The Dynamics of Free and Confined Quantum Tops

Robert M. Dimeo, Division 856,  NIST Center for Neutron Research, MSEL

The molecules in many molecular solids behave like tiny tops whose rotation is hindered through the interaction with nearby molecules. Since these are nano-scale tops, their rotational dynamics are governed by quantum mechanics, thus they behave differently from the macroscopic toy tops we know. One startling example of this is a phenomenon known as quantum rotational tunneling. Since the molecular rotor is hindered, it needs a fixed amount of energy (usually thermal) to rotate. At temperatures below 10 K there is not enough energy to cause the molecule to rotate...yet it does! This phenomenon is known as rotational tunneling.

The rotational tunneling frequency, which can be measured directly with inelastic neutron scattering, is extremely sensitive to changes in the local molecular environment. Therefore rotational tunneling spectroscopy is an excellent probe of local disorder. The ability to probe local disorder is an important tool especially when studying adsorbed species. Rotational tunneling spectroscopy of adsorbed molecular species can provide direct information on finite-size effects, surface interactions, and disorder which have fundamental implications in understanding nanostructured materials, tribology, and adhesion, for instance.

The work presented here describes the results of an investigation of the effects of confinement and surface chemistry on the rotational dynamics of methyl iodide, a textbook example of a one-dimensional quantum rotor. The measurements were performed using state-of-the-art neutron spectroscopy at the NIST Center for Neutron Research.