Nonresonant light scattering

In the most general case of the nonresonant fifth-order light scattering measurement, five off-resonant optical pulses are injected, as shown in Fig. 1. Two pairs of pulses are used to create two consecutive vibrational coherence states, and the fifth pulse is scattered by the temporal and spatial grating thus created. The controlled delay times between the first two pairs of pulses and between the second pair of pulses and the final pulse are... 

Molecular absorption or light scatter can be corrected for by several methods. If the interfering salt composition is known, it may be possible simply to add the salt or salts to standards and make a direct comparison with calibration curves. It is relatively simple to measure the background absorbance. One way is to choose a nonresonance line from the lead hollow cathode lamp or another lamp that is not absorbed by lead and which occurs at least two bandpasses from the resonance... 

Unless y 2 = q (nonresonant Raman), measurement with circularly polarized light is needed to determine the three invariants. The results are usually expressed as the depolarization ratio / (O/, vi)/4"(0/] Vi). For nonresonant Raman scattering this equals 3/4 for a = is lower otherwise, and reaches zero if a = a ( ) (totally symmetric vibration). In resonant Raman, can be different from zero, and the depolarization ratio can exceed 3/4 ( anomalous polarization ). 

Even more elegantly, the local resolution is improved by irradiation with very intense focused femtosecond laser pulses outside the absorption range of the fluoro-phore (e.g., in the near-infrared). The very intense focus of the laser beam—and only this—will excite the fluorophore by nonresonant two-photon absorption. Artifacts by scattered primary radiation are ruled out and the local resolution is comparable to a confocal microscope. In addition, the damage of the sample by laser light absorption is reduced to a minimum. 

A. Resonance Raman Spectrum. A normal (nonresonance) Raman spectrum of a sample results from an inelastic scattering of an incident light source (e.g., a laser) at frequency vq, where... 

We must remark that the amplitude of these processes is generally weak compared to the direct exciton-photon amplitude, owing to the small libration amplitudes (of the order of 1 °) at low temperatures. It is still smaller when the incident light polarization is parallel to the molecular transition dipole. For instance, in anthracene-crystal excitation, we expect the exciton-photon-phonon contribution to be more important for the a than for the b polarization. On the contrary, these processes become much more important in nonresonant excitations, in Raman scattering for instance (cf. Section II.D). 

Raman scattering is a two-photon process of intrinsically low efficiency in terms of the number of photons scattered per exciting photon irradiating the sample. Typical nonresonant Raman cross-sections are of the order of 10 cm per molecule. With conventional instrumentation, this condition means that a signal-to-noise ratio usually below 10" is obtained. The Raman-scattered signal, which is usually less than 1 ppm of the incident light, from a given sample is proportional to... 

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