  In this paper, the chaotic internal degrees of freedom of a benzene molecule
adsorbed on a graphite substrate, their interplay with thermal noise, and their
effects on the diffusion and drift are investigated analytically by making use
of the presence of two different time scales as well as by molecular-dynamics
simulations. The effects of thermal noise are investigated, and it is found
that noise does not significantly alter the dynamics of the internal degrees of
freedom, yet affects the friction and diffusion of the center of mass.
Qualitative and quantitative theoretical predictions for the friction and
diffusion of the molecule on the substrate are made and are compared to
molecular-dynamics simulations. Contributions to the friction and diffusion
from the finite heat bath as well as the slow dynamics of the center of mass
are formally identified. It is shown that the torsion in benzene, which
dominates the nonlinear coupling, significantly affects the friction of the
molecule on the surface. The results compare favorably with recent results from
He/neutron spin echo experiments on this system. Based on the analytical and
numerical results, some suggestions are made for experimental conditions under
which the effects of internal degrees of freedom might be observable.
