Measurement of proton momentum distributions by eV neutron scattering

The method of measuring proton momentum distributions by neutron Compton scattering (NCS) is described. Results on a hydrogen bonded potassium dihydrogen phosphate (KDP) single crystal are presented. For the first time NCS measurements are used to reconstruct the proton momentum, wavefunction and Born-Oppenheimer potential. The results provide clear evidence that the ferroelectric transition in KDP is mediated by a quantum tunnelling transition in the hydrogen bonds of the structure. There is a narrowing of the proton momentum distribution as the temperature is raised through the transition, which is consistent with quantum tunnelling between two sites above, but not below, the transition. The separation of the potential wells, derived from the data, is consistent with diffraction data and the calculated transition frequencies are consistent with infrared data. (C) 2002 Elsevier Science B.V. All rights reserved.