Stokes scatter

For > 0, one has absorption for < 0, emission. Multiphoton absorption and emission fall into this class. The Class I Raman spectroscopies clearly exliibit a net absorption of energy in Stokes scattering and a... 

The upper sign is for anti-Stokes scattering, the lower for Stokes scattering. The factor in the parentheses is... 

Raman scattering and (b) anti-Stokes Raman scattering. In Stokes scattering, tlie cluomophore is initially in the ground vibrational state, g, and oi > CO2. hr spontaneous anti-Stokes scattering, the cluomophore must be initially m an excited vibrational state,/ Also note that in (b), M2 is (arbitrarily) defined as being greater than... 

First order stimulated Stokes scattering experiences an exponential gain in intensity as the fields propagate tlirough the scattering medium. This is given by the expression [75]... 

The reason why the spacings are equal, and not the 1-0, 2-1, 3-2,... anharmonic intervals, is explained in Figure 9.21. The laser radiation of wavenumber Vg takes benzene molecules into the virtual state Fj from which they may drop down to the v = level. The resulting Stokes scattering is, as mentioned above, extremely intense in the forward direction with about 50 per cent of the incident radiation scattered at a wavenumber of Vg — Vj. This radiation is sufficiently intense to take other molecules into the virtual state V2, resulting in intense scattering at Vg — 2vj, and so on. 

The quantum mechanical view of Raman scatering sees a radiation field hvo inducing a transition from a lower level A to a level n. If vnlc is the transition frequency, then the inelastically scattered light has frequency v0 — v t. That is, the molecule removes energy hv k from an incident photon. This process corresponds to Stokes scattering. Alternatively, a molecule under-... 

Fiq. 2. The quantum mechanical view of Raman scattering, (a) Stokes scattering process (b) anti-Stokes scattering process (8). 

At room temperature the thermal population of vibrational excited states is low, although not zero. Therefore, the initial state is the ground state, and the scattered photon will have lower energy than the exciting photon. This Stokes shifted scatter is what is usually observed in Raman spectroscopy. Figure la depicts Raman Stokes scattering. 

Stokes number (Stk), 22 57, 23 184, 190 in depth filtration theory, 77 340 Stokes Raman scattering, 27 322 Stokes scatter, 76 485-486 Stokes shifts, 20 512 Stomach poison insecticides, 74 339... 

Grochala, Kudelski, and Bukowska [228] have described the anion-induced charge-transfer enhancement in S E RS and S E RRS spectra of rhodamine 6G on Ag electrode as a function of the electrode potential, upon addition of chloride and citrate anions. In a very recent paper, Brolo et al. [229] have discussed the ratio of the surface-enhanced anti-Stokes scattering to the surface-enhanced Stokes-Raman... 

Figure 7.15 Fringe structure of the anti-Stokes scattering observed by the interference of two Raman excitations. Open circles are observed data and solid lines are sine functions fitted to the observed data, (a) The delay is scanned around 10 ps. (b) The delay Tjj g is scanned around 500 ps. In both cases, the probe pulse is irradiated at 1 ns after the first excitation. The intensity is normalized by the signal intensity when only the single IRE pulse is irradiated. Reproduced with permission from Ref. [43]. Copyright 2013 by the American Physical Society.

For any vibrational mode, the relative intensities of Stokes and anti-Stokes scattering depend only on the temperature. Measurement of this ratio can be used for temperature measurement, although this application is not commonly encountered in pharmaceutical or biomedical applications. Raman scattering based on rotational transitions in the gas phase and low energy (near-infrared) electronic transitions in condensed phases can also be observed. These forms of Raman scattering are sometimes used by physical chemists. However, as a practical matter, to most scientists, Raman spectroscopy means and will continue to mean vibrational Raman spectroscopy. 

Rayleigh scattering V / Stokes scattering anti-Stokes scattering... 

Figure 10.1. Comparison of normal (top) and surface-enhanced (bottom) Raman scattering. The top panel shows the conversion of incident laser light of intensity /(vl) into Stokes scattered light /NRS, which is proportional to the Raman cross section and the number of target molecules N in the probed volume. In the bottom panel

The coherent anti-Stokes scattering of a probing pulse generating radiation with frequency a> = m > + mq and wave vector kA is depicted in... 

Figure 1 Schematic representation of a time-resolved coherent Raman experiment, (a) The excitation of the vibrational level is accomplished by a two-photon process the laser (L) and Stokes (S) photons are represented by vertical arrows. The wave vectors of the two pump fields determine the wave vector of the coherent excitation, kv. (b) At a later time the coherent probing process involving again two photons takes place the probe pulse and the anti-Stokes scattering are denoted by subscripts P and A, respectively. The scattering signal emitted under phase-matching conditions is a measure of the coherent excitation at the probing time, (c) Four-photon interaction scheme for the generation of coherent anti-Stokes Raman scattering of the vibrational transition.

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