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.

We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface-sensitive and nondestructive technique shows that the CH3-terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 Å), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.

Low energy (0–12 eV) electron impact on condensed amorphous H2O and D2O films is shown to induce electron stimulated desorption of H− and D−, respectively, via dissociative electron attachment. The onsets for H− and D− detection are at 5.5 eV, with a maximum yield for anion desorption at ∼7.4 eV. The kinetic energy distributions of the desorbing anions are peaked near 0 eV, indicating that the anions suffer post-dissociation collisions at or near the surface, with a large probability of anion trapping on the surface. The present results provide direct information on the dissociation products, prior to the interferences of subsequent reaction processes in the condensed film.

We have succeeded in obtaining infrared spectra of molecules adsorbed on the surface of clusters. The method is based on the photodissociation spectroscopy technique developed in our laboratory for the study of cluster beams and on a simple but effective way to prepare mixed clusters in which an IR chromophore is attached to the surface of a nonabsorbing host cluster. The possible extension of this technique to the study of molecular spectroscopy at the surface of clusters large enough to simulate crystal and liquid surfaces is also discussed.

nitrogen heterocycles to self-induce an effect analogous to an internal heavy atom effect in the presence of CDs has been rep~r t e d .~J~ Certain nitrogen heterocycles possessing lone-pair electrons have a tendency to deactivate through radiationless intersystem crossing. As a direct consequence of the electrondonating ability of the ring nitrogen in ACR, deactivation of the ACR molecule, if manifested through the lowest n i * singlet state, would necessarily involve transitions due to the nitrogen heteroatom. Such a mechanism seems plausible, especially given the involvement of unprotonated ACR in the quenching scheme. However, if the nitrogen heteroatom on ACR is oriented in the B-CD cavity in close proximity to the glycosidic oxygens, it seems that the role of the nitrogen heteroatom would be more compatible as one of electron acceptor. Indeed, fluorescence lifetime and absorbance quenching studies of 10-methylacridinium chloride in various nucleosidic environments demonstrate the electronaccepting ability of ACR dyes.9 Lifetime decay times were also used to demonstrate the decreased fluorescence decay time of ACR resulting from adjustment of pH with increasing concentrations of NaOH. ConclusionIn view of our findings, the enhanced quenching of ACR's fluorescence may be attributed to the specific inductive effects due to the location of nitrogen within the ring system. It seems certain that the driving force for complexation of ACR with 0-CD involves the direct interaction of the nitrogen heteroatom with (39) Turro, N. J.; Cox, G. S.; Li, X. Photochem. Phorobiol. 1983,37 (2), 149.the higher electron density provided by the glycosidic oxygens. Additionally, it is apparent that the mechanism of deactivation for ACR is affected by the relative energy differences between SI, S2, and T3, all of which depend on the solvent matrix. The probable enhancement of intersystem crossing as a result of interactions with P-CD, in turn, decreases the number of fluorescing species in solution, resulting in the observed quenching. Furthermore, the presence of a heteroatom within the ring system not only affects the overall solubility of the compound in aqueous systems but also appears to be responsible for the relatively weaker binding constants calculated for ACR to &CD. In contrast, the binding of 0-CD to structurally similar compounds that do not incorporate a heteroatom is often considerably stronger.The neutral species, as determined through pH studies, appears to be most important to the quenching observed for ACR, particularly if compared to protonated heterocycles, such as indole, which exhibit fluorescence enhancement upon addition of BCD. This ability of ACR to reflect changes in the microenvironmental polarity and pH of the 0-CD system suggests its potential application as a probe in other organized media.Low-energy (0-20 eV) electron impact on thin condensed hydrocarbon films is observed to produce, via dissociative attachment and dipolar dissociation, significant yields of H-and much lower yields of CH; ( n = 1-3) ...