Visualization and measurement of quantum rotational dynamics

An introduction to quantum rotational tunneling and libration is presented with an emphasis on obtaining a qualitative understanding of this phenomenon through visualization of the dynamics, simple approximations, and measurements. The tunneling and librational dynamics of small molecular rotors are discussed using a very simple model of the rotational potential. Numerical calculations of the evolution of probability packets are carried out for the low-lying states and the connection is made between the quantum and classical librational dynamics. Finally, we present measurements of these quantum rotations using inelastic neutron scattering and show in particular how neutron scattering measurements of the ground state tunnel splitting and first librational transition can be used to characterize the magnitude and shape of the potential hindering the motion of the rotor. Some conceptual and computational problems are included that are suitable for undergraduate students. (C) 2003 American Association of Physics Teachers.