Alkali atoms at surfaces show unexpected behaviour arising from the strong interadsorbate forces and coupling of electronic and nuclear motion [2,3]. In the present work we present new results giving additional insight into both the dynamics and the scattering.

High resolution quasielastic helium atom scattering (QHAS) experiments provide a unique tool to study adsorbate dynamics[1] since3He spin-echo measurements give the surface correlation in time through the intermediate scattering function (ISF), I(ΔK,t). The functional form of the ISF has a well defined analytic form for simple systems, however for real systems the form is typically complex and challenging to predict. For example the shape of the ISF can take a multifunctional form due to summation effects from multiple species or clusters moving with the same or different modes, and a convolution effect from differences in the local appearance of the potential energy landscape to the adsorbate for the length scale under study.

Previous investigations of the diffusion of alkali metals on metal surfaces have revealed unexpected phenomena. For sodium on Cu(100), a 'new' mode of aperiodic motion, perpendicular to the surface, was discovered[2]. For caesium, the observed dynamics on the same surface were found to be qualitatively different to sodium[3], indicating a critical balance of adsorbate-substrate and adsorbate-adsorbate interactions, determining the behaviour for each adsorbate.

In this work we present detailed helium-3 spin-echo QHAS measurements of the dynamics of sodium adsorbed on Cu(111) at low to moderate coverages. We observe a QHAS broadening with the presence of a complex lineshape which shows a momentum transfer variation. The functional form is compared with theoretical models[4] for different potential energy landscapes. The variation of the QHAS broadening with temperature and momentum transfer are studied to access information on the dynamic phenomena on the Cu(111) surface, exploring in particular the generality of perpendicular motion, and changes in the adsorbate-substrate/adsorbate-adsorbate force balance.

[1] A.P. Jardine, S. Dworski, P. Fouquet, G. Alexandrowicz, D. J. Riley, G. Y. H. Lee, J. Ellis, W. Allison, Science 304, 1790 (2004).

[2] G. Alexandrowicz, A. P Jardine, H. Hedgeland, W. Allison, J. Ellis, Phys. Rev. Lett 97, 156103 (2006)

[3] A P Jardine, G Alexandrowicz, H Hedgeland, R D Diehl , W Allison and J Ellis, J. Phys.: Cond. Matter 19 305010 (2007)

[4] R. Martinez-Casado, J.L. Vega, A.S Sanz, S. Miret-Artes, J. Chem. Phys.126,194711-5 (2007)