ELSEVIER Surface Science 392 (1997) L7 LI0 .... surface science Surface Science Letters Double 2p electron excitation in low-energy Ne + single scattering from a Si surface: an energy loss study G. Manic& F. Ascione, N. Mandarino, A. Bonanno, P. Riccardi, P. Alfano, P. Zoccali, A. Oliva, M. Camarca, F. Xu * Dipartimento di Fisica, Universitgt della Calabria. 87036 Arcavacata di Rende (CS), ltalv Received 31 March 1997; accepted for publication 9 July 1997 Abstract Results of 1950 eV Ne + single scattering from a Si surface show that all the backscattered Ne + ions have suffered severe inelastic energy loss and the total inelasticity in the close binary encounter amounts to 45 +4 eV. This value is well beyond that required for one-electron excitation from the neutralized Ne°, but agrees well with that for the double 2p electron excitation of a neutralized Ne to 2p4(1D)3s 2. These findings contrast the re-ionization model proposed by Souda et al. [Phys. Rev. Lett. 75 (1995) 3552; Surf. Sci. 363 (1996) 139], and confirm that the formation of autoionization states which decay far away from the surface is the main mechanism for Ne* scattered from Si. © 1997 Elsevier Science B.V. Kewvords: Atom-solid scattering; Ion-solid interactions; Low energy ion scattering; Noble gases: Silicon Charge exchange plays a crucial role in the quantitative determination of surface composition using of ion scattering spectroscopy (ISS). The classical resonant and Auger capture mechanisms for surface neutralization and ionization have been extensively studied ever since the pioneering work of Hagstrum some 40 years ago [1,2], and quite a good understanding of these phenomena has been achieved [3 ]. On the other hand, electron excitation in close encounters can also play an important role in determining the final charge state of back- scattered projectiles [4-6]. The Fano-Lichten- Barat molecular orbital curve-crossing model [7], originally developed for describing core electron excitation in gas-phase atomic collisions, is now * Corresponding author. Fax: (+ 39) 984 493187: e-mail: xu@fis.unical.it 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 (97)00575-X widely adopted with great success in also interpre- ting the results of ion-surface collisions [8-11 ]. Very recently, Souda et al. [12,13] reported a study of 2 keV Ne + and Ne ° scattering from Mg, A1, and Si surfaces. They observed that for an incidence angle of c~ = 30 ° and a scattering angle of 0=60 C~, the Ne + lineshape and intensity are virtually the same for both neutral and charged projectiles, and concluded that one-electron excita- tion is predominant over two-electron excitation in Ne. They attributed this difference to the results of gas-phase experiments, where two-electron exci- tation is the main output to a so-called "band effect" of the energy-level crossings. This conclu- sion is also in direct contrast to many other experimental results [5,8, 10,1 1]. In this Letter, we report a study on inelastic Ne + single scattering from a Si surface. We show