Observation of “forbidden modes” in neutron scattering measurements

Extra peaks have been observed at lower energy transfer in neutron scattering measurements of the [00ζ]L branch of the phonon dispersion relation in rubidium. They are explained in terms of the relaxed resolution, normal to the scattering plane, of the triple-axis spectrometer, and the fact that the L and T branches in the [00ζ] direction lie close together. The possibility of similar or related observations in other systems is briefly examined.