Molecular ions in cluster bombardment: what clues do the molecular dynamics simulations provide?

A mixed cluster ion beam to enhance the ionization efficiency in molecular secondary ion mass spectrometry

RATIONALE

Chemical modification of a rare gas cluster ion beam (GCIB) to increase the intensity of desorbed molecular ions in secondary ion mass spectrometry experiments relative to the pure Ar cluster.

METHODS

Doping of the GCIB by mixing small concentration levels (1-3% relative partial pressure) of CH4 into the Ar gas driving the cluster ion source.

RESULTS

Mass spectra measured on a trehalose film using the doped GCIB exhibit enhanced molecular ion signals. From depth profiling experiments, the results are shown to arise from an increase in the ionization efficiency of the sputtered molecules rather than a change in the sputtering yield of neutral species.

CONCLUSIONS

Tuning of the chemistry of mixed clusters is suggested as a general approach to enhancing the ionization probability of sputtered molecules.

Evidence for the formation of dynamically created pre-formed ions at the interface of isotopically enriched thin films

A novel approach to elucidate the ionization mechanism for the [M + H]+ molecular ion of organic molecules is investigated by molecular depth profiling of isotopically enriched thin films. Using a model bi-layer film of phenylalanine (PHE) and PHE-D8, the results show formation of an [M + D]+ molecular ion for the non-enriched PHE molecule attributed to rearrangements of chemical damage due to successive primary ion impacts. The [M + D]+ ion is observed at the interface for 19.9nm in the enriched-on-top system and 9.9nm for the enriched-on-bottom system. This ion formation is direct evidence for dynamically created pre-formed ions as a result of chemical damage rearrangement induced by previous primary ion bombardment events.