HYDROGEN-BOND DYNAMICS IN BENZOIC-ACID CRYSTALS

We report quasi-elastic neutron scattering (QENS) and optical spectros copic measurements of the proton motion in the hydrogen bond of benzoi c acid crystals, a system in which proton transfer occurs in a near-sy mmetric double-well potential and both coherent and incoherent proton tunneling have been measured. Inter-well relaxation is characterized b y a correlation time that determines the width of the QENS line. Our n ew measurements cover the transition from the classical to the quantum regime. The optical spectroscopic data, using new dopant molecules, e xtend measurements of the proton dynamics to very low temperatures, wh ere coherent tunneling is observed. Theoretical methods, based on a pe rturbative instanton approach, have been developed to describe tunneli ng in multidimensional potential energy surfaces and are used to model our observations. (C) 1998 Elsevier Science B.V. All rights reserved.