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Langmuir 1985,1, 294-300
Ion Fractions and Collision Dynamics of 1-3-keV Ne+, Ar+, and Kr+ Scattering from an Yttrium Surface Ranjit Kumar, Jie-Nan Chen, and J. Wayne Rabalais* Department of Chemistry, University of Houston, University Park, Houston, Texas 77004 Received October 4, 1984 TOF spectra of scattered primary and recoiled neutrals and ions for 1-3-keV Ne+, Ar+, and Kr+ bombardment of a polycrystalline yttrium surface are presented. The spectra are analyzed in terms of classical dynamics into events that consist of single ( S S ) and multiple (MS) scattering of primary ions and direct (DR) and surface (SR) recoiling of surface atoms. Ion fractions Y at a 45O scattering angle, determined by measurement of spectra for neutrals plus ions and neutrals alone, are 2.7% for Ne+, 30.2% for Ar+, and 9.5% for Kr+ at 3 keV and decrease with decreasing energy. The data support an ion/surface interaction mechanism in which the ion trajectory is divided into three parts: (i) the incoming trajectory, where there is efficient Auger (AN) and resonant (RN) neutralization with the surface bands, (ii) the violent collision, where electron promotions in the close interatomic encounter can result in reionization, (iii) the outgoing trajectory, where AN and RN processes can reneutralize the nascent ions. Plots of In Y vs. l/ul0, where ul0 is the normal component of the outward velocity, yield straight lines in accordance with neutralization on the outgoing trajectory. The intercepts at l / u l 0 = 0 indicate 82% and 100% reionization of Ar and Kr, respectively, in the violent collision. Classical scattering calculations are performed on these systems and used to discuss the interatomic potentials at the distances of closest approach, interpenetration of electron shells, and electron promotion in quasi-molecules formed during the close encounter.
Introduction In scattering of kiloelectronvolt ion beam from surfaces, it is well-known' that both neutrals and ions are observed in the scattered flux. The fraction of particles scattered as ions Y, defined as the ratio of the number of ions to the total number of particles scattered into a certain solid angle, has been shown2 to be dependent on several parameters. These include the ion energy (or velocity), the scattering angle and trajectory, orientation of the target surface, the electronic structure of the solid surface, ion, and the corresponding neutralized ion, and adsorbate coverage of the surface. Ion fractions have been measured for relatively few systems. Most measurements have been made for nobel gas ions scattering from metal surfaces;"'6 such Y values range from essentially zero to ca. 50%. Scattering of alkali ions from metals17yields very high Y values (99% for K+on Cu).'* Very few Y values have been measured from nonmetal surfaces; in noble gas scattering from saltlgand oxidemsurfaces, it has been shown that Y (1) Suurmeijer, E. P. Th. M.; Boers, A. L. Surf. Sci. 1973,43,309. (2) Eckstein, W. In "Inelastic Particle-surface Collisions";Taglauer, E., Heiland, W., Ma.;Springer-Verlag: Berlin, 1981. (3) Buck, T. M.; Chen, Y. S.; Wheatley, G. H.; van der Weg, W. F. Surf. Sci. 1976,47,244. (4) Buck, T. M.; Wheatley, G. H.; Verheij, L. K. Surf. Sci. 1979,90, 635. ..~ (5) Ecketein, W.; Molchanov, V. A.; Verbeek, H. Nucl. Instrum. Methods 1978,149,599. (6) Eckatein, W.; Matachke, F. E. P. Phys. Rev. B 1976, 14, 3231. (7) Luitiena, S. B.; Alma, A. J.: Suurmeiier. E. P. Th. M.: Boers, A. L. Surf.. Sci. isso, a, 631 ind 652.' (8) Luitjens, S. B.; Algra, A. J.; Boers, A. L. Surf. Sci. 1979,80,566. (9)Agamy, S. A.; Robinson, J. E. Surf. Sci. 1979,90,648. (IO) Kumar,R.; Mmtz, M.H.; Schultz, J. A.; Rabalaia, J. W. Surf. Sci. 1983,130,L311. (11) Kumar, R.; Mmtz, M. H.; Rabalais, J. W. Surf, Sci. 1984,147,15. (12) Kumar,R.; Rabalais, J. W. Surf. Sci. 1984,147,37. (13) Nizhnaya, S. L.; Parilis, E. S.; Verleger, V. K. Radiat. Eff. 1979, 40. ~. , 23.
(14) Kiahinevsky, L. M.; Parilis, E. S.; Verleger, V. K. Radiat. Eff. 1976,29,215. (15) Kishinevsky, L. M.; Parilis, E. S. Sou. Phys.-JETP (Engl. Traml.) 1969,28,1020. (16) Schneider, P. J.; Eckstein, W.; Verbeek, H. N u l . Instrum. Methods 1982,194,387. (17) Boers, A. L. Nucl. Instrum. Methods 1984,B2, 253. (18) Algra, A. J.; Loenen, E. V.; Suurmeijer, E. P. Th. M.; Boers, A. L. Radiat. E f f . 1982,60, 173. (19)Rabalais, J. W.: Schultz, J. A.: Kumar, R.: Murrav. P. T . J. Chem. Phys. 1983, 78,5250.
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can be high and can differ for scattering collisions with different atoms of the compound. Interpretation of ion fraction data is based on the nature of the ion-target interatomic potentials and the electronic transitions occurring along the scattering trajectory. The behavior of Y, and hence the nature of the neutralization and/or ionization process, must be known and understood in order to place such analytical techniques as ion scattering spectrometry2'p22 (which uses energy analysis of scattered ions), secondary ion mass spectrometry (SIMS)23 (which uses mass analysis of sputtered ions), and bombardment-induced light emission24(which uses intensity analysis of photons emitted from the collision) on a quantitative basis. Since the existing theoretical modela used to describe ion-surface neutralization have recently been reviewed by Boers,26we present here only a brief synopsis of the most widely accepted models. Beginning in 1954, Hagstrum published a series of elegant experimental and theoretical papers%treating the ejection of Auger electrons stimulated by low-energy (
