Sputtering of parent-like ions from large organic adsorbates on metals under keV ion bombardment

Abstract

Thin films of hydrocarbon molecules, unsaturated fatty acid and low molecular weight polystyrene deposited on different metal substrates (silver, copper and gold) were bombarded by 15 keV Ga ions and the secondary ions were mass- and energy-analysed by means of a time-of-flight secondary ion mass spectrometer. The samples were studied in order to evidence the effects of different substrates and coverages on the emission of the parent and cationised molecular ions, and to gain a better understanding of the large molecular ion emission processes. Ion beam degradation studies were realised for fundamental purposes too. In general, the kinetic energy distributions of metal-cationised molecules are broad in comparison with those of the parent ions, and of the smaller polystyrene fingerprint ions. In addition, the velocity distributions of the parent ions and of the metal-cationised molecules are similar. Parent ions of aromatic molecules are, on average, more energetic than those of aliphatic molecules. In the case of metal-cationised molecules, the three hypotheses of emission of a preformed complex, recombination in the selvedge and metastable decay of larger aggregates are critically reviewed in comparison with the experimental data. The recombination hypothesis cannot account for the whole set of observations. On the other hand, the very different evolutions of the parent ions and of the metal-cationised molecules in the degradation experiments cannot be explained solely in the frame of metastable decay reactions, although the kinetic energy measurements show that a significant fraction of the parent-like ions are produced in the vacuum. The augmentation of the secondary ion kinetic energy with increasing molecule size for triacontane monomers and dimers, and for silver-cationised polystyrene oligomers, is in disagreement with the sputtering by a single cascade atom, too. Finally, the discussion outlines the conditions that must be satisfied to model the experimental observations and proposes a view of the sputtering of these large molecular cations based on multiple collision processes and possible subsequent dissociation in the vacuum.