A MOLECULAR THEORY FOR THE RANK DEPENDENCE OF ORIENTATIONAL RELAXATION IN BROWNIAN DIPOLAR LATTICE

The rank dependence of orientational relaxation is investigated in mod el Brownian dipolar lattice using the projection operator formalism. T he contribution of long range dipolar interactions to the rate of sing le particle orientational relaxation is evaluated perturbatively in te rms of the polarity parameter of the system. Analytical expressions, e xact up to second order, for the general rank dependent, single partic le orientational correlation function are presented. It is found that the frequency term in the equation of motion of the single particle or ientational correlation function does not depend on dipolar interactio ns when evaluated up to the second order term. The memory function, on the other hand, derives significant contributions from the dipolar in teractions. Relaxation of orientation of rank l is found to become non exponential due to coupling with those of ranks l - 1 and l + 1, and t his becomes more pronounced in the limit of strong interaction. The th eoretical predictions are corroborated well by the known computer simu lation results. (C) 1998 American Institute of Physics.