CARS resonant

A spontaneous Raman spectra is shown in Figure 2.8d in which the on- and off-resonant frequencies are indicated. The DNA bundles are observed at the resonant frequency, as shown in Figure 2.8a, while they cannot be seen at the off-resonant frequency in Figure 2.8b. This indicates that the observed contrast is dominated by the vibrationally resonant CARS signals. Figure 2.8c shows a cross-section of Figure 2.8a denoted by two solid arrows, which were acquired with a 5 nm step. The FWHM of... 

Weak Raman-resonant CARS, i.e., when y(3,r << can be amplified... 

The detection sensitivity being limited at 100 to 1000 ppm for usual gases in flames using background cancellation, an effort has been undertaken in order to understand resonance enhancement mechanisms and in order to apply resonance CARS to trace species detection. The theory is now well understood (8) and an encouraging experimental verification has been reported with detectivity gains of 100 to 1000 (9). However, numerous experimental problems remain to be solved, among which are saturation and laser stability problems (10). 

Figure 3 Femtosecond nondegenerate CARS in liquids (a) Coherent probe scattering signal versus delay time open circles, dashed curve nonresonant scattering of CCU yielding the instrumental response function and the experimental time resolution of 80 fs full points, solid line resonant CARS signal from the CH3-mode of acetone at 2925 cue1, obtaining T2/2 = 304 3 fs. (b) Ratio of experimental and calculated scattered data of (a) for acetone versus delay time the small experimental error of the data points extending over 6 orders of magnitude is noteworthy.

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... 

As an example of resonance CARS studies in solids, for which a linear resonance Raman study has been impossible to perform because of simultaneous strong luminescence, we consider here investigations on colour zones in substituted diacetylene crystals... 

Figure 6.1-23 Resonance CARS spectra of a substituted diacetylene single crystal (FBS-DA) at 10 K. The pump wavelength Xp used is labeled for each spectrum, (a) and (b) show CARS spectra of the P-colour zone, and (c)-(l) those for the Y-coiour 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 (Materny and Kiefer, 1992b).

Figure 6.1-24 Resonance CARS excitation of the u (= 850 cm ) vibration of the permanganate ion doped in a KCIO4 single crystal. The absorption spectrum at T = 15 K shows the vibronic structure of the Mn04-ion. CARS excitation with tar. = 18 050 cm and los = 17200 cm" gives rise to a twofold resonance since the intermediate states of the CARS process coincide with the sharp electronic transitions. The CARS excitation profile peaks particularly for this double resonance (see linear scale). Further resonances can be seen in the CARS excitation profile plotted on a logio scale (Adapted from Leuchs and Kiefer, 1993a, b).

As already noted, Schneider and co-workers were the first to report nanosecond time-resolved Raman spectra of spirooxazine derivatives (including the spironaphthoxazine), using resonance CARS.18 Two years later, the same group... 

U. Kliiter, W. Hub, and S. Schneider, Resonance CARS spectra of metastable merocyanines produced by UV-photolysis of spirooxazines, Springer Proc. Phys. 41, 152-155 (1985). 

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. 

For electronically resonant CARS/CSRS, where the resonance conditions, = W2) and 3)5 are also satisfied in... 

Figure 11. Resonant CARS intensity versus delay time r = 0.7 psec 1, = 0.75 psec 1.

Figure 12. Resonant CARS intensity versus delay time r = 0.07 psec 1, T, = 0.75 psec 1. Comparison with Fig. 11 shows that the resonant CARS signal is insensitive to the rate of IVR on the excited-state potential surface.

From Equation 13.5 it is clear that the resonant CARS intensity varies with temperature through its dependence on the number density N as well as population factor and Raman line width Tj, which are functions of temperature. The total CARS intensity also depends on the non-resonant contribution, which in turn varies with species concentration and temperature. These features are the basis of temperafure and species concentration measurements using the CARS technique. 

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