Partial conversion of neutron incoherent into coherent scattering by NH 3 quantum rotators

Abstract

Neutron diffraction experiments on powders are used to investigate the quantum mechanical behaviour of uniaxial rotators, i.e. NH 3-molecules in Ni(NH 3) 6(PF 6) 2, Co(NH 3) 6(PF 6) 2 and Ni(NH 3) 6I 2. A diffuse temperature dependent component of the total scattering, modulated ∼ sin Qr H-H/ Qr H-H, is observed at a temperature of ≃ 20 K and below. This observation is due to the fact, that on cooling the sample to kT ∼ B (rotational constant) the rotational ground state is strongly populated. The ground state of a uniaxial free rotator, however, is completely polarized. As a consequence the scattering of those NH 3 groups being in the ground state becomes coherent and is observable in diffraction experiments.