PHONON-ASSISTED ASYMMETRIC TUNNELING IN A DOUBLE-WELL POTENTIAL - CONSEQUENCES FOR NEUTRON-SCATTERING AND DIFFUSION

We investigate the dynamics of a light interstitial atom in a double-w ell potential and extend the small polaron theory to include tunneling transitions from the ground state to excited states as well as asymme try between sites. Starting from a generalized spin-boson Hamiltonian we use the resolvent of the time-development operator to derive formul as for transition rates and neutron-scattering structure factors. We s how how with only two parameters the model allows for a quantitative d escription of hydrogen diffusion in bcc metals up to high temperatures . Asymmetry between the wells strongly reduces the mobility, particula rly at low temperatures.