Development of the 2-Dimensional Quasielastic Light Scattering Spectrometer

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The purpose of this research project is to develop a 2-dimensional quasielastic light scattering (2D-QELS) spectrometer, which enables us to obtain detailed information on the microscopic fluctuation of various materials. The measurement method for 2D-QELS is based on a different technique producing 2-dimensional (2D) spectra from other 2D spectroscopies such as 2D-NMR and IR.In 1993, we assembled the 2D-QELS spectrometer which consisted of a laser, a high-voltage sinusoidal wave generator, a cell with parallel-plate electrodes, an optical mixing system in the heterodyne mode, and a detector unit with a photomultiplier and a photon counter. And some perparatory measurements for 2D-QELS were performed by using a simpler system with a photodiode and an A/D converter. In 1994, we made an interface unit between the 2D-QELS spectrometer and a personal computer, and incorporated a software program based on an extended Wiener-Khinchin theorem, which enabled us to obtain the 2D spectra directly from the scattered light intensity without calculating the autocorrelation function. Then the 2D-QELS spectrometer completed was applied to standard samples such as a polystirene latex and polystirene sulfonate in aqueous solutions. As a result, it was found that we could measure the phase delta of the complex electrophoretic mobility mu^*=muexp (idelta), which was unobtainable by the conventional method, determine the value of the amplitude mu more acculately, and detect the apparent diffusion constant D_<app> in the frequency range wider than the conventional method. The new measurement method used in 2D-QELS is expected to be applicable to the time-dependent flucuations on various kinds of periodically stationary systems.