How do surface energy gaps and image states affect the neutralization of Li+ in the scattering by metal surfaces?

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

San Sebastian

Workshop; 30th Brandt Ritchie Workshop (BRW); 2013

The fine details of the electronic structure of metal surfaces, such as energy band gaps, and intrinsic and image surface states, 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 in the scattering of positive Li ions by Cu and noble metal surfaces are a manifestation of the effectof the band structure peculiarities on the charge exchange between the projectile ions and the target surface. The presence of band gaps affects the ion level width, while the interaction of the ion state with the surface states, if the small energy dispersion on k//is neglected, leads to bonding and anti-bonding states. These effects are crucial in the case of ion energy levels close to the Fermi energy, which is the case of Li+. In this work we analyze the energy dependence of the neutral fraction of Li+backscattered by Cu (111) and Cu (100) surfaces within an extended Anderson model and by using a time dependent Green functions technique. The Hamiltonian 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. [1]. Our results show that: -The presence of an energy gap and the Fermi level falling within it explains the experimental trends observed in the case 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 an accurate description of the very high neutral fraction observed at low projectile energies.