An
understanding of hydrogen diffusion on metal surfaces is important
not only for its role in heterogeneous catalysis and hydrogen fuel
cell technology but also because it provides model systems where tunneling
can be studied under well-defined conditions. Here we report helium
spin–echo measurements of the atomic-scale motion of hydrogen
on the Ru(0001) surface between 75 and 250 K. Quantum effects are
evident at temperatures as high as 200 K, while below 120 K we observe
a tunneling-dominated temperature-independent jump rate of 1.9 ×
109 s–1, many orders of magnitude faster
than previously seen. Quantum transition-state theory calculations
based on ab initio path-integral simulations reproduce the temperature
dependence of the rate at higher temperatures and predict a crossover
to tunneling-dominated diffusion at low temperatures. However, the
tunneling rate is underestimated, highlighting the need for future
experimental and theoretical studies of hydrogen diffusion on this
and other well-defined surfaces.
We report measurements of the acoustic and optical phason dispersion curves associated with the lattice of partial dislocations on the reconstructed (111) surface of gold. Our measurements of these low energy (<0.5 meV) weakly dispersive modes have been enabled by the very high resolution of the novel helium spin-echo technique. The results presented here constitute the first measurement of the phason dispersion of misfit dislocations, and possibly of excitations associated with any type of crystalline dislocations.
We describe a model of the dynamical temperature evolution in a solenoid winding. A simple finite element analysis is calibrated by accurately measuring the thermally induced resistance change of the solenoid, thus obviating the need for accurate knowledge of the mean thermal conductivity of the windings. The model predicts quasi thermal runaway for relatively modest current increases from the normal operating conditions. We demonstrate the application of this model to determine the maximum current that can be safely applied to solenoids used for helium spin-echo measurements.
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