Energy-transfer, electron-transfer, and atom/group-transfer resulting from low-energy ion-surface collisions characterize hydrocarbon, fluorocarbon, and mixed self-assembled monolayers

Abstract

Organic thin films (alkanethiolates chemisorbed on gold) were employed in low-energy (eV) ion-surface collisions to validate the technique as a surface analysis tool and to further investigate processes associated with ion-surface interactions. Low-energy ion surface collisions of small polyatomic and atomic ions with self-assembled monolayers (SAMs) ascertain the chemical composition, structure, and quality of SAMs utilizing four processes: energy transfer (fragmentation of projectile ions: surface-induced dissociation (SID)), electron transfer (neutralization of the projectile ions), atom/group transfer (reaction between the projectile ion and atom/groups from SAMs), and chemical sputtering. Low-energy ion-surface collisions were used to investigate newly synthesized fluorinated compounds where the degree of fluorination of the thiolate tail group increases. Data indicate that substitution of CH₃ with CF₃ as the terminal group has a substantial influence on energy transfer, electron transfer, and atom/group transfer. Slight penetration into a depth of SAM films is illustrated by the formation of certain ion-surface reaction products (a result not observed for previously characterized Langmuir-Blodgett (L-B) films). A novel neutralization mechanism for reaction between methyl cation and hydrocarbon and fluorocarbon SAMs was established. Ion neutralization (besides direct electron transfer) results from a hydride ion transfer, methyl anion transfer, or fluoride transfer between hydrocarbon and fluorocarbon SAMs and incoming methyl cations. Experimental ion-surface and ion-molecule data support the ion neutralization mechanism originally proposed by ab initio and thermochemical calculations. Ion-surface processes were also used to characterize three mixed SAM systems (system 1: hydroxyl/hydrocarbon mixed SAMs and systems 2 and 3: fluorocarbon/hydrocarbon mixed SAMs). The mixed SAMs were prepared from binary thiol solutions and uniform solutions of asymmetrical disulfides. These ion-surface data can be useful for qualitative (identification of the sample's chemical composition) and quantitative analysis (calculation of the surface concentration of a chemical species for a mixed SAM). An in-line Sector-Time-of-Flight (TOF) tandem mass spectrometer with low-energy ion-surface collisions was characterized. Research involved testing the versatility of the instrument in terms of effective ion activation (peptide fragmentation) and surface analysis of organic thin films. This prototype will aid further implementation of SID into commercial TOF instruments for efficient ion activation and surface analysis capabilities.