Grazing collision of keV protons on LiF correlation between energy loss and electron emission J. Villette * , J.P. Atanas, H. Khemliche, M. Barat, V. Morosov, P. Roncin Laboratoire des Collisions Atomiques et Mol eculaires, Universit e Paris-Sud Orsay, Bat. 351, CNRS URA 281, 91405 Orsay Cedex, France Abstract Grazing interaction of low energy protons with LiF(1 0 0) is studied using a new coincidence technique combining energy loss, charge state analysis and electron spectroscopy. Correlation between the scattered projectile energy loss and the number of emitted electrons points to an energy loss mechanism not leading to electron emission. Detailed analysis of the energy loss spectra, which show well-resolved structures, suggests that this mechanism corresponds to the population of surface excitons. Moreover the correlation between the projectile ®nal charge state and the number of emitted electrons sheds new light on the major role played by the negative ion at the surface. Electron removal from the valence band proceeds mainly through formation of H  at halogen sites, whereas electron emission results from the detachment of these negative ions at subsequent F  sites. Ó 1999 Elsevier Science B.V. All rights reserved. 1. Introduction In collisions of ions on metals, the processes responsible for projectile energy loss and electron emission are now well understood. Excitation of conduction electrons from the Fermi level to the neighboring empty levels of the conduction band explains the large stopping power of all metals. On the other hand, the excitation of conduction elec- tron to the vacuum requires few eV and explains the appearance of a threshold in the ion velocity that depends on the metal work function. Re- cently, investigations of energy loss and electron emission have been extended to large band gap insulators. Both energy loss and electron emission should exhibit a threshold behavior at relatively high collision energy since the band gap has to be overcome for every electronic excitation. In that respect, it is dicult to understand the sustained secondary electron emission [1] in H  and Ar  collision on LiF at energies below 1 keV, where the threshold for gold target appears. More recently, it was shown that energy loss remains substantial even at an incident energy as low as 600 eV [2], i.e. much lower that the threshold expected from di- rect excitation of valence electrons across the band gap. We present recent results from coincidence measurements of electron emission and energy loss for grazing incidence interaction of low energy protons with LiF(1 0 0). The data are then inter- preted in view of recent high-resolution data [3]. Nuclear Instruments and Methods in Physics Research B 157 (1999) 92±97 www.elsevier.nl/locate/nimb * Corresponding author. E-mail: villette@lcam.u-psud.fr 0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 4 3 4 - 6