Rotational States of NH4+in KBr Crystal. A Complementary Study by Inelastic Neutron Scattering and Calorimetric Measurements

Rotational tunneling energy states of the NH 4 + ion doped in KBr crystal have been reinvestigated so as to obtain decisive evidence on which symmetry is the most likely one for the orientation of the ions in the lattice. The inelastic neutron scattering experiments at 1.8 K uncovered the existence of two characteristic bands in the energy range ε<8 meV, which leads to the conclusion that the model of energy states for C 3 v symmetry is exclusively acceptable for this system. The Q -dependence of the scattering features was also shown to be reasonably consistent with the model. Moreover, both the “equilibrium” and “instantaneous” heat capacities obtained between 4 and 10 K yielded the confirmation of the C 3 v symmetry. The rate of conversion between the nuclear spin symmetry species of NH 4 + was examined.