Dynamic light scattering with single-mode receivers: partial heterodyning regime.

A frequent source of errors in dynamic light-scattering experiments is partial heterodyning caused by scattering on large particles or imperfections of the sample cell. With a conventional two-pinhole receiver it is impossible to distinguish its effect from the effects of a finite detector area and detector nonlinearity. However, an accurate data analysis is feasible when a single-mode light receiver is employed. We present formulas for single-mode autocorrelation and cross-correlation functions that include a local oscillator and an incoherent background of arbitrary strength and take into account detector nonlinearity (e.g., dead time) up to second order. A simple but accurate method for the determination of the nonlinearity parameters and the effective number of receiver modes is also provided. The success of the data-evaluation procedure is demonstrated by the measurement of the hydrodynamic radius of latex in the presence of deliberately added local-oscillator or incoherent-background contributions.