C-O Bond Dissociation and Induced Chemical Ionization Using High Energy (CO₂)_n⁺ Gas Cluster Ion Beam

A gas cluster ion beam (GCIB) source, consisting of CO₂ clusters and operating with kinetic energies of up to 60 keV, has been developed for the high resolution and high sensitivity imaging of intact biomolecules. The CO₂ molecule is an excellent molecule to employ in a GCIB source due to its relative stability and improved focusing capabilities, especially when compared to the conventionally employed Ar cluster source. Here we report on experiments aimed to examine the behavior of CO₂ clusters as they impact a surface under a variety of conditions. Clusters of (CO₂)_n⁺ (n = 2000~10,000) with varying sizes and kinetic energies were employed to interrogate both an organic and inorganic surface. The results show that C-O bond dissociation did not occur when the energy per molecule is less than 5 eV/n, but that oxygen adducts were seen in increasing intensity as the energy is above 5 eV/n, particularly, drastic enhancement up to 100 times of oxygen adducts was observed on Au surface. For Irganox 1010, an organic surface, oxygen containing adducts were observed with moderate signal enhancement. Molecular dynamics computer simulations were employed to test the hypothesis that the C-O bond is broken at high values of eV/n. These calculations show that C-O bond dissociation occurs at eV/n values less than the C-O bond energy (8.3 eV) by interaction with surface topological features. In general, the experiments suggest that the projectiles containing oxygen can enhance the ionization efficiency of surface molecules via chemically induced processes, and that CO₂ can be an effective cluster ion source for SIMS experiments. Graphical Abstract.