Paul A. Rowntree

Molecular order at the surface of an organic monolayer studied by low energy helium diffraction

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 A), 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.

Infrared spectroscopy at the surface of clusters: SF6 on Ar

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.

Electron stimulated desorption via dissociative attachment in amorphous H2O

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.

Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams

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 ...

Dissociative electron attachment to condensed and adsorbed halomethanes

Dissociative electron attachment (DEA) processes have been observed and characterized for submonolayer and multilayer quantities of CCl4, CDCl3, CD2Cl2, CH3Cl, and CH3Br adsorbed on metallic substrates and submonolayer quantities adsorbed onto Kr spacer layers. Anion yields (Br−, Cl−, H−, D−) are reported for incident electron energies from 0 to 12 eV; no desorbing polyatomic anion fragments were detected. The results are compared to the gas‐phase DEA analogs and demonstrate the perturbations on the DEA process that are introduced by the presence of the highly polarizable environment (molecular solid+metal substrate). We also report the first observation of H−(D−) produced by DEA of halomethanes containing hydrogen (deuterium).

Electron-stimulated desorption of anions from condensed CF4

Low-energy (<20 eV) electron attachment to condensed CF4 is associated with the desorption of F− ions via dissociative electron attachment and dipolar dissociation. The present results are compared with the corresponding processes in the gas phase and from CF4 clusters.

Low energy helium diffraction studies of CH3Br overlayers physisorbed on C(0001), NaCl(001), and LiF(001) surfaces

Low energy helium diffraction has been used to determine the unit mesh parameters of overlayers of CH3Br physisorbed on C(0001), NaCl(001), and LiF(001) at ≊35 K. CH3Br forms a uniaxially commensurate overlayer on C(0001) with unit mesh parameters 4.26 A×6.75 A. On NaCl(001), CH3Br forms a high coverage and a low coverage phase. The high coverage phase is incommensurate and has unit mesh parameters 4.54 A×6.73 A, whereas the low coverage phase is commensurate with a ((2)1/2×3(2)1/2)R45° unit mesh. The structure of CH3Br/LiF(001) is essentially the same as that of the high coverage phase of CH3Br/NaCl(001) with unit mesh parameters 4.52 A×6.71 A. The unit mesh parameters (with the exception of low coverage CH3Br/NaCl ) are very similar to the lattice parameters of the a‐b [or (001)] plane of bulk crystalline CH3Br at ≊153 K. By analogy with the bulk crystal, it is likely that there are two molecules per unit mesh and that the CH3Br dipoles are nearly perpendicular to the surface and antiferroelectrically o...

Oxidation of hydrogen-passivated silicon surfaces induced by dissociative electron attachment to physisorbed H2O

Abstract The oxidation of hydrogenated silicon surfaces has been induced by irradiation of physisorbed water molecules with low energy electrons ( E i =0–20 eV), and characterized using XPS. These results demonstrate that the initial step of the chemisorption process in the low incident energy regime is the resonant formation of the dissociative 2 B 1 and 2 A 1 transient (H 2 O) − states.

Energy and charge transfer at rare-gas surfaces via substrate excitation and core-excited resonances

Abstract The electron simulated desorption of D − and H − from submonolayer D 2 O and H 2 O on Kr and Xe rare-gas substrates is significantly enhanced at incident electron energies 0.1–0.4 eV below the substrate excition energies. This enhancement is interpreted to result from the formation of core-excited resonances at the substrate surface followed by transfer of the energy and charge to the absorbate.

Dynamics of surface reactions induced by low-energy electrons Oxidation of hydrogen-passivated Si by H2O

Abstract The dynamics of surface chemical reactions induced by low energy electrons have been studied using a model system of water physisorbed on a hydrogenated surface of silicon. The onset of the reaction has been observed at electron energy E P = 5.2 ± 0.2 eV and associated with dissociative electron attachment to the adsorbed water molecules. The cross-section of the surface oxidation exhibits a strong maximum at E P ∼ 11 eV in contrast to the reported gradual rise of the cross-section for bulk ice radiolysis with the increasing electron energy. This discrepancy has been explained by a selective quenching of the excited and positively charged states of adsorbed water molecules due to the resonance charge exchange between adsorbate and substrate. Particularly, we show that excited negative ions of water can be formed at the surface by coupling of the substrate electrons to neutral excited states of adsorbed water molecules. A simple model has been proposed in which the kinetics of the surface chemical reaction induced by electrons are related to the rates of the radical generation in the adsorbed film, their recombination and reaction with the substrate.