The pressure dependence of rotational states in (NH4)2 PdCl6

The pressure dependence of the ground-state tunnel splitting and the energy of transition to the first excited librational state of the NH+4 ion in (NH4)2 PdCl6 has been measured with inelastic neutron scattering. A Grüneisen parameter for the tunneling transitions γtun=−13 has been obtained. The relation between the rotational states and the intermolecular interaction is based on a model theory of Hüller and Raich. The influence of additional higher order terms in the rotational potential is discussed. An exponent n=11±2 of the r-dependent interaction V∼r−n is derived.