BOND-SELECTIVE DISSOCIATION OF ALKANETHIOL BASED SELF-ASSEMBLED MONOLAYERS ABSORBED ON GOLD SUBSTRATES, USING LOW-ENERGY-ELECTRON BEAMS

We have conducted a study of electron-stimulated reactions in butaneth iol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorb ed onto Au/mica substrates, using in situ infrared spectroscopy to qua ntify the processes; the electron dose dependence of the depletion of various C-H stretching modes has permitted the determination of the fi rst dissociation cross sections for electron stimulated reactions in s elf-assembled organic monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0-15 eV regime is shown to proceed pr incipally via dissociative electron attachment, thus confirming previo us work that directly measured Hz desorption yields during irradiation . The dissociation probabilities exhibit a well-resolved maximum at 10 eV, with a full-width at half-maximum of similar to 4 eV. Unlike prev ious studies, our spectroscopic investigation shows that the dehydroge nation is not uniformly distributed throughout the organic film, but i s strongly localized near the methyl terminations of the film. The dis sociation cross sections at this interface increase rapidly with incre asing chain length. We have shown that these increases are not due to the interaction of the dissociative anionic state with the film via ch arge-induced dipole forces, nor are they due to interactions with the metal substrate via charge-image charge forces. Our results are consis tent with a dipole-image dipole quenching model, whereby the excited s tate lifetimes are reduced from an estimated similar to 26 fs (for a g as-phase electron-alkane collision) to similar to 2-10 fs, depending o n the chain length. These distance-dependent lifetimes cause the disso ciation yields for short-chain systems to be significantly lower than long-chain systems, and it is predicted that the electron-induced diss ociation cross sections for alkanethiol monolayers should show much st ronger isotopic dependencies than found with the gas-phase alkane spec ies. (C) 1998 American Institute of Physics.