ROTATIONAL TUNNELING AND LIBRATIONAL EXCITATIONS - THE LOSS OF QUANTUM COHERENCE

We study theoretically the dynamics of rotational tunnelling systems b y combining tunnelling transitions between the wells and librational t ransitions within the wells in a unified treatment. Our method is basi cally not restricted to a perturbative treatment of the coupling of th e rotor to its environment. The line shift and line widths are shown t o be due to incoherent librational excitations while damping in the tu nnelling process itself is subordinate. For weak coupling, we find tha t both the temperature dependence of the line width and line shift obe y an Arrhenius law with an activation energy given by the librational energy HBARomega0. This is in agreement with the experimental data.