ISOTOPIC FRACTIONATION OF NEGATIVE-IONS PRODUCED BY CS SPUTTERING IN A HIGH-INTENSITY SOURCE

The isotopic compositions of negative ions produced by a high-intensit y sputter source from a wide range of elements as been studied. In all cases of atomic negative ions, the lighter isotope is enhanced with r espect to the bulk composition of the sputter material. Fractionations range from 25% per amu for lithium to almost zero for iridium. The ob served fractionations are approximated by the expression f = (100/DELT Am)(square-root (M(H)/M(L)) - 1), here M(L) and M(H) are the masses of the lighter and heavier isotopes. With the exception of lithium, simi lar results were obtained with both elemental and compound sputter tar gets and fractionation appears to be essentially matrix independent. M olecular beams showed significantly less fractionation than atomic bea ms from the same materials; within errorS, Molecular fractionations ar e consistent with zero. Sputter theory is in excellent agreement with the results obtained with atomic ions, but is unable to account for th e near absence of fractionation observed with molecular beams.