C.C. Perry

Face specificity and the role of metal adatoms in molecular reorientation at surfaces

Abstract Using reflection absorption infrared spectroscopy (RAIRS), the coverage-dependent reorientation of the benzoate species on the (110) and (111) faces of copper is compared and contrasted. Whereas on Cu(110) benzoate reorients from a flat-lying to an upright orientation with increasing coverage, on Cu(111), at all coverages, benzoate is aligned normal to the surface. The formation of periodic, flat-lying copper–benzoate structures has been attributed to the availability of metal adatoms, which differs dramatically between the (111) and (110) faces. We discuss the face specificity of molecular orientation by comparing calculated formation energies of adatom vacancies from ledges and kink sites on (100), (110) and (111) faces. Further support for this model is given by the evaporation of sodium, either by pre- or post-dosing, onto low-coverage benzoate/Cu(111), which induces benzoate to convert from a perpendicular to a parallel orientation. Likewise, coevaporation of Cu while dosing benzoic acid onto the Cu(111) surface also results in a majority of flat-lying benzoate species. Finally, for adsorption on the p(2×1)O/Cu(110) reconstruction, benzoate occurs only as the upright species, which is consistent with reducing the copper mobility and availability on the (110) face. We therefore suggest the possible role of metal adatoms as a new mechanism in controlling adsorbate orientation and therefore face specificity in surface reactions.

Surface reactions of vapor phase titanium atoms with halogen and nitrogen containing polymers studied using in situ X-ray photoelectron spectroscopy and atomic force microscopy

Abstract The surface reactions of vapor phase Ti with polytetrafluorethylene (PTFE), polyvinylchloride (PVC) and nitrogen-modified polyethylene (PE) have been studied using in situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM). Titanium reactions with PTFE and PVC surfaces lead to the simultaneous formation of a titanium halide salt and TiC. Titanium reactions with both PTFE and PVC also produced significant morphological changes at the polymer surface. During Ti metallization of PTFE, defluorination did not produce any CF or CF 3 species normally observed during defluorination reactions of metal atoms with PTFE. The amount of titanium fluoride present at the metal–polymer interface was enhanced by post-metallization X-ray irradiation. Results on the Ti surface reactions are also compared and contrasted with Fe and Cu reactivity with PTFE and PVC. The reactions of Ti with nitrogen-modified PE lead to the simultaneous formation of TiN and TiC. Experimental evidence, however, suggests that the different nitrogen containing functional groups present in the nitrogen-modified PE were not equally reactive towards Ti metallization.

Metallization of poly(vinylchloride) by Fe, Ni, Cu, Ag, and Au

The evolution of the metal-polymer interface during the physical vapor deposition of Fe, Ni, Cu, Ag, and Au on poly(vinylchloride) (PVC) has been studied in situ using x-ray photoelectron spectroscopy. At low metal exposures, all of the metals studied reacted with PVC, forming a metal chloride. For those metals with more than one stable oxidation state, the salt produced corresponded to the metal’s lowest stable oxidation state, e.g., CuCl instead of CuCl2. Higher metal exposures resulted in deposition of a metallic overlayer. For Fe, Ni, Cu, and Au the variation in the metal’s reactivity (measured by the metal/metal-chloride ratio as a function of metal coverage), was observed to increase in the order Fe≈Ni>Cu>Au.

Kinetics of electron-induced decomposition of CF2Cl2 coadsorbed with water (ice): A comparison with CCl4

The kinetics of decomposition and subsequent chemistry of adsorbed CF(2)Cl(2), activated by low-energy electron irradiation, have been examined and compared with CCl(4). These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF(2)Cl(2) and CCl(4) dissociation increase in an H(2)O (D(2)O) environment (2-3x), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H(2)O, using 180 eV incident electrons, are measured (using TPD) to be 1.0+/-0.2 x 10(-15) cm(2) for CF(2)Cl(2) and 2.5+/-0.2 x 10(-15) cm(2) for CCl(4). RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl(-) and F(-) anions in the halocarbon/water films and production of H(3)O(+), CO(2), and intermediate compounds COF(2) (for CF(2)Cl(2)) and COCl(2), C(2)Cl(4) (for CCl(4)) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation.

Interaction of chlorine radicals with polyethylene and hydrocarbon thin films under vacuum conditions: a comparison with atomic oxygen reactivity

Abstract The surface reactions of atomic chlorine and oxygen with hydrocarbon-based polymers and organic thin films under vacuum conditions have been investigated with in situ X-ray photoelectron spectroscopy (XPS). The interaction of chlorine radicals (Cl( 2 P)) with polyethylene (PE) under vacuum conditions produces a partially chlorinated layer containing both CCl and CCl 2 groups whose concentration was maximized at the surface. Compared to higher-pressure photochlorination experiments where the flux of chlorine atoms is higher, the maximum extent of PE chlorination as measured by the C:Cl XPS ratio and the evolution of the C(1s) region was reduced in the present study while the surface selectivity of the reaction was enhanced. This influence of chlorine atom flux on the extent of chlorination and surface selectivity can be rationalized by a simple stochastic model of the PE chlorination process that incorporates steric effects associated with the production of mono and dichlorinated carbon atoms as well as cross-linking reactions between carbon-containing radicals. During the reaction of PE with atomic oxygen (O( 3 P)), a concentration gradient of oxygen-containing carbon functionality is also observed in the near surface region. Experiments carried out on hydrocarbon thin films based on self-assembled monolayers (SAMs) reveal that chlorination proceeds without erosion. In contrast, the incorporation of new carbon containing-oxygen functionalities during reactions of hydrocarbon films with atomic oxygen occurs in competition with carbon erosion.

