STUDY ON MECHANISM OF CLUSTER FORMATION THROUGH ELECTRONIC SPUTTERING INDUCED BY SWIFT HEAVY IONS

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The yield and emission energy distribution of secondary ions have been systematically measured for Al Si AlィイD22ィエD2OィイD23ィエD2 and SiOィイD22ィエD2 bombarded by Si, Cu and Ag ions at an impact energy region of MeV, where the electronic stopping power dominates over the nuclear stopping power. Singly charged cluster ions as well as multiply charged monatomic ions were observed in the case of the insulator targets. Dominant species of the cluster ions were Si(SiOィイD22ィエD2)ィイD1+ィエD1ィイD2xィエD2 and SiO(SiOィイD22ィエD2)ィイD1+ィエD1ィイD2xィエD2 (x is an integer )for SiOィイD22ィエD2 and (AlィイD22ィエD2OィイD23ィエD2)ィイD1+ィエD1ィイD2xィエD2, AlO(AlィイD22ィエD2OィイD23ィエD2)ィイD1+ィエD1ィイD2xィエD2and AlィイD22ィエD2OィイD22ィエD2(AlィイD22ィエD2OィイD23ィエD2)ィイD1+ィエD1ィイD2xィエD2 for AlィイD22ィエD2OィイD23ィエD2. For the conductive targets only singly and multiply charged monatomic ions were observed. The obtained emission energy for atomic ions depends not only on projectiles but also on the electric charge q. The energy distribution of the atomic ions emitted from the conductive targets is asymmetric extending to a high-energy side and is rather high compared with that for the insulators. The energy distribution for cluster ions is very narrow compared with that for the atomic ions. The mean energy for the cluster ions hardly depends on the size of cluster. These results combined with the yield dependence on the electronic stopping power show that the atomic ions are produced from the hot track region through the sequential process of ionization and recoil caused by the projectiles and forced to move by the electric repulsive force. The cluster ions are formed in the periphery of the track region.