Dynamic Light Scattering and Fluorescence Correlation Spectroscopy

HISTORICAL REVIEW 1869 J. Tyndall performed the first experimental studies on light scattering from aerosols. He explained the blue color of the sky by the presence of dust in the atmosphere. 1871 Lord Rayleigh presented the theory of scattering from assemblies of non-interacting particles that were sufficiently small compared with the wavelength of light. According to Rayleigh, scattering by a gas occurs because of the fluctuation of the molecules around a position of equilibrium. Rayleigh obtained the formulae that explained the blue color of the sky as being due to the molecules in the atmosphere preferentially scattering blue light in comparison to red light. 1906 L. Mandelshtam raised the question on the nature of scattered light once again. He pointed out that Rayleigh's arguments do not fully explain the scattering phenomenon. According to Mandelshtam, light scattering is also due to the random fluctuations of molecules near the position of equilibrium. As a result of random fluctuations of order λ 3 , where λ is the scattering wavelength, the number of macromolecules will vary with time. It follows from the Mandelshtam theory that the translational and rotational diffusion coefficients of macromolecules could be obtained from the spectrum of the scattered light. In 1924 , L. Mandelshtam described theoretically and experimentally what he termed combination light scattering, which later was renamed Raman scattering. As early as 1926 he recognized that the translational diffusion coefficient of macromolecules can be obtained from the spectrum of the light they scatter. However, at that time, lack of spatial coherence and monochromaticity in conventional light sources rendered such experiments impossible. 1914 L. Brillouin predicted a doublet in the frequency distribution of scattered light due to scattering from thermal sound waves in a solid. This doublet was later named after him. E. Gross performed a series of light-scattering experiments on liquids in 1932. He observed not only the Brillouin doublet but also a central peak, the position of which was unshifted. In 1933 L. Landau and G. Placzek gave a theoretical explanation of the central line from a thermodynamic point of view and calculated the ratio of the integrated intensity of the central line to that of the doublet. 1916 M. von Smoluchowski gave the first theoretical description of the amplitude and the temporal decay of number fluctuations in a diffusion system.