Proton quantum coherence observed in water confined in silica nanopores

Deep inelastic neutron scattering measurements of water confined in nanoporous xerogel powders, with average pore diameters of 24 and 82 angstrom, have been carried out for pore fillings ranging from 76% to nearly full coverage. DINS measurements provide direct information on the momentum distribution n(p) of protons, probing the local structure of the molecular system. The observed scattering is interpreted within the framework of the impulse approximation and the longitudinal momentum distribution determined using a model independent approach. The results show that the proton momentum distribution is highly non-Gaussian. A bimodal distribution appears in the 24 angstrom pore, indicating coherent motion of the proton over distances d of approximately 0.3 angstrom. The proton mean kinetic energy < E-K >(W) of the confined water molecule is determined from the second moment of n(p). The < E-K >(W) values, higher than in bulk water, are ascribed to changes of the proton dynamics induced by the interaction between interfacial water and the confining surface.