MOLECULAR DIPOLE CHAINS - EXCITATIONS AND DISSIPATION

As a model for orientational excitation of molecular arrays, we examin e the excitation behavior and energy flow patterns in a model system. The model is simply a chain of classical point dipoles with fixed mass center, rotating in a plane containing the intermolecular axis, and i nteracting by the classical dipole potential. At low energies, the dis persion relation is quite different from that for a phonon system, sho wing a flat frequency maximum at k = 0. Correlation function analysis shows a significant transition from the low-energy regime in which the local dipole motion is predominantly oscillatory (with periodic corre lation functions and Fourier components that maximize at a well-define d oscillation frequency), to a high-energy situation in which a Raylei gh peak occurs in the k = 0 Fourier component, and finite frequency re sponse occurs only for higher wave vector. Physically, this transition occurs for thermal energy roughly equal to the typical magnitude of t he local dipolar interaction. Thus for energies above this transition, the kinetic energies are high enough that the local motion is predomi nantly rotatory rather than oscillatory. These changes are also seen i n the frequency moments and Kirkwood g-factors. The simulations show t hat initial energy deposited in one rotating dipole passes down the ch ain almost like a solitary wave, reflecting off of the free chain end and then traversing the chain again.