Isotope Shift of a Low-Lying Lattice Resonant Mode

Abstract

A sharp and very low-frequency lattice absorption due to a lithium resonant mode has been found in KBr:LiBr. A large frequency shift with lithium isotopes has been observed. The experimental results can be understood with a simplified lattice model in which both nearest-neighbor force constants and the mass of the impurity are varied. It is concluded that the resonant mode comes from a remarkable softening of the forces between the impurity and the surrounding lattice.