In order to put photon-correlation spectroscopy in its proper context, it is instructive to consider the various regimes of spectroscopy and the physical situations about which they provide information. Conventional spectroscopy of scattered light before the advent of the laser had two principal limitations. Firstly, the best resolution obtainable with interferometric methods was of the order of 0.1 to 1 cm−1, in the case of a grating spectrometer and 10 MHz to 100 MHz in the case of a Fabry-Perot interferometer. Since the observed frequency shift of scattered light is essentially inversely proportional to the scale of the scattering process involved, it is apparent that these instruments were limited to studying atomic and molecular processes or, at best, such high frequency co-operative processes as scattering from acoustic phonons. Slower, long range processes, such as critical phenomena, processes involving large bodies, such as the diffusion of macromolecules, or processes involving bulk motion as in anemometry can only be studied by using the high resolution attainable with the application of the techniques of light beating, or its more recent derivative, photon correlation spectroscopy.
KeywordsWind Tunnel Autocorrelation Function Intensity Fluctuation Scattered Field Heterodyne Detection
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