Chain Length Effects for Cluster Ion Formation during High Energy Ion/Surface Collisions with Self-Assembled Monolayer Surfaces

A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) was used to investigate chain length effects for the formation of cluster ions. The clusters were formed from high energy monatomic ion collisions with hydrocarbon self-assembled monolayer (SAM) surfaces that were deposited onto gold substrates. A wide range of n -alkanethiols was used to make the SAM surfaces, which included both odd and even hydrocarbon chain length thiols. The mass range of the TOF−SIMS made it possible to monitor all of the cluster ions that were formed by the high energy ion/surface interactions. The focus of this work was on the formation of high mass gold−sulfur and gold−adsorbate cluster ions. Mechanisms for the formation of these large cluster ions are proposed, which build upon a precursor cluster mechanism for high energy collisions with hydrocarbon SAM surfaces. 1 The formation of high mass cluster ions is a function of the chain length of the alkanethiol chemisorbed to the gold surface. It has been demonstrated that both the attenuation of the ions by steric effects and van der Waals interactions dictate their formation.