The use of highly organized molecular films as electron scattering targets: spectroscopic and desorption measurements of selective bond rupture in organic films

In-situ FT-IR spectroscopic and desorption detection schemes have been used to observe, characterize and quantify electron-induced modifications of self-assembled monolayers (SAMs) of organic molecules chemisorbed to highly ordered metal surfaces. In the case of n-alkanethiol SAMs/Au(111) surfaces, the cross-sections for C---H bond rupture at the terminal methyl groups have onsets with incident energies near E_i7 eV and well-resolved maxima at E_i10 eV, indicating that dissociative electron attachment is the primary step of the excitation-dissociation process. The systematic differences between the depletion of the ---CH₃ and ---CH₂-functional groups on the surface, detected using infrared spectroscopy, show that molecular desorption of entire n-alkane chains is not a significant process, despite the S---Au(111) bond being the weakest of the system. The dissociation cross-sections increase with increasing chain length, indicating that the dissociation dynamics are strongly affected by the distance to the metal substrate, presumably via a dipole-damping process; we have used this behaviour to estimate that the excited state lifetimes for these systems are 2–10 fs. Chemisorption of “target” species to stable metal substrates appears to be a general approach for the study of electron-molecule interactions and condensed phase processes in molecular systems at ambient temperatures.