Adiabatic Rotational Splittings and Berry's Phase in Nuclear Quadrupole Resonance.

A new effect in nuclear quadrupole resonance (NQR) spectroscopy is predicted and observed. Using the sup 35Cl NQR spectrum of NaClO sub 3 as a test case, it is shown that rapid rotation of the crystal while observing the spectrum induces splittings in the NQR lines. Since the NQR spectrum of a static sample is independent of the sample orientation, the observed splittings contradict the usual assumption that the magnetic resonance spectrum of a rotating sample is simply the orientationally averaged spectrum. In addition, the splittings contain structural information, in particular regarding the directions of local electric field gradients at nuclear positions, and allow the determination of the absolute sign of the nuclear gyromagnetic ratio. The splittings represent the first experimental demonstration in magnetic resonance of a physical phenomenon now called "Berry's phase", namely the path dependence of the phase of a quantum mechanical state when it evolves under a slowly changing Hamiltonian. The splittings are also interpreted as resulting from a fictitious magnetic field that appears in a rotating frame of reference. Conditions under which Berry's phase will have observable effects on magnetic resonance spectra of rotating samples are discussed.