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 (B P2) CH3C6H4C6H4CH2CH2-SH deposited on Au(111) substrate. Neutral molecules were detected by laser postionization mass spectrometry. Only molecular fra gments were detected from ion-bombarded systems. The mass spectra obtained for sputtered and gas phase fragments indicate that molecules recorded duri ng ion bombardment were indeed emitted from the surface and were not the re sult of photofragmentation induced by the ionizing laser beam. From experim entally 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 distribu tions become broader and shift to longer flight times. The shift is more pr onounced for molecules from BP2 and increases with the mass of the recorded molecular fragment. We postulate that the emission of molecules is initiat ed by processes which gently break molecular bonds (e.g., chemical reaction s, secondary electrons). The formed fragments are loosely bound to the surf ace and can be removed by evaporation. At the investigated temperature rang e (170-350 K), the observed emission delay is attributed to the time requir ed for the molecule to evaporate from the surface and is not influenced by the bond breaking rate. (C) 2001 Elsevier Science B.V. All rights reserved.