Detection of Large Cluster Ions by Ion‐to‐ion Conversion

Abstract

Gold cluster ions in the mass range of 4×104–2×106 u were produced by bombarding a thin film of gold with 252Cf fission fragments. The gold film which covered a carbon–aluminium substrate formed islets having a mean diameter of 44 Å. 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. They were investigated by means of a tandem time-of-flight (TOF) instrument. In the first TOF section, the cluster ions gained a kinetic energy of z×40 keV (where z is the charge state) and hit a converter, from where secondary ions and electrons were ejected. The most efficient converter material found so far is cesium iodide which particularly emits Cs+, I− and cluster ions. Less efficient were the contaminated or clean metals Au, Ag, Cu and Pb and the alloy AgMgO. At an energy of z×40 keV, the mean secondary electron yield from CsI was 0.011 per cluster ion. The detection efficiency for a cluster ion by ion-to-ion conversion 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 4×104 and 2×106 u/z was determined and compared with a corresponding mass distribution of the gold-islets covering the substrate. The mean charge state of the negatively charged clusters was estimated to be 1–2 and the mean mass z×295 000 u.