Perturbation theory approach to rotational tunneling systems

Diagrammatic perturbation theory is used to consider the problem of the cou pling of a molecular group libration to the lattice vibrations in solids ex hibiting rotational tunneling detected by inelastic neutron scattering. The technique is applied to a molecular group of symmetry C-3 in presence of t he hindering potential of the solid. The spectral density for transitions b etween rotational tunneling states in the ground librational level is obtai ned as a function of temperature. Low temperature results are presented for different phonon spectrum parameters. They are used to check the assumptio n that this coupling is the origin of the peculiar behaviour of thr: line s hifting and broadening as a function of temperature in the INS spectra of s uch systems.