Hydrogen tunneling or hopping - how can we know?

Quantum tunneling processes are often of interest in the consideration of hydrogen transport. The nuclear magnetic resonance (NMR) spin-lattice relaxation time T-1 provides a particularly convenient signature to distinguish between hopping and tunneling processes. Using as examples, deuterated and normal benzoic acid (BA), and hydrogen glasses of the form R1-x(NH4)(x)H2AsO4 (RADA-X), typical NMR signatures of hopping/tunneling processes will be discussed. A unified treatment is presented which demonstrates the extraction of potential barrier 'V', quantum-well asymmetry 'A', and both hopping and tunneling rate constants. Three regions are discussed; (1) a 'high'-temperature (HT) hopping region, (2) a 'low-temperature' (LT) incoherent tunneling regime, and finally (3) at very low temperature, a region where T-1 is essentially temperature-independent. This last region is discussed in terms of coherent quantum tunneling. (C) 1999 Published by Elsevier Science B.V. All rights reserved.