Molecular ionization probability in cluster-SIMS

Abstract

The ionization efficiency of molecular species sputtered under bombardment with cluster ion beams is generally assumed to be rather low, but hard experimental evidence for this notion is still lacking. In order to estimate the prospects for possible sensitivity improvements in molecular secondary ion mass spectrometry (SIMS) experiments, the author therefore reports on recent experiments utilizing laser postionization (LPI) in order to determine an absolute ionization probability value for a few organic molecules sputtered under C60 cluster ion bombardment. The results show that the molecular ionization efficiency obtained under these bombarding conditions is of the order of 10−3, leaving at least 2 orders of magnitude for possible enhancement of the physical or a chemical ionization mechanism leading to the formation of (quasi-) molecular secondary ions. Analyzing the measured molecular LPI signal, the author demonstrates that about the same headroom exists for improvement of the currently achieved postionization efficiency. Using trehalose as a model system, the author then illustrates the level of improvement achieved by different attempts to utilize projectile induced surface chemistry in order to enhance the ionization efficiency. The results may be useful to guide new developments aimed at enhancing the detection sensitivity in molecular cluster-SIMS experiments.