FRAGMENTATION OF CLUSTERS INDUCED BY COLLISION WITH A SOLID-SURFACE -COMPARISON OF ANTIMONY AND BISMUTH CLUSTER IONS

Mass-selected antimony cluster ions Sb-n(+) (n = 3-12) and bismuth clu ster ions Bi-n(+) (n = 3-8) are allowed to collide with the surface of highly oriented pyrolytic graphite at energies up to 350 eV. The resu lting fragment ions are analysed in a time-of-flight mass spectrometer . Two main fragmentation channels can be identified. At low impact ene rgies both Sb-n(+) and Bi-n(+) cluster ions lose neutral tetramer and dimer units upon collision. Above about 150 eV impact energy Sb-3(+) b ecomes the predominant fragment ion of all investigated antimony clust ers. The enhanced stability of these fragment clusters can be explaine d in the framework of the polyhedral skeletal electron pair theory. In contrast, Bi-n(+) cluster scattering leads to the formation of Bi-3(), Bi-2(+) and Bi+ with nearly equal abundances, if the collision ener gy exceeds 75 eV. The integral scattering yield is substantially highe r in this case as compared to Sb-n(+) clusters.