Onset of 3D Collective Surface Diffusion in the Presence of Lateral Interactions:Na/Cu(001)

Abstract

3He spin-echo measurements are used to follow the picosecond motion of sodium atoms on a copper (001) substrate. 2D correlated motion arising from repulsive adsorbate interactions is observed for coverages as low as 0.04 ML. At coverages greater than 0.05 ML there is a pronounced onset of motion perpendicular to the surface. The perpendicular motion is thermally activated and seems related to the basic translational hopping diffusion process. The correlated motion is modeled successfully using a molecular dynamics simulation and a dipolelike lateral interaction. A simple model which relates the apparent height of the atom with its local coverage is shown to reproduce the experimental observations.

Potential energy surface of alkali atoms adsorbed on Cu(001)

“…11 More recently, the much improved resolution of the 3 He spin-echo technique 12 has allowed for a deeper analysis of the diffusion process at larger coverage. 13 This study interpreted in terms of collective phenomena a coverage dependence of Na surface diffusion previously reported and unexplained: 14,15 indeed, the diffusion process involves nontrivial electrondensity rearrangements, 16,17 a phenomenon further showing the complexity of such systems. Additionally to the information on the given studied system, high-precision HAS measurements may act as a reference to other experimental and theoretical methods.…”

“…The value of E dd has been previously adopted as a measure of local concentration of adsorbates for generic distributions of alkali atoms. 13,16 As already done for Na, 21 we define a refined adsorption energy by subtracting such dipole-dipole contribution, using the value of obtained by the corresponding DFT calculation…”

Theoretical approaches in adsorption: alkali adatom investigations

“…Interpretation of adsorption experiments such as the ones presented by the Cambridge group and obtained by 3 He spin echo scattering measurements [19,43] can be improved by coupling them to ab initio investigations. With this purpose, we have calculated the adsorption sites and the energetics of Na/Cu(001) at low coverages, in the framework of DFT.…”

Stochastic theory for lineshape broadening in quasielastic He atom scattering with interacting adsorbates

“…1 QHAS has been applied to different metal/metal and adsorbate/metal systems, the diffusion of Na adatoms ͑at different coverages͒ on Cu͑001͒ being one of the most extensively systems studied so far. [2][3][4][5][6][7][8][9][10][11][12] At low coverages, this system has been theoretically described within the so-called single adsorbate approximation and analyzed in terms of the motional narrowing effect, 4,7 and also within the Kramers turnover theory 5 and the dephasing theory. 6 In particular, the motional narrowing effect governs the broadening of the lineshapes as a function of the friction, the parallel momentum transfer, the lattice structure, and the jump dynamics.…”