ROTATIONAL TUNNELING AND LIBRATIONAL EXCITATIONS - THE LOSS OF QUANTUM COHERENCE

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