High resolution mass spectrometry of liquid metal ion sources

Abstract

The mass spectra of In, Ga, and Au liquid metal ion sources (LMIS) exhibit large molecular ion clusters with the maximum observed number of atoms/cluster ranging from 6 for Ga to 12 for In. Au ion sources emit a large percentage of its ions in the form of clusters while the ratios of ion clusters to atomic ions for Ga and In sources are one part in 103 and 104, respectively. Emission characteristics (intensity, energy distributions, and deficits) of the emitted ions of different charge and mass states were measured as a function of ion source temperature, current and emission angle. The effects of these parameters on the emitted ion velocity distribution may yield information on the mechanism of ion formation and the observed energy spread. In all of the sources tested the (dI/dΩ) distribution of the molecular clusters was broad at low energy deficits and narrow at high deficits indicating different regions of formation. However, the atomic ion angular distributions for high and low ion energy deficits are nearly equal in width, but show distinct differences in shape. Models of ion formation are presented.