Light Scattering as a Probe of Fast-Reaction Kinetics: The Depolarized Spectrum of Rayleigh Scattered Light from a Chemically Reacting Medium

Abstract

The spectral distribution of the depolarized component of light scattered from a dilute solution of molecules in dynamic chemical equilibrium between two states differing in optical anisotropy and dynamics is calculated. It is assumed that the molecules undergo isotropic translational and anisotropic rotational diffusion. It is furthermore assumed that the duration of a molecular transformation is much less than the characteristic times for rotational and translational diffusion. It is shown that in the most general case considered, the spectrum consists of 20 superposed lines with widths dependent on the translational diffusion coefficients and components of the rotational diffusion tensors of the two species and the backward and forward chemical rate constants. The relative strengths of the lines depend on the optical anisotropies and rotational diffusion tensor components of the two species as well as on their equilibrium concentrations. When the reaction is so fast that rotational and translational diffusion contributions to the linewidth may be ignored altogether, the spectrum reduces to a single Lorentzian line with half-width independent of scattering angle and proportional to the sum of the backward and forward rate constants for the chemical transformation.