A SEARCH FOR DOUBLY-CHARGED NEGATIVE CLUSTER IONS IN SPUTTERING

The emission of negative cluster ions sputtered under cesium impact from various elemental and multicomponent specimens (B, C, Al, Si, Cu, LiF, MgF2, CdS, and InP) was investigated by means of secondary-ion mass spectrometry. In all cases abundant yields of singly charged cluster ions were observed in the mass range covered by the instrument (less-than-or-equal-to 280 amu). On the other hand, doubly charged negative clusters could be detected only for carbon, C-7(2-) being the smallest one, in agreement with previous observations by Schauer et al. [Phys. Rev. Lett. 65 (1990) 625]. The intensities of C(n)2- ions exhibit odd-even oscillations and even-numbered clusters are more intense than the neighboring odd-numbered ones. Apart from abundance distributions, energy spectra were recorded for selected species. These data revealed the existence of fragmentation processes which dissipate the clusters' internal energy and lead to more stable daughter ions by the evaporation of atomic or diatomic species.