Rotational states of the NH+4 ion in (NH4)2SnCl6 by inelastic neutron scattering

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Abstract
The tunnel splitting of the librational ground state of a NH+4 ion in (NH4)2 SnCl6 has been analyzed by ’’incoherent’’ inelastic neutron scattering. Ground state splittings of 2.96±0.04 and 1.51±0.03 μeV have been observed at T=6 K and are interpreted as transitions between A?T and T?E spin symmetry states, respectively. With increasing temperature the observed tunnel splitting shifts towards smaller energies but fails to show significant lifetime broadening. The first excited librational state is observed at 13.4±0.5 meV. An excitation with E?30 meV probably corresponds to the second excited librational state. Above 70 K classical reorientational motion gives rise to quasielastic scattering. From the temperature dependence of the quasielastic linewidth an activation energy EA=590±30 K is obtained. A three dimensional rotational potential is discussed in view of the above data. The temperature dependence of the tunneling states is compared with current theories.