The dynamics of a light interstitial atom in a double-well potential were investigated, and the small polaron theory was extended so as to include tunnelling transitions from the ground state to excited states, as well as the asymmetry between sites. Starting from a generalized spin-boson Hamiltonian, the resolvent of the time-development operator was used to derive formulae for the transition rates and neutron-scattering structure factors. It was shown that, using just 2 parameters, the model permitted a quantitative description to be given of H diffusion in body-centered cubic metals up to high temperatures. An asymmetry between the wells strongly reduced the mobility; especially at low temperatures.

Phonon-Assisted Asymmetric Tunnelling in a Double-Well Potential: Consequences for Neutron Scattering and Diffusion. S.Dattagupta, H.R.Schober: Physical Review B, 1998, 57[13], 7606-14