Forced Rayleigh scattering

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Forced Rayleigh scattering (FRS) is an experimental method in physics and chemistry based on light scattering and is usually used to measure diffusion on length scales of roughly 10 µm. [1] [2] [3] [4] Many FRS experiments have been carried out on thermal and mass diffusion in liquids, and thermophoresis (Soret effect) has been measured in polymer solutions. [5]

The FRS method employs the interference fringes of two absorbed coherent “pump” laser beams within the sample to create a spatially sinusoidal variation in the temperature and, if dye molecules are present, the population of long-lived excited dye states. A third “probe” laser is incident at a wavelength at which the (complex) index of refraction n is different in the bright and dark regions of the interference pattern, so that the periodic variation in n serves as a diffraction grating for the probe beam. A photodetector is used to measure the diffracted light, and after the pump beams are turned off, the detected signal decays at an exponential rate proportional to the relevant diffusion coefficient(s).

FRS studies of molecular mass diffusion are somewhat more involved than studies of thermal diffusion or thermophoresis because the ground and excited dye states may have different diffusion coefficients. [6] [7] When this is the case, two gratings differing in phase by 180 degrees are present: the excited state grating and a complementary ground-state grating. The diffracted signal is then a combination of two exponential decays rather than one. Although unique determination of the two separate diffusion coefficients is not easy, the average of the two diffusion coefficients can be obtained from the diffracted signal without difficulty. [8] [9]

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References

  1. ↑ Blanco, Pablo; Wiegand, Simone (2010-03-04). "Study of the Soret Effect in Monosaccharide Solutions". The Journal of Physical Chemistry B. American Chemical Society (ACS). 114 (8): 2807–2813. doi:10.1021/jp910331a. ISSN   1520-6106. PMID   20136092.
  2. ↑ Pohl, D. W.; Schwarz, S. E.; Irniger, V. (1973-07-02). "Forced Rayleigh Scattering". Physical Review Letters. American Physical Society (APS). 31 (1): 32–35. Bibcode:1973PhRvL..31...32P. doi:10.1103/physrevlett.31.32. ISSN   0031-9007.
  3. ↑ Eichler, H.; Salje, G.; Stahl, H. (1973). "Thermal diffusion measurements using spatially periodic temperature distributions induced by laser light". Journal of Applied Physics. AIP Publishing. 44 (12): 5383–5388. Bibcode:1973JAP....44.5383E. doi:10.1063/1.1662160. ISSN   0021-8979.
  4. ↑ H. Eichler, P. Gunter, and D. W. Pohl, Laser-Induced Dynamic Gratings. Springer-Verlag, Berlin, 1986.
  5. ↑ Köhler, W. (1993). "Thermodiffusion in polymer solutions as observed by forced Rayleigh scattering". The Journal of Chemical Physics. AIP Publishing. 98 (1): 660–668. Bibcode:1993JChPh..98..660K. doi: 10.1063/1.464610 . ISSN   0021-9606.
  6. ↑ Park, Sangwook; Sung, Jungmoon; Kim, Hongdoo; Chang, Taihyun (1991). "Complementary grating effect in forced Rayleigh scattering". The Journal of Physical Chemistry. American Chemical Society (ACS). 95 (19): 7121–7124. doi:10.1021/j100172a004. ISSN   0022-3654.
  7. ↑ Eitoku, Takeshi; Terazima, Masahide (2008). "Protein Diffusion Probed by the Transient Grating Method with a New Type of Photochromic Molecule". Photochemistry and Photobiology. Wiley. 84 (4): 869–873. doi:10.1111/j.1751-1097.2008.00315.x. ISSN   0031-8655. PMID   18346090.
  8. ↑ Lee, J. A.; Lodge, T. P. (1987). "Polymer-probe interactions in forced Rayleigh scattering measurements of probe diffusion in poly(vinyl acetate) solutions". The Journal of Physical Chemistry. American Chemical Society (ACS). 91 (22): 5546–5548. doi:10.1021/j100306a010. ISSN   0022-3654.
  9. ↑ Spiegel, D. R.; Marshall, A. H.; Jukam, N. T.; Park, H. S.; Chang, T. (1998). "Measurement of mass diffusion coefficients using nonexponential forced Rayleigh scattering signals". The Journal of Chemical Physics. AIP Publishing. 109 (1): 267–274. Bibcode:1998JChPh.109..267S. doi: 10.1063/1.476560 . ISSN   0021-9606.
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