ENHANCEMENT OF NUCLEAR-SPIN CROSS-RELAXATION IN METAL-HYDROGEN SYSTEMS

A cross-relaxation model has previously been proposed to explain the anomalous frequency dependence of proton relaxation rates R1 in several metal-hydrogen systems at low temperatures. We report two experiments that test and unambiguously confirm the cross-relaxation model. The first experiment uses large-amplitude field modulation to sweep the proton resonance through the inhomogeneously broadened spectrum of metal resonances. The second involves rotation of the sample to sweep the metal resonances through the proton line. In both experiments, the protons are brought into spin thermal contact with an increased number of metal nuclei. Sample rotation rates of only 10 Hz increase the proton R1 by a factor of 50 for the TaHx system. A simple model explains the variation of R1 with rotation speed. Both techniques reveal increased proton relaxation rates R1, unique signatures of the cross-relaxation model. A model is presented that explains the weaker than linear temperature dependence of R1. The role of disorder on a length scale of 500, angstrom or less is emphasized.