Coverage dependence of quantum tunneling diffusion of hydrogen and deuterium on Ni(111)

Quantum Diffusion of H on Pt(111): Step Effects

“…Between the two flat samples A and B, the diffusion coefficients coincide with each other at high temperatures, but display some difference at low temperatures. The transition temperature for the leveling off around 95 K is lower but comparable with those observed on nickel and tungsten surfaces by FEM and LOD [1][2][3][4][5][6][7][8][9][10][11][12], but significantly higher than the value on Cu(100) measured by STM [13]. Since quantum tunneling opens a new diffusion channel in addition to the classical one, the data shown in Fig.…”

“…The nonequivalence of the adsorption sites can come from (i) small-polaron effect; (ii) adsorbateadsorbate interaction; and (iii) surface defect-mediated adsorption energy changes. While the first two may occur even for perfect surfaces [11], the last one can create a large variety of nonequivalent sites depending on the defect density, the defect type, and the distance away from defects. As discussed in Ref.…”

“…Experimentally, while similar behaviors were observed by the same group using the same technique for H on a number of other metal surfaces [2 -7], only one report using a different technique, scanning tunneling microscopy (STM), has confirmed the weak temperature dependence of surface (tracer) diffusivity for H on Cu(100) [13], but at a diffusion rate about 6 -9 orders of magnitude slower (10 ÿ19 cm 2 = sec vs 10 ÿ10 -10 ÿ13 cm 2 = sec). Using a linear optical diffraction (LOD) technique that probes diffusion over a length scale of microns, Zhu and his coworkers found an activated quantum diffusion with a strong temperature dependence for H on Ni(111) [11,12] and Ni(100) [8], drastically different from what has been observed by Gomer and his coworkers on the same systems [7]. These differing and apparently contradictory observations, using different techniques, need to be resolved in order to remove any doubts on the experimental observation of quantum surface diffusion.…”

“…DOI: 10.1103/PhysRevLett.97.166101 PACS numbers: 68.35.Fx, 68.35.Dv, 68.43.Jk, 82.20.Xr Because of the small mass of the hydrogen atom and the weakly corrugated potential energy surface, diffusion of hydrogen on metal surfaces may take both classical overbarrier hopping and quantum mechanical under-barrier tunneling. Quantum effects in surface diffusion are of broad interest and have been intensively studied both experimentally [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and theoretically [16 -26]. The first observation of a nearly temperature independent surface (chemical) diffusivity of H on W(110) in the low temperature regime by Gomer and coworkers [1] using field emission microscopy (FEM) has stimulated a great theoretical effort [16 -26] because of the observed unexpected different behavior from the light atom diffusion in bulk metals, for which a much stronger temperature dependence was found [27].…”

Two-bath model for activated surface diffusion of interacting adsorbates

“…19 A straightforward generalization of the effective frequencies can be done for the two-bath model. In particular, this new approach could provide a theoretical framework to better interpret the coverage dependence of tunneling diffusion experiments 41 within a quantum Markovian formalism. 42 Many times, in surface diffusion LMD simulations surface barriers are only changed with the coverage in order to reproduce experimental data, keeping the corresponding frequencies fixed.…”