Power spectra observed in laser scattering from moving, polydisperse particle systems in flames—I. Theory

Abstract

Theoretical expressions are derived for the photocurrent power spectra observed with laser scattering from moving, polydisperse particle systems. The analysis refers to self-beating of scattered radiation (to which we refer as the homodyne spectrum) and beating between the laser beams scattered in the same direction from two, equally-intense incident laser beams which arrive at symmetrical angles with respect to the scattering direction (to which we refer as an example of heterodyne spectroscopy or, more briefly, as the heterodyne spectrum). When we use a spherically-symmetric Gaussian distribution for the illumination intensity in the coherence volume (scattering region), we obtain a Voigt profile for the photocurrent power spectrum in the general case. We have used the theoretical relations derived for moving systems in order to estimate the maximum values of the particle diameters which are measurable for Stokes-Einstein diffusion when radiation is scattered from He-Ne or Ar-ion lasers.