Chain-length-dependent branching of irradiation-induced processes in alkanethiolate self-assembled monolayers

The effect of X-ray and low-energy (50 eV) electron irradiation on short-chain alkanethiolate (AT) self-assembled monolayers (SAMs) on Au(111) was studied by synchrotron-based high resolution X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy. As a test system, a SAM of hexanethiolate (C6) was used, whereas an analogous long-chain film, dodecanethiolate (C12) SAM, was taken as a reference. While both the C6 and C12 films exhibited a full range of irradiation-induced reactions characteristic of AT SAMs on coinage metal substrates, the branching of the reactions in these two systems was distinctly different. Whereas in the case of C12/Au, the dominant processes were decomposition of the alkyl chains and capture of the released alkylsulfide moieties in the aliphatic matrix, desorption of the complete molecular species emerging after the cleavage of the thiolate-gold bond prevailed in the case of C6/Au. This behavior was explained by higher volatility of the released C6 species and the reduced chemical activity of the alkyl matrix in C6/Au as compared to C12/Au. This matrix contained far less active sites than C12/Au due to enhanced quenching of the primary dissociative excitations by the dipole-image dipole interaction with the substrate. The efficiency of this distance-dependent process is especially high for short-chain AT SAMs.