.6-8. Because it contains two tunable (uji,us) laser sources it is also suitable to perform resonance CARS... 

Resonances betwen Raman active molecular vibrations and rotations and the difference frequency combination 0)1 —0)2 can also occur. When the four-wave mixing process 2o)i — 0)2 or o)i+o)2 — 0)2 is used with these resonances it is termed coherent anti-Stokes Raman scattering (CARS). The resonant enhancement that occurs in the generated intensity as the pump frequencies are tuned through the two-photon difference-frequency resonance forms the basis of CARS spectroscopy (see Section IV.A). 

Figure 1.3 HD rotational and vibrational state distributions measured for the H + D2 reaction at a collision energy of 1.3 eV. The energy is determined by the recoil energy of the H atom in the photodissociation of HI at a wavelength where it dissociates primarily to ground state I atoms. The experimental results shown [adapted from D. P. Gerrlty and J. J. Valentini, J. Chem. Phys. 81, 1298, (1984) and Valentin and Phillips (1989)] used CARS spectroscopy to determine the state of HD. E. E. Marinero, C. T. Rettner, and R. N. Zare, J. Chem. Phys. 80,4142 (1984) used resonance enhanced multiphoton ionization, REMPI, for this purpose. The figure also shows curves. Those on the left are the so-called linear surprisal representation, see Section 6.4. The plot on the right shows the same experimental data on a logarithmic scale. The curves [adapted from N. C. Blais and D. G. Truhlar, J. Chem. Phys. 83, 2201 (1985)] are a dynamical computation by the method of classical trajectories. Section 5.2.

Local analysis can be carried out only by the LIF and CARS methods. They have the highest time resolution. The CARS method or any other method of Raman spectroscopy are imiversal but have low sensitivity. Therefore, it is reasonable to use resonance Raman spectroscopy because its sensitivity is higher. Due to a low sensitivity, Raman spectroscopy is combined with a pulse source of generation of active... 

---