Molecular dipole chains: Excitations and dissipation

As a model for orientational excitation of molecular arrays, we examine 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 interacting by the classical dipole potential. At low energies, the dispersion relation is quite different from that for a phonon system, showing 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 correlation functions and Fourier components that maximize at a well-defined oscillation frequency), to a high-energy situation in which a Rayleigh peak occurs in the k = 0 Fourier component, and finite frequency response occurs only for higher wave vector. Physically, this transition occurs for thermal energy roughly equal to the typical magnitude of the local dipolar interaction. Thus for energies above this transition, the kinetic energies are high enough that the local motion is predominantly rotatory rather than oscillatory. These changes are also seen in the frequency moments and Kirkwood g-factors. The simulations show that initial energy deposited in one rotating dipole passes down the chain almost like a solitary wave, reflecting off of the free chain end and then traversing the chain again.