We have conducted a study of electron-stimulated reactions in butanethiol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorbed onto Au/mica substrates, using in situ infrared spectroscopy to quantify the processes; the electron dose dependence of the depletion of various C-H stretching modes has permitted the determination of the first dissociation cross sections for electron stimulated reactions in self-assembled organic monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0-15 eV regime is shown to proceed principally via dissociative electron attachment, thus confirming previous 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 previous studies, our spectroscopic investigation shows that the dehydrogenation is not uniformly distributed throughout the organic film, but is strongly localized near the methyl terminations of the film. The dissociation cross sections at this interface increase rapidly with increasing chain length. We have shown that these increases are not due to the interaction of the dissociative anionic state with the film via charge-induced dipole forces, nor are they due to interactions with the metal substrate via charge-image charge forces. Our results are consistent with a dipole-image dipole quenching model, whereby the excited state lifetimes are reduced from an estimated similar to 26 fs (for a gas-phase electron-alkane collision) to similar to 2-10 fs, depending on the chain length. These distance-dependent lifetimes cause the dissociation yields for short-chain systems to be significantly lower than long-chain systems, and it is predicted that the electron-induced dissociation cross sections for alkanethiol monolayers should show much stronger isotopic dependencies than found with the gas-phase alkane species.