SIMS study of the mechanism of cluster formation during ion bombardment of alkali halides

Abstract

The secondary ion mass spectra (SIMS) of the salts NaX (X=F,Cl,Br,I) and CsX (X=F,I,ClO₄) are reported. A UHV spectrometer system designed for SIMS studies is described in detail. Large secondary ion clusters of high mass, e.g., Na₁₄F₁₃⁺(569 amu) and Cs₄I₃⁺(913 amu), whose minimum lifetimes are of the order of tenths of milliseconds, have been detected. The long series of detected cluster ions of the type M_nX_n−1⁺ indicates that there is a high degree of stability associated with these clusters. The intensities I of the different series of observed clusters of the type M_nX_n−m⁺ (m=−2,−1,0,1,2) vary as a function of cluster mass M according to I=aM^b, where b takes values from −1.98 to −9.02 for various series. The kinetic energy distributions of the clusters have a maximum in the region 1–2 eV which, in general, shifts to lower energy and sharpens with increasing cluster size. A clustering mechanism which is qualitatively consistent with all of the experimental observations is proposed. This mechanism involves formation of a bound cluster from a collection of secondary particles moving through the selvedge region of the surface as a result of a single primary ion collision. The formation of a tightly bound cluster after the particles are moving through the selvedge region accounts for the molecular rearrangement observed during sputtering.