Stokes Raman effect

It was predicted in 1923 by Smekal and shown experimentally in 1928 by Raman and Krishnan that a small amount of radiation scattered by a gas, liquid or solid is of increased or decreased wavelength (or wavenumber). This is called the Raman effect and the scattered radiation with decreased or increased wavenumber is referred to as Stokes or anti-Stokes Raman scattering, respectively. 

The present study demonstrates that the analytic calculation of hyperpolarizability dispersion coefficients provides an efficient alternative to the pointwise calculation of dispersion curves. The dispersion coefficients provide additional insight into non-linear optical properties and are transferable between the various optical processes, also to processes not investigated here as for example the ac-Kerr effect or coherent anti-Stokes Raman scattering (CARS), which depend on two independent laser frequencies and would be expensive to study with calculations ex-plictly frequency-dependent calculations. 

Since the Raman scattering is not very efficient (only one photon in 107 gives rise to the Raman effect), a high power excitation source such as a laser is needed. Also, since we are interested in the energy (wavenumber) difference between the excitation and the Stokes lines, the excitation source should be monochromatic, which is another property of many laser systems. 

Figure 2.52 Schematic representation of the transitions giving rise to the Raman effect. GS = ground electronic state, ES = excited electronic state, VS = virtual electronic stale, R = Rayleigh scattering, S = transitions giving rise to Stokes lines, AS = transitions giving rise to Anti-Stokes lines, RRS = transitions giving rise to resonance Raman.

In order to extend the range of 2laser excitation, both CARS (Coherent Anti-Stokes Raman Scattering) and CSRS (Coherent Stokes Raman Scattering) are used. In both cases <03 = 2003 -U2 In the CARS mode 0)3 > wj > (03 in the CSRS mode <02 > (1)3. One-photon resonance effects are the same in both cases as described later. Phase matching is also the same in both cases with 3 = 2 ... 

The first observation of the stimulated Raman effect was reported by Woodbury and Ng 215) j e effect was then thoroughly studied by several authors 216-218) and its theoretical background developed 219.220) (see also the review articles by Zubov et a/.22D). The stimulated Raman effect can be described as a parametric process where the coupling between a light wave at the Stokes frequency (Os and an optical phonon (vibrational wave) at cOy is produced by a pump field at col = (Oj + ojy. 

There are the further advantages that rotational lines can be studied and that fluorescent substances can be investigated by the inverse Raman effect. Benzene and other molecular liquids have been studied by this method by McQuillan and Stoicheff 232) jhe required continuum radiation was anti-Stokes emission produced by passing the laser beam in liquid toluene. 

Liu, C., Huang, Z., Lu, E. Zheng, W., Hutmacher, D. W., and Sheppard, C. 2006. Near-field effects on coherent anti-Stokes Raman scattering microscopy imaging. Opt. Express 15 4118-31. 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

Not long after the discovery of the stimulated Raman effect in liquids 63> it was also detected in single crystals 64), namely diamond, calcite, and a-sulfur. Only much later could it be shown that the effect can also be observed in crystal powders 651. The stimulated Raman effect 99 > is excited by giant-pulse lasers with a power of several MW. The strongest Raman lines of a substance are amplified until their intensity is of the same order of magnitude as that of the exciting line furthermore second, third, etc. Stokes lines of the fundamentals in question are observed with twice, thrice, etc. the frequency shift. 

The inverse Raman effect was detected in liquids 93> soon after the discovery of the stimulated Raman effect. When a medium is irradiated simultaneously by intense monochromatic light from a giant-pulse laser and by a continuum, sharp absorption lines are observed on the anti-Stokes side of the laser line, and under special conditions also on the Stokes side 94 >. McLaren and Stoicheff 95) used the intense fluorescence from a dye solution excited by frequency-... 

Fig. 8.12 The Raman effect. Monochromatic light of frequency vQ is scattered by a sample, either without losing energy (Rayleigh band) or inelastically, in which a vibration is excited (Stokes band), or a vibra-tionally excited mode in the sample is de-excited (anti-Stokes band). The spectrum is that of the light scattered by the sample. The energy level diagrams illustrate that the scattering process occurs via highly unstable states of high energy.

Table II Space- and Time-Resolved Measurements from Inelastic Light Scattering. All methods are suitable for nonequilibrium conditions. Here, RS refers to Raman scattering, CARS to coherent anti-Stokes Raman spectroscopy, and RIKES to Raman-induced Kerr effect.

At higher frequencies, the laser-Raman effect affords, in principle, the possibility of detecting non-thermal excitation of vibrations. These would be found from a higher than thermal ratio of anti-Stokes to Stokes lines. The Raman effect in biological systems has recently been reviewed by Webb (21). Unfortunately only two relevant measurements have been carried out, so far, but both demonstrate non-thermal excitation. A difficulty affecting reproducibility arises here from the effect of a laser beam on a biological system as discussed in (21), in the case of individual cells. The best way to avoid this appears to be the use of a flow instrumentation so that each cell is subjected to the laser beam for a very short period only (22). 

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