Modification and stability of aromatic self-assembled monolayers upon irradiation with energetic particles

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4′-methyl-biphenyl-4yl)-ethanethiol (BP2, CH ₃-C ₆H ₄C ₆H ₄CH ₂CH ₂-SH), phenyl mercaptan (PEM, C ₆H ₅CH ₂CH ₂-SH), and 4′-methyl-biphenyl-4- thiol (BP0, CH ₃-C ₆H ₄C ₆H ₄-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.