Investigations of molecular depth profiling with dual beam sputtering

In this study, the feasibility of molecular depth profiling using dual beam sputtering of biological materials is examined. The model system is a 402-nm Langmuir Blodgett multilayer film consisting of 149 monolayers of barium arachidate (AA). The thin film was initially subjected to pre-bombardment with a 15-keV Au⁺ beam. Subsequently, an imaging depth profile experiment was performed on the pre-irradiated sample using a 40-keV C₆₀ ⁺ beam. An extremely low erosion rate under Au⁺ bombardment is found on this model system. In the subsequent C₆₀ ⁺ depth profiles, surprisingly large molecular ion signals are detected at the gold pre-irradiated surface. These signals then rapidly decay to nearly zero, indicating a damaged sub-surface layer being generated by the Au⁺ pre-bombardment. The thickness of the damaged layer is found to increase with increasing gold ion fluence and saturate at about 100 nm at 6 × 10¹⁴ Au⁺/cm². This altered layer thickness is significantly larger than the value of ∼50 nm obtained on a trehalose film pre-bombarded with a Ga⁺ source. The results also show that the damage caused by the Au⁺ beam can be removed by C ₆₀⁺ sputtering and that the molecular information is restored after damage removal.