SURFACE STRUCTURE BY ANGLE-RESOLVED SECONDARY ION MASS SPECTROMETRY

STEVEN PAUL HOLLAND, Purdue University

Abstract

The technique of angle-resolved secondary ion mass spectrometry (SIMS) is shown to be applicable to the determination of substrate and adsorbate overlayer structures. The technique evolved as a result of classical dynamical calculations which model the effect of an energetic ion impinging on a metal microcrystallite. The angular distribution of the ejected particles is indicative of the substrate orientation and of the adsorbate position. The first empirical test of the model's angular distribution predictions reveals that copper and oxygen ions ejected from Cu(001)c(2x2)-O exhibit azimuthal anisotropy. From the relative spacial distributions of the two ions, the oxygen adsorbate site can be ascertained. Determination of adsorbate height may also be feasible with a more complete secondary ion distribution analysis. Cluster ions are observed to exhibit angular anisotropy as well. The angular distributions of Ni('+) and Ni(,2)('+) ejected from Ni(001) are presented. The Ni(,2)('+) ion exhibits greater anisotropy than does the monomer ion, although both have maximum intensity along the same crystallographic directions. The experimental results are well reproduced by the classical dynamical calculations. The latter predicts that the peak in the dimer distributions arises from a unique ejection mechanism in which ejected next nearest neighbors along a close packed row recombine in the near surface region. Angle-resolved SIMS may be applicable to the determination of alloy surface structures by analysis of ejected cluster particles. A systematic variation of the adsorbate height above the substrate would highlight the sensitivity of angle-resolved SIMS to that spacing. Therefore, angle-resolved secondary ion experiments are performed for the calcogens on Ni(001). The calcogens are reported to reside at different heights above the substrate. Both oxygen and sulfur ions are seen to exhibit azimuthal anisotropy. Differences in their angular distributions are indicative of different ejection mechanisms, which may be correlated to adsorbate height. The oxidation of Ni(001) is seen to have dramatic effect on the Ni('+) and Ni(,2)('+) angular distributions while not appreciably changing the O('-) azimuthal anisotropy. A unique angle-resolved SIMS instrument is employed to study Ni(001)c(2x2)-CO. Experimental distributions of Ni('+) resulting from variations in the azimuthal angle, the polar angle, and the secondary ion energy correlated well with the distributions predicted by the classical dynamical model when the CO is adsorbed in an atop site. Comparison with the calculation results for CO bonded in a twofold bridge site indicates that the azimuthal distribution is most sensitive to the adsorbate site. The azimuthal distributions of the high kinetic energy CO are also sensitive to the adsorbate site.

Degree

Ph.D.

Subject Area

Analytical chemistry

COinS