Excited-Atom Production by Electron Bombardment of Alkali-Halides
- PDF / 583,139 Bytes
- 9 Pages / 420.48 x 639 pts Page_size
- 15 Downloads / 210 Views
EXCITED-ATOM PRODUCTION BY ELECTRON BOMBARDMENT OF ALKALI-HALIDES
R. E. Walkup, Ph. Avouris, and A. P. Ghosh, IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, N.Y. 10598
ABSTRACT We present experimental results which suggest a new mechanism for the production of excited atoms and ions by electron bombardment of alkali-halides. Doppler shift measurements show that the electronically excited atoms have a thermal velocity distribution in equilibrium with the surface temperature. Measurements of the absolute yield of excited atoms, the distribution of population among the excited states, and the dependence of yield on incident electron current support a model in which excited atoms are produced by gas-phase collisions between desorbed ground-state atoms and secondary electrons. Similarly, gas-phase ionization of ground-state neutrals by secondary electrons accounts for a substantial portion of the positive ions produced by electron bombardment of alkali-halides. INTRODUCTION Bombardment of solids by energetic beams of electrons or photons is generally accompanied by the ejection of particles from the surface. An understanding of the basic mechanisms responsible for this electron- or photon-stimulated desorption (ESD or PSD) is of substantial current interest [11. Many recent studies have focused on alkali-halide substrates because these materials have relatively simple geometric and electronic structures, and because they exhibit a rich variety of desorption phenomena. It has been established that the desorbed particles are principally ground-state halogen atoms and molecules, and ground-state alkali atoms [2 - 61, with a small yield of electronically excited alkali atoms [7 - 131, and a smaller yield of ions [14 - 16]. Mechanisms for the desorption of ground-state neutrals have been qualitatively understood for some time [2 - 6,17 - 19]. The yield of ground-state atoms is remarkably large [-13], - 10 neutral atoms desorb from NaC1 per incident electron at 1 keV incident energy and T=570 K. Such large yields are due to the fact that the formation of defects in the bulk , followed by defect diffusion to the surface results in thermal desorption of neutral atoms. Plausible mechanisms for ion desorption have been proposed [201. However, trajectory calculations [21] indicate that the most simple mechanisms based on repulsive Coulomb forces may fail due to the ability of the ions in the lattice to re-arrange. Thus the ion desorption mechanisms have not been firmly established. The mechanism for excited alkali-atom desorption has also remained an open question. In this article, we discuss experimental results which suggest a new mechanism for the production of excited-atoms due to electron bombardment of alkali-halides. The experiments include measurements of velocity distributions of the excited-atoms by Doppler-shift techniques. These measurements provide direct insight into the excitedstate formation mechanism. In addition, we discuss systematic measurements of the Mat. Res. Soc. Symp. Proc. Vol. 75
Data Loading...
