SECONDARY ION MASS-SPECTROMETRY OF MOLECULAR-SOLIDS - CLUSTER FORMATION DURING ION-BOMBARDMENT OF FROZEN WATER, BENZENE, AND CYCLOHEXANE

The secondary ion mass spectra (SIMS) of H2O, C6H6, and C6H12frozen at 77 K are reported. The spectra are gen-±erated by subjecting the molecular surface to 0.5-3-keV He+ions and mass analyzing the positive and negative secondary clusters in a quadrupolc mass filter. The most abundant clusters in the H2O spectrum were of the type H(H2O)n+, n-1, 2,…,51, although other low-intensity clusters were also observed. The benzene and cyclohexane spectra exhibited clusters of the type CnHm+, n = 1, 2,…, 32 and m = 1, 2,…, 10. The intensities I of the different types of cluster series observed fromH2O vary as a function of the number of li2O molecules n in the cluster according to I = aebnwhere a and b are constants. The kinetic energy distributions of the clusters have a maximum in the region 1.0-3.3 eV which, in general, shifts to lower energyand sharpens with increasing cluster size. A general clustering mechanism which is qualitatively consistent with all of the experimental observations is proposed. This mechanism involves thermal activation by the primary ion impact and irreversibleadiabatic expansion to relieve the nonequilibrium situation. Bound clusters are formed from collections of secondary particlesmoving through the selvedge region of the surface. The formation of a tightly bound cluster only as the particles are movingthrough the surface accounts for the molecular rearrangement observed during sputtering. © 1979, American Chemical Society. All rights reserved.