Low-Energy Collisions of Pyrazine and d6-Benzene Molecular Ions with Self-Assembled Monolayer Surfaces: The Odd−Even Chain Length Effect

Abstract

The low-energy reactive collisions of two independent probe ions, pyrazine and d6-benzene, illustrate an odd−even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfaces prepared from alkanethiols ranging from CH3(CH2)10SH to CH3(CH2)17SH chemisorbed to polycrystalline gold are shown to exhibit an odd−even effect where the orientation of the terminal methyl group determines the reaction behavior of the thin film. X-ray photoelectron spectroscopy shows the surfaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au−SR) on the SAM surface. When 30 eV incident ions were used, the extent of hydrogen addition to the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an odd chain length SAM surface, the terminal methyl group exposes a C−H bond perpendicular to the surface, increasing hydrogen addition reactivity for the probe ions. The low-energy p...