Surface Channeling Experiments at 20 MeV and Resonant Coherent Excitation ofN6+Ions

Abstract

Surface channeling of 21.8 and 23 MeV N 61 ions of a Pt(110) surface is experimentally verified. At 21.8 MeV resonant coherent excitation is observed leading to enhanced ionization when the ions scatter along the ͓110͔ surface half channels. [S0031-9007(97)04307-X] PACS numbers: 79.20.Rf, 34.50.Dy, 61.85. + p Channeling was first observed in a computer simulation [1]. The phenomenon was explained theoretically by a guided motion of the fast ions by the planar or cylindrical potentials formed by the planes or the strings of atoms in a single crystalline solid [2,3]. Experimental verification of the effect was found in high energy ion beam experiments typically in the 0.5 to 1 MeV range [4]. Channeling was developed into a useful tool for the analysis of solid state properties [5]. In comparison, surface channeling plays a minor role due to the problems of preparing surfaces with sufficiently large terraces [6]. The steps on terraced surfaces allow the penetration of the fast particles which can then leave the surface again; in such way the surface channeling is in an intriguing way mixed with bulk or subsurface channeling. Furthermore, in surface channeling the conditions of proper or hyperchanneling have to be fulfilled [7,8]. In bulk channeling these terms describe the effect when the ions traveling through a solid stay within one planar or axial channel. Note that the 3D potential in a solid which governs the motion of the ions is not necessarily a closed surface; i.e., a particle can wander between different channels without violating the channeling conditions. At a surface, however, normal channeling means penetration into the bulk. In order to avoid the penetration very small grazing angles are necessary. In 1965 Okorokov proposed that the surface of a single crystal provides a periodic potential which should cause the resonant excitation of atoms which scatter along the surface with the "right" velocity [9]. If the atom velocity y n r d, where hn r DE ij an atomic excitation energy and d is the atomic distance in, e.g., a chain of atoms, the atom feels a periodic disturbance and a resonant coherent excitation (RCE) can be observed. In the case of highly charged, fast ions the excitation leads to enhanced ionization, which is hence the signature of RCE. The first verification of the RCE was experiments with highly charged, hydrogenlike ions channeling through Au axial channels at energies in the 10 to 30 MeV range [8]. The effects observed are quantitatively understood [10]. More recently planar channeling was also used for the RCE of fast ions [11]. In the case of axial channeling it is the energy of the ions which is varied to find the resonances. In the case of planar channeling the energy is kept constant and the tilting angle to the channels is varied. The results provide detailed insight of the interaction of fast ions with solids. Items to be included in the theoretical treatment are the change of the binding energy of the electronic states in question, the dynamic screening of the fast ions, and t...

Negative ion formation in the scattering of atoms and ions from dielectric surfaces

“…If the velocity is high enough, this oscillating field could induce transitions between electronic states of the projectile. This, so-called, resonant coherent excitation or Okorokov process (Okorokov 1965, Datz et al 1978, Kupfer et al 1981, Kimura et al 1996, Kimura and Mannami 1998, Garcia de Abajo and Echenique 1996, Hatke et al 1997 has been observed for the 1s → 2p transition in atomic hydrogen grazingly scattered at an LiF(001) surface (Auth et al 1997. In the same way oscillating fields might lead to electron loss by transitions from the bound states to the continuum states above the vacuum level (Garcia de Abajo et al , 1994.…”