Low-Energy Collisions of Pyrazine and d_6-Benzene Molecular Ions with Self-Assembled Monolayer Surfaces: The Odd-Even Chain Length Effect

摘要

The low-energy reactive collisions of two independent probe ions, pyrazine and d_6-benzene, illustrate an odd-even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfces prepared from alkanethios ranging from CH_3(CH_2)_(10)SH to CH_3(CH_2)__(17)SH chemisorbed to polycrystlaline gold ae shown to exhibit an odd-even effect where the orientation of the terminal methyl group determines the reaction beavior of the thin film. X-ray photoelectron spectroscopy showw the srufaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au-SR) on the SAM surfce. When 30 eV incident ions were used, the extent of hyrogen additionto the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an od 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 probe ions also reacted with the surfaces to from alkyl adition production products. The alkyl additin products also showed an odd-even effect. In this case, the even chain length SAM surfaces were more reactive than the odd chain length surfaces. For an even SAM surface, the terminal C-C bond is oriented quasi-perpendicular to the Au surface, allowing more direct assess to the carbon atom on the terminal methyl group and increasing its reactivity.