Measurement of the 3D Born-Oppenheimer potential of a proton in a hydrogen-bonded system via deep inelastic neutron scattering:: The superprotonic conductor Rb3H(SO4)2

We report the first direct measurement of the proton 3-D Born-Oppenheimer potential in any material. The proton potential surfaces in the hydrogen-bonded superprotonic conductor Rb3H(SO4)(2) are extracted from the momentum distribution measured using Deep Inelastic Neutron Scattering (DINS). The potential has a single minimum along the bond direction, which accounts for the absence of the antiferroelectric transition seen in the deuterated material. The measured potential is in qualitative agreement with phenomenological double Morse potentials that have been used to describe hydrogen bonds in other systems.