Emission of neutral molecules from ion-bombarded thiol self-assembled monolayers

We have investigated ion-stimulated desorption of neutral molecules emitted from 8 keV Ar⁺ ion-bombarded self-assembled monolayers (SAMs) of phenethyl mercaptan (PEM) C₆H₅CH₂CH₂-SH and 2-(4′-methyl-biphenyl-4yl)-ethanethiol (BP2) CH₃C₆H₄C₆H₄CH ₂CH₂-SH deposited on Au(111) substrate. Neutral molecules were detected by laser postionization mass spectrometry. Only molecular fragments were detected from ion-bombarded systems. The mass spectra obtained for sputtered and gas phase fragments indicate that molecules recorded during ion bombardment were indeed emitted from the surface and were not the result of photofragmentation induced by the ionizing laser beam. From experimentally obtained time-of-flight (TOF) distributions, it was determined that the majority of desorbed neutral molecules leave the surface with very low translational energies. As the sample temperature is reduced, the distributions become broader and shift to longer flight times. The shift is more pronounced for molecules from BP2 and increases with the mass of the recorded molecular fragment. We postulate that the emission of molecules is initiated by processes which gently break molecular bonds (e.g., chemical reactions, secondary electrons). The formed fragments are loosely bound to the surface and can be removed by evaporation. At the investigated temperature range (170-350 K), the observed emission delay is attributed to the time required for the molecule to evaporate from the surface and is not influenced by the bond breaking rate.