Mass and energy analyses of an AuSi liquid metal ion source

Abstract

Mass and energy analyses of ions emitted from an AuSi eutectic alloy liquid metal ion source (LMIS) were carried out using a tandem double-focusing mass spectrometer. In the investigated ion current range between below 1 mu A and 10 mu A a great variety of molecules and clusters were detected. The field strength at the apex of the Taylor cone was estimated from post-ionisation probability to be 20 V nm-1 for silicon ions and 30 V nm-1 for gold ions at 10 mu A emission current. The energy spread of different ion species depends on the charge-to-mass ratio and on the total extracted ion current. For the ion current dependence of the energy spread a nearly two thirds power dependence was obtained. The energy width varies between 10 eV and 100 eV for Au ion species and between 10 eV and 20 eV for Si ion species. The energy deficit decreases with increasing ion current for Au and Si ion species. The authors assume a decrease of the field-dependent activation energy of field evaporation together with growing asymmetry in the energy distributions.