Influence of primary ion species on the secondary cluster ion emission process from SAMs of hexadecanethiol on gold

In order to investigate the secondary cluster ion emission process of organo-metallic compounds under keV ion bombardment, self-assembled monolayers (SAMs) of alkanethiols on gold are ideal model systems. In this experimental study, we focussed on the influence of the primary ion species on the emission processes of gold-alkanethiolate cluster ions from a hexadecanethiol SAM on gold. For this purpose, we carried out time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements using the following primary ion species and acceleration voltages: Ar⁺, Xe⁺, SF₅⁺ (10 kV), Bi⁺, Bi₃⁺(25 kV), Bi₃²⁺, Bi₅²⁺, Bi₇²⁺ (25 kV).It is well known that molecular ions M⁻ and gold-alkanethiolate cluster ions Au_xM_y⁻ with M = S-(CH₂)₁₅-CH₃, x − 3 ≤ y ≤ x + 1, x, y > 0, show intense peaks in negative mass spectra. We derived yields Y_SI exemplarily for the molecular ions M⁻ and the gold-hexadecanethiolate cluster ions Au_y+1M_y⁻ up to y = 8 and found an exponentially decreasing behaviour for increasing y-values for the cluster ions.In contrast to the well-known increase in secondary ion yield for molecular secondary ions when moving from lighter to heavier (e.g. Ar⁺ to Xe⁺) or from monoatomic to polyatomic (e.g. Xe⁺ to SF₅⁺) primary ions, we find a distinctly different behaviour for the secondary cluster ions. For polyatomic primary ions, there is a decrease in secondary ion yield for the gold-hexadecanethiolate clusters whereas the relative decrease of the secondary ion yield ξ_Y with increasing y remains almost constant for all investigated primary ions.