NH3 on the MgO{100} surface: spectroscopic evidence of coupled internal and centre of mass quantum rotation

High-resolution neutron spectroscopy was used to measure the rotational spectra of isolated NH3 molecules adsorbed on MgO{100} surfaces. The energies of the four lowest rotational levels were analyzed first by the single-particle model and secondly by a model of coupled rotation of the molecule and its centre of mass. Only the latter potential surface can be reproduced with reasonable accuracy on the basis of semi-empirical pair interaction potentials. Thus this system is among the first examples of coupled internal and off-centre rotation in the quantum regime. The off-centre site of the NH3 and the coupled motion are confirmed by a recent ab initio calculation.