Quasielastic light scattering by flexible polymers in the presence of a sinusoidal driving field: internal relaxation modes

Application of an applied electric field to a system of charged molecules superimposes a constant drift velocity on the random thermal motions of the molecules. The spectral density of light scattered from this system is frequency shifted by an amount proportional to the electrophoretic mobility of the molecule. Current theories consider only the application of a square-wave field and the effect on the center-of-mass motion of the molecule. A theory is developed in the present communication that considers the effect of a sinusoidal field on the internal motions of random coils. If the applied frequency is greater than ω_D=μEKcos(θ/2), then the center-of-mass motion remains random whereas internal modes may be “driven” by the applied field. Furthermore, the amplitude of the Doppler-shifted peak position diminishes if ωτ_m > 1, where τm is the relaxation time of the mth mode. It is possible, therefore, to obtain precise values for the number of relaxation modes present and their characteristic relaxation times.