Surface diffusion of hydrogen atoms on Cu(111) studied by optical diffraction from hydrogen density patterns formed on removable templates of xenon monolayers

We formed grating-like patterns of hydrogen adatom density on Cu(111) by using complementary patterns of xenon adatom density as the template. The template was preformed by laser-induced thermal desorption of a Xe monolayer on Cu(111) using the interference pattern of two coherent laser pulses. By following the evolution of hydrogen density gratings on Cu(111) from 153 to 183 K with linear optical diffraction, we found that the diffusion of hydrogen atoms on Cu(111) in this temperature range is the classical over-barrier hopping characterized by an activation energy barrier E_diff = 6.4 Kcal/mol (or 279 meV) and a pre-exponential factor D₀ = 2.0 × 10 ^-3 cm²/s. Based on the potential model proposed by Basdescu and coworkers for hydrogen on Ni(111), the WKB tunneling coefficient between the first vibrational excited states of a hydrogen adatom on Cu(111) is expected to be at least one order of magnitude smaller than that on Ni(111), indicating that the classical over-barrier hopping of hydrogen atoms could directly cross over to under-barrier tunneling between ground states on Cu(111).