Berlin 2001 – scientific programme
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AMPD: EPS AMPD
AMPD 5: Sitzung 5
AMPD 5.7: Talk
Wednesday, April 4, 2001, 12:10–12:35, H105
Energy loss of slow projectiles grazingly scattered from a LiF(001) surface: Optical phonon excitation and skipping motion — •A.G. Borisov1, P. Roncin1, H. Khemliche1, A. Momeni1, J. Villette1, H. Winter2, and A. Mertens2 — 1Laboratoire des Collisions Atomiques et Moléculaires, Unité Mixte de Recherche CNRS-Université Paris Sud UMR8625, bât 351, 91405 Orsay Cedex, FRANCE — 2
When atomic projectiles collide with solid matter excitations in target and projectile give rise to stopping phenomena. For slow ions in the keV range charge exchange, electronic excitations and binary collisions with target atoms ("nuclear stopping") dominate the dissipation of the projectile energy [1]. While nuclear stopping can be described in a straightforward manner by classical trajectory calculations, the description of other energy loss channels is more complicated.
Recently insulating surfaces of ionic alkali-halide crystals have attracted much of experimental and theoretical interest. In contrast to well studied metal targets [2], the electronic structure of alkali-halides is characterized by a broad band gap (up to 14 eV for LiF) separating the valence and conduction bands. This band gap reduces the probability of electronic excitations for slow projectiles. This opens the possibility to study energy loss processes other than electronic excitations, such as energy losses to the collective mode of the lattice vibration (optical phonon). Moreover, owing to the small number of electronic excitations per projectile trajectory, one can basically perform a "translational energy spectroscopy" [3] study of the projectile-surface charge transfer processes.
Experimentally, the choice of a grazing scattering geometry allows one to minimise the nuclear stopping and to study the effects of charge transfer, electron-hole production and collective excitations on the energy loss. Resent results on the threshold in the stopping of slow projectiles [4], the discrete energy loss structures [4,5], and the population of surface excitons [5] have clearly demonstrated connection between energy loss of slow projectiles and charge transfer phenomena. As a further important energy loss channel for slow charged projectiles, the stopping by excitations of optical phonons [6] is discussed. It is demonstrated that, in the regime of skipping motion, the projectile stopping by excitations of optical phonons gives rise to "odd" progression of the energy loss peaks usually ascribed to the subsurface channeling [7].
[1] J.F. Ziegler, J.P. Biersack, and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon Press, NY, 1985).
[2] P.M. Echenique, F. Flores, and R.H. Ritchie, Solid State Commun. 43, 229 (1990).
[3] H.P. Winter, J. Phys. Chem. 99, 15448 (1995).
[4] C. Auth et al, Phys. Rev. Lett. 81, 4831 (1998).
[5] P. Roncin et al, Phys. Rev. Lett. 83, 864 (1999).
[6] A.G. Borisov et al, Phys. Rev. Lett. 83, 5378 (1999).
[7] J. Villette et al, Phys. Rev. Lett. 85, 3137 (2000).