Molecular depth profiling in ice matrices using C ₆₀ projectiles

The prospects of molecular sputter depth profiling using C ₆₀ ⁺ projectiles were investigated on thick ice layers prepared by freezing aqueous solutions of histamine onto a metal substrate. The samples were analyzed in a ToF-SIMS spectrometer equipped with a liquid metal Ga ⁺ ion source and a newly developed fullerene ion source. The C ₆₀ ⁺ beam was used to erode the surface, while static ToF-SIMS spectra were taken with both ion beams alternatively between sputtering cycles. We find that the signals both related to the ice matrix and to the histamine are about two orders of magnitude higher under 20-keV C ₆₀ than under 15-keV Ga bombardment. Histamine related molecular signals are found to increase drastically if the freshly introduced surface is pre-sputtered with C ₆₀ ions, until at a total ion fluence of about 10 ¹³ cm ^-2 the spectra are completely dominated by the molecular ion and characteristic fragments of histamine. At larger fluence, the signal is found to decrease with a disappearance cross section of approximately 10 ^-14 cm ² , until at total fluences of about 10 ¹⁴ cm ^-2 a steady state with stable molecular signals is reached. In contrast, no appreciable molecular signal could be observed if Ga ⁺ ions were used to erode the surface.