Direct observation of tunneling in KDP using Neutron Compton scattering.

Neutron Compton Scattering measurements of the momentum distribution of hydrogen in KDP just above and well below the ferro-electric transition temperature show clearly that the proton is tunnelling in the high temperature phase. The double well potential extracted directly from the data is consistent with neutron diffraction data, and the vibrational spectrum is in substantial agreement with infrared absorption measurements.