Anharmonicity and Tunnelling on the Potential Energy Surface of Ammonia

We request beam-time on the VESUVIO spectrometer in order to characterise the Potential Energy Surface of the Ammonia molecule through a neutron Compton scattering measurement. Ammonia (NH3) molecular dynamics is affected by a strong hydrogen bonding between molecules, and by the typical “umbrella” Nitrogen spatial inversion, i.e., the connection between two minima in the potential energy surface through quantum tunnelling. We propose to investigate the single-particle dynamics measuring the nuclear momentum distributions and mean kinetic energies of Hydrogen and Nitrogen nuclei in Ammonia. The measurement of both protonated and deuterated Ammonia at different temperatures and across the solid-liquid phase transition (solid at T = 5K and 185 K, liquid at 195 K) will highlight anharmonicity in the potential energy surface and the importance of tunnelling on the nuclear dynamics.