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.