IMAGE STATE INFLUENCE ON CHARGE EXCHANGE IN Li+-METAL SURFACES COLLISIONS

F. BONETTO; EVELINA A. GARCÍA; C. GONZÁLEZ; E. C. GOLDBERG

Rosario

Encuentro; V Reunión Nacional de Sólidos; 2013

The fine details of the electronic structure of metal surfaces, such as energy band gaps, and intrinsic and image surfacestates, are crucial for energy transport and photochemical reactions. In particular, they play a key role in photo dissociation,photo desorption as well as in catalytic reactions. The high neutral fractions at low incoming energies measured inthe scattering of positive Li ions by Cu and noble metal surfaces are a manifestation of the effect of the band structurepeculiarities on the charge exchange between the projectile ions and the target surface. The presence of band gaps affectsthe ion level width, while the interaction of the ion state with the surface states, if the small energy dispersion on k// isneglected, leads to bonding or anti-bonding like states. These effects are crucial in the case of ion energy levels close tothe Fermi level, which is the case of Li+.In this work we analyze the energy dependence of the neutral fraction of Li+ backscattered by Cu(111), Cu(100), Ag(111)and Ag(100) surfaces within an extended Anderson model and by using a time dependent Green functions technique. TheHamiltonian parameters are calculated by considering a linear expansion on atomic orbitals of the solid bulk-like states,in terms of the short range atom-atom interactions and of the density matrix of the solid obtained from DFT calculations.For computing the image surface states we use the metal potential given by Chulkov et al.1Our results show that:- The presence of an energy gap and the Fermi level falling within it explains the experimental trends observed in thecase of Li+ scattered by Cu (111).- The interaction of the ion state with the Cu(100) first image surface state (lifetime of 40 fs) leads to a more accuratedescription of the very high neutral fraction observed at low projectile energies.- The contribution of the image state is not sufficient to adequately describe the experimental neutralization rates obtainedin Li+-Ag systems. However, and mainly at low energies, this contribution represents an improvement in the calculationsince it tends to decrease the difference between theoretical and experimental results.Referencias1E. V. Chulkov, V. M. Silkin, and P. M. Echenique, Surface Science 437, 330 (1999).1