Quantum molecular tunnelling in solids: Inelastic neutron scattering study

摘要

Molecular tunnelling at low temperature is a very sensitive tool to probe the molecular environment in the solid state. The rotational excitations of NH_4~+ ions in dilute solutions in metal alkali halide lattices at temperatures below 15K have been studied by inelastic neutron scattering technique. At very low ammonium ion concentration, c (c→0), INS spectra show sharp tunnelling lines representing isolated almost free rotors. On increase of c it transforms to an orientational glass phase. It has been shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variations of the intensity profiles with increase of c have been explained by a model based on Gaussian distributed barrier heights.