Dynamics of self-interstitial atoms in bcc metals

We present calculations of the local frequency spectrum of 〈110〉-split-interstitial atoms in bcc metals with the use of the Green's-function method. The dumbbell vibrates with low-frequency resonant modes and high-frequency localized modes with little contribution from the host frequencies. In agreement with experiment the spectrum leads to enhanced mean-square thermal displacements of the dumbbell atom. The frequencies of the librational modes B1g and B2g provide a consistent explanation of the observed changes in the shear moduli of irradiated Mo. The long-range migration of the interstitial atoms is discussed in view of the obtained resonance modes.