DETECTION OF LARGE CLUSTER IONS BY ION-TO-ION CONVERSION

Gold cluster ions in the mass range of 4 x 10(4)-2 x 10(6) u were prod uced by bombarding a thin film of gold with Cf-252 fission fragments. The gold him which covered a carbon-aluminium substrate formed islets having a mean diameter of 44 Angstrom. It turned out that nearly each second fission fragment penetrating an islet desorbed this islet as a whole. Most of these desorbed gold grains were negatively charged. The y were investigated by means of a tandem time-of-flight (TOF) instrume nt. In the first TOF section, the cluster ions gained a kinetic energy of z x 40 keV (where z is the charge state) and hit a converter, from where secondary ions and electrons were ejected. The most efficient c onverter material found so far is cesium iodide which particularly emi ts Cs+, I- and cluster ions. Less efficient were the contaminated or c lean metals Au, Ag, Cu and Pb and the alloy AgMgO. At an energy of z x 40 keV, the mean secondary electron yield from CsI was 0.011 per clus ter ion. The detection efficiency for a cluster ion by ion-to-ion conv ersion was found to decrease from 99.7% to 96.5% in the available mass range. After correction for detection efficiency the complete mass-to charge distribution between 4 x 10(4) and 2 x 10(6) u/z was determine d and compared with a corresponding mass distribution of the gold-isle ts covering the substrate. The mean charge state of the negatively cha rged clusters was estimated to be 1-2 and the mean mass z x 295 000 u.