In the present paper, an ion microprobe is used to study the fragmentation regularities of Al,' and Si,' cluster ions sputtered under ion bombardment. The results have shown that the ions formed during cluster fragmentation, along the path from the target to the detector, transform the mass spectra of both charged and neutral components and the energy distribution of the parent cluster and monomer ions.Recently, a great many publications have appeared concerning the study of the properties of clusters.' Similar attention has been paid to both the investigation and the development of methods of small-particle intensive flux generation for thin film production with a given structure and stoichiometry. Surface sputtering by accelerated heavy ions seems to be the most promising technique for small particle production. Numerous experiments on the mass spectrometry of sputtering products have shown that the flows of cluster ions could be generated with a number of atoms up to n -100, depending on the target material and sputtering conditions, e.g. molecular ions of M(MX),+ type with n > 100. ' In addition, the high fraction of the charged cluster component in sputtering products makes it easy for the experiments to be carried out, because additional equipment is not needed for the cluster ionization for further mass spectrometric analysis.The nature of the process of sputtering of solid surfaces as cluster ions has not been verified yet. Recent investigations have revealed that cluster ions leaving the bombardment region could undergo some unimolecular decays as a result of excess energy accumulated by a cluster during its f o r m a t i~n .~.~ Research on these decays allows the nature of cluster ion formation to be understood better and provides information on both the stability and structure of the ions formed. In terms of the applied problems of SIMS, the consideration of metastable ions in the mass spectra of sputtered ions could improve the accuracy of qualitative and quantitative analyses.In the present paper, an ion microprobe with a mass spectrometer of sector type operating in double focusing mode is described. The apparatus has been developed for studying the regularities of unimolecular decay of cluster ions scattered under ion bombardment of solids. The apparatus features high concentration sensitivity owing to the special measures taken for effective collection and formation of sputtered ion flux (Fig. 1).Noble gas ions generated by a duoplasmatron (1) after acceleration and following mass separation in a Wien filter (3) bombard the sample by an ion beam 1 Figure 1. Scheme of ion microprobe: 1, duoplasmatron ion source; 2, condensor lens; 3, Wien filter; 4, intermediate lens; 5, deflector; 6, objective lens; 7 sample; 8. immersion lens; 9, dynamic transfer system; 10, entrance slit; 11, electromagnet; 12, electrostatic analyser; 13, energy slit; 14, electron multiplier; 15, counter; 16, plotter register; 17, grid system.