Dynamics of strongly coupled model rotational-translational systems.

Abstract

The power spectra of different dynamic correlation functions of a single one-dimensional rotor coupled to a bath of harmonic oscillators have been studied using molecular dynamics and, to some extent, Mori theory. The latter results, obtained in the lowest nontrivial order of approximation, are found to be unreliable over most of the parameter range studied. The molecular dynamics results are compared both to experiment and to our earlier results for the one-dimensional oscillator which is an example of a linear system. We find that the present nonlinear system retains some of the general spectral features of the linear one. However, the nonlinear coupling strongly affects the low-frequency end of the velocity power spectrum ${P}_{v}$(\ensuremath{\omega}), particularly in the strong-coupling limit \ensuremath{\alpha}\ensuremath{\rightarrow}1. The weak divergence in ${P}_{v}$(\ensuremath{\omega}) as \ensuremath{\omega}\ensuremath{\rightarrow}0 for \ensuremath{\alpha}=1 present in the oscillator case is absent in the rotor system. For nonzero \ensuremath{\alpha} (0\ensuremath{\alpha}1) we find that, for temperatures smaller than the adiabatic barrier height, the qualitative structure of the sine and the cosine power spectral functions are relatively insensitive to \ensuremath{\alpha}. The velocity power spectrum, however, is strongly \ensuremath{\alpha} dependent and can in principle distinguish between the strong (\ensuremath{\alpha}\ensuremath{\rightarrow}1)- and weak (\ensuremath{\alpha}\ensuremath{\rightarrow}0)-coupling regimes.