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 H2 desorption yields during irradiation. The dissociation probabilities exhibit a well-resolved maximum at 10 eV, with a full-width at half-maximum of ∼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 ∼26 fs (for a gas-phase electron-alkane collision) 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.

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This paper is part of an ongoing study of the structure sensitivity of ammonia adsorption on platinum. In the work here, the adsorption of ammonia on Pt ( 111) and pte 210) are compared using temperature programmed desorption (TPD), single-reflection infrared OR), electron energy-loss spectroscopy (EELS), and molecular-beam techniques. In addition, the adsorption of ammonia on pte III), (5 X 20)Pt( 100), (2 X I )Pt( 110), Pt(21O), and Pt(211) are compared with TPD. It is found that the TPD spectrum of ammonia is similar on Pt(lll), Pt(lOO), Pt ( 110), Pt ( 210), and Pt (211 ). There was always a molecular peak between 310 and 370 K, another peak at 170 K, and a multilayer peak at 120 K. The sticking probability was found to vary by a factor of 10 from Pt (210) to Pt ( III ) . However, the infrared spectrum of ammonia adsorbed on Pt (111) is virtually identical to the EELS of ammonia on Pt (210) after accounting for a difference in the sensiti vity of IR and EELS. Analysis of the TPD data indicates that the surface concentration of ammonia is a factor of 100-200 higher on Pt(21O) than on pte 111) at 670 K. By comparison, Pt(21O) was found to be at least a factor of 100 more active than pte 111) for ammonia dissociation at 670 K. There is more work to be done. However, the results so far indicate that the vaariations in sticking probability and residence time play an important role in the variations in rate.

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