DYNAMICS OF THE DAMPED ROTATIONAL TUNNELING SYSTEM - THE EFFECT OF LIBRATIONAL EXCITATIONS

We present a description of the dynamics of rotational tunneling systems in which tunneling transitions between the wells and librational transitions within the wells are combined in a unified treatment. Our method does not depend on a perturbation expansion of the coupling of the rotor to its environment. We show that the line widths and line shifts in the structure factor for neutron scattering are dominated by incoherent librational excitations while damping in the tunneling process itself is subordinate. For weak coupling, we find that both the temperature dependence of the line width and line shift obeys an Arrhenius law where the activation energy is given by the librational energy hBARomega0. This is in agreement with the experimental data.