Translational to Vibrational Energy Conversion during Surface-Induced Dissociation of N-Butylbenzene Molecular Ions Colliding at Self-Assembled Monolayer Surfaces

Abstract

Translational to vibrational (T→V) energy conversion in the course of inelastic collisions of n-butylbenzene molecular ions with thiolate self-assembled monolayer (SAM) gold surfaces is studied to better understand internal energy uptake by the hyperthermal projectile ions. The projectile ion is selected by a mass spectrometer of BE configuration and product ions are analyzed using a quadrupole mass analyzer after kinetic energy selection with an electric sector. The branching ratio for formation of the fragment ions m/z 91 and m/z 92, measured over a range of collision energies, is used to estimate the average internal energy with the aid of calculations based on unimolecular dissociation kinetics [Rice–Ramsperger–Kassel–Marcus (RRKM) theory]. The measured T→V conversion efficiencies (the fraction of the laboratory kinetic energy converted into internal energy) are 11∼12% for dodecanethiolate SAM (H-SAM) and 19∼20% for 2-perfluorooctylethanethiolate SAM (F-SAM), respectively, over ranges of a few 10s of eV. The values are similar to those reported earlier for other thermometer molecules undergoing surface collisions. Chemical sputtering leading to ionization of the surface is a prominent feature of the surface-induced dissociation (SID) spectra of n-butylbenzene acquired using the H-SAM surface but not the F-SAM surface because of the lower ionization energy of the former.