INELASTIC NEUTRON SCATTERING STUDIES OF TUNNELLING PROCESSES IN MOLECULAR CRYSTALS
Roberto Caciuffo
Istituto
Nazionale per la Fisica della Materia, Dipartimento di Scienze
dei Materiali e della Terra, Universita di Ancona, Via Brecce
Bianche, I-60131 Ancona, Italy.
E-mail: rgc@popcsi.unian.it
Keywords: neutron scattering; molecular tunnelling;
magnetization tunnelling
High resolution neutron spectroscopy has been widely used in the past twenty years to study single-particle rotations in molecular crystals, giving a wealth of useful information on the correlation between the macroscopic properties of a molecular solid and the orientational freedom of its constituents [1,2]. A large number of systems has been investigated in which the rotating groups experience a relatively strong barrier to reorientation. In these cases, the low temperature dynamics can be described in terms of torsional excitations, taking into account that the degeneracy of the torsional levels is lifted by tunnelling between different potential minima. The tunnelling splitting is very sensitive to the barrier height, decreasing rapidly as the barrier height increases. Therefore, its experimental determination generally allows an accurate estimation of symmetry and magnitude of the potential energy surface, which is in turn strongly dependent on the molecular conformation in the solid and on the nature of the binding forces.
In this talk, I will present the results obtained from inelastic neutron scattering experiments on the quantum dynamics of supramolecular complexes containing ensembles of weakly interacting molecular groups [3]. The analysis of these results provide structural information which cannot be obtained from diffraction experiments when sufficiently large single crystal samples are not available.