Vibration and diffusion of Cs atoms on Cu(001)

Abstract

3He spin-echo (3HeSE) dynamics measurements of low coverages of Cs on Cu(001) both reveal quasi-elastic broadening of the helium beam due to aperiodic transport on the surface, and extend measurements of the previously observed low energy acoustic phonon mode, at coverages between 0.014 and 0.056 ML and temperatures of 130 and 80 K. The low energy phonons and quasi-elastic broadening occur on similar timescales and we separate the contributions by converting the spin-echo measurement to the energy domain. Langevin molecular dynamics simulations reproduce the variation of the quasi-elastic peak width, phonon position and amplitudes with momentum transfer, temperature and coverage. The main features in the experimental data require a potential corrugation of 20 ± 2 meV and a friction parameter of 1/40 ps−1. Our results indicate that the Cs dynamics are dominated by dipole–dipole repulsion and produce strongly correlated motion. However, contrary to previous expectations the transport proceeds through jump like behaviour within the Cs overlayer, and Cs moves much more freely than other alkali metals on copper. The unusual behaviour that we see requires three critical components; strong interadsorbate forces, a weak but finite substrate corrugation, and low adsorbate–substrate friction. Together, these key features manifest themselves as a distinct signature in the intensity distribution across the energy/momentum exchange spectrum.

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

“…47 As for HAS experiments on Cs/ Cu͑001͒, owing to low experimental amplitude at large momentum transfer, it has not been possible to determine the top/bridge energy ratio; therefore, a substrate potential with a single parameter, E T ϵ E B =20Ϯ 2 meV, has been proposed. 46 This value is intermediate between our estimates for the bridge and top energies, 13 and 25 meV, respectively. One has to notice that the PES corrugation for alkalis on Cu͑001͒, on the order of a few tenths of eV, is about the same magnitude as the chemical accuracy searched and not commonly achieved by exchange-correlation functionals such as PBE.…”

“…45 As for Cs adatoms, their diffusion would be dominated by the strong repulsive interaction with the neighbors. 46 …”

Surface dynamics and friction of K/Cu(001) characterized by helium-3 spin-echo and density functional theory

“…The values indicate a very high mobility of adsorbed potassium atoms similar in magnitude to that seen for the system of Cs/Cu͑001͒. 9 In common with Cs, we see a momentum-transfer dependency that does not follow a simple sinusoidal single jump shape 22 but a shape that is rather flat. A flatter form for the ⌬K variation is indicative of multiple jumps, 31,32 typically caused by a low adsorbate- substrate friction.…”

“…The effective activation energy may underestimate the true, adiabatic barrier; 30 nevertheless, the value obtained in the present work necessarily reflects a relatively small corrugation energy landscape, significantly lower than sodium 8 and slightly higher than that seen with caesium. 9 In Fig. 5 we present the momentum-transfer dependency of ␣ in the ͓100͔ direction for = 0.018, 0.032, 0.045, and 0.056.…”

“…8͒ and Cs/Cu͑001͒. 9 Previous studies of K/Cu͑001͒ have focused mainly on adsorbate structure, vibrations, and adsorption kinetics, [10][11][12][13][14][15][16][17][18] rather than adsorbate diffusion. Low-energy electrondiffraction ͑LEED͒ investigations of K/Cu͑001͒ have mapped a series of growth phases seen during adsorption.…”