Mechanistic Insight into Gas Cluster-Induced Sputtering of Kilodalton Molecules Using Kinetic Energy Distribution Measurements

Recent progress in organic secondary ion mass spectrometry (SIMS) relied essentially on the development of new cluster beams, especially large atomic and molecular clusters formed via the adiabatic expansion of a gas in vacuum (Arn+, [CH4]n+, [CO2]n+, [H2O]n+). Although computer simulations and a few experimental investigations have shed light on certain fundamental aspects of large cluster-induced molecular desorption, the analytical application of these new beams usually preceded the detailed understanding of their interaction with surfaces. Here, to gain insight into the molecular emission process, the axial kinetic energy distributions (KEDs) of secondary ions emitted from organic films bombarded with Ar330–5000+ and Bi5+ clusters were measured using a reflectron time-of-flight secondary ion mass spectrometer. Irganox 1010 was chosen as a model kilodalton molecule because a large body of SIMS data involving this molecule already exists in the literature. Our results show that the axial KED of Irganox molecular and fragment ions varies as a function of the scaled kinetic energy E/n of the Ar cluster projectile and so does the fraction of ions produced above the surface via unimolecular dissociation, which exhibit an energy deficit with respect to the full acceleration provided at the entrance of the spectrometer. Below a few tens of eV/atom, the KEDs of ions such as (M – H)− (m/z 1175) become gradually narrower and their formation via metastable decay in the gas phase above the surface intensifies. Interestingly, the molecular ion M•+ (m/z 1176) is essentially produced in the gas phase with both Arn+ and Bi5+ cluster beams. Specifics of the observed KEDs of negative and positive molecular ions are discussed in comparison with results of other experiments recently reported in the literature and of molecular dynamics (MD) computer simulations using a coarse-grained representation of kilodalton organic molecules. The measurement of the axial kinetic energy of molecular and fragment ions sputtered from Irganox 1010 under argon and bismuth cluster bombardment, complemented with MD simulations, offers a clearer microscopic view of the emission process.