We studied the surface diffusion of hydrogen on Ni(lOO) by linearly diffracting a probe laser beam from a monolayer grating of adsorbed hydrogen. In the temperature range from 110 to 161 K, the Arrhenius plot shows a significant level off of the diffusion coefficient D{T) at around 144 K. Such a deviation from the behavior of a thermally activated diffusion suggests the onset of quantum tunneling diffusion.PACS numbers: 68.35.Fx, 66.10.Cb, 68.35.Ja One of the most interesting aspects of the transport of massive particles in condensed media is quantum tunneling motion. The process is most sensitive to various interactions and their interplay. The motion of the hydrogen atom and its isotopes in solids has attracted considerable attention in the past two decades [1,2]. Because the characteristic vibrational energies of hydrogen atoms in or on a solid are much closer to the substrate phonon energies, the screening effect is expected to be much stronger. The isotope dependence of the quantum motion of hydrogen should be the strongest of all stable elements because of the large mass ratios. As chargeless particles, hydrogen atoms may be one of the best systems for investigation of transport phenomena in the strong scattering limit, which will complement the investigation on electrons and electromagnetic radiation [3][4][5][6]. Theoretical studies and indirect experimental evidences indicate that the wave functions of hydrogen atoms on metals have sufficiently large spreads that the transport and vibrational properties may behave more like band electrons [7]. Experimentally, hydrogen atoms are relatively easily dissolved into many metals [1]. They also readily adbsorb onto the surfaces of many materials [8].However, direct experimental observation of the quantum motion of hydrogen atoms is rather scarce. The mobilities of hydrogen atoms in bulk solids have not been found other than those characteristics of thermally activated diffusions. The theories of quantum transports of hydrogen atoms such as the ''small-polaron'' theories and narrow-band particle models, all predicting power-law behaviors for diffusion coefficients at low temperatures, have yet to be tested against experimental observation [1,9,10].Recently, Gomer and co-workers measured the surface diffusion coefficients D(T) of hydrogen and its isotopes on W and Ni single crystals using a field-emission microscope (FEM) [11,12]. They found that the Arrhenius plots of D(T) level off at temperatures as high as 130 K and that D(T) exhibits large and yet reduced (from a rigid lattice model) isotope dependence, indicating the onset of quantum tunneling motion and a large mass renormalization effect. Even more interesting is that in a large temperature range from roughly 100 down to 30 K D(T) shows little temperature dependence. These results present a new case which is not yet explained satisfactorily by the existing theories [13]. On the experimental side. Comer's reports stand alone and the progress with the field-emission microscopy has been slow. This is mainl...

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