Electron stimulated C–F bond breaking kinetics in fluorine-containing organic thin films

Abstract By monitoring the variation in the fluorine content and the distribution of CF x (x=0–3) species in a semifluorinated self-assembled monolayer (CF-SAM) upon prolonged X-ray irradiation, the electron stimulated C–F bond breaking kinetics in fluorine-containing organic films has been determined. At short irradiation times, X-ray irradiation induced changes in the films chemical composition are consistent with the presence of C–F, C–C and S–Au bond cleavage events. In contrast, C–F bond breaking is identified as the dominant kinetic process for longer X-ray exposures. The kinetics of X-ray induced defluorination are consistent with a first-order decay process mediated by a series of consecutive C–F bond breaking events (e.g., CF→C) whose rate constants are in excellent agreement with a stochastic model of defluorination.

Electron stimulated reactions of methyl iodide coadsorbed with amorphous solid water.

The electron stimulated reactions of methyl iodide (MeI) adsorbed on and suspended within amorphous solid water (ice) were studied using a combination of postirradiation temperature programmed desorption and reflection absorption infrared spectroscopy. For MeI adsorbed on top of amorphous solid water (ice), electron beam irradiation is responsible for both structural and chemical transformations within the overlayer. Electron stimulated reactions of MeI result principally in the formation of methyl radicals and solvated iodide anions. The cross section for electron stimulated decomposition of MeI is comparable to the gas phase value and is only weakly dependent upon the local environment. For both adsorbed MeI and suspended MeI, reactions of methyl radicals within MeI clusters lead to the formation of ethane, ethyl iodide, and diiodomethane. In contrast, reactions between the products of methyl iodide and water dissociation are responsible for the formation of methanol and carbon dioxide. Methane, formed as a result of reactions between methyl radicals and either parent MeI molecules or hydrogen atoms, is also observed. The product distribution is found to depend on the films initial chemical composition as well as the electron fluence. Results from this study highlight the similarities in the carbon-containing products formed when monohalomethanes coadsorbed with amorphous solid water are irradiated by either electrons or photons.

Reactivity of Cu with poly(tetrafluoroethylene) and poly(vinyl chloride): Effect of pre- and post-metallization modification on the metal/polymer interface

The reactivity of Cu with poly(tetrafluoroethylene) (PTFE) and poly(vinylchloride) (PVC) during thermal evaporation, as well as the effect of pre- and post-metallization Ar+ ion and x-ray mediated surface modification treatments on the metal/polymer interface, have been studied using in situ x-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM). During thermal evaporation, copper was unreactive on PTFE but reacted with PVC to form CuCl. Pretreatment of PTFE or PVC surfaces by Ar+ ion or x-ray irradiation did not modify the chemical reactivity of the polymer surface during subsequent Cu deposition, although significant morphological changes were observed on PTFE by AFM. In contrast, post-metallization modification of the Cu/PTFE interface by Ar+ ion or x-ray irradiation lead to the production of CuF2, and increased the yield of CuCl in the Cu/PVC system. In either the Cu/PTFE or Cu/PVC systems, the maximum concentration of copper halide formed and dependence upon treatment time w...

Effect of chemical composition on the neutral reaction products produced during electron beam irradiation of carbon tetrachloride/water (ice) films

The neutral reaction products formed during electron beam irradiation of carbon tetrachloride/water (ice) films have been studied as a function of the films initial CCl4 ∶ H2O ratio using a combination of reflection absorption infrared spectroscopy, mass spectrometry and X-ray photoelectron spectroscopy. When the initial CCl4 ∶ H2O ratio was high, the dominant reaction products in the film were C2Cl4 and a partially chlorinated carbonaceous film (CClx) formed as the result of carbon–carbon coupling reactions in the film. In these CCl4 rich films, chlorine is partitioned mainly into the gas phase while CO is the dominant carbon-containing gas phase species. As the CCl4 ∶ H2O ratio decreases, CO2 becomes an increasingly important reaction product at the expense of species generated from carbon–carbon coupling reactions, while chlorine is increasingly partitioned as HCl in the film, producing H3O+ and Cl−. The production of both H2 and O2 from electron stimulated reactions associated with H2O are suppressed in CCl4/H2O films, although oxygen is more efficiently quenched in the presence of CCl4.

Complementary vibrational and reflectance anisotropy spectroscopic determination of molecular azimuthal orientation

Abstract Azimuthal orientations of 9-anthracene carboxylate (9-AC) on clean and p(2×1)O/Cu(110) surfaces were determined from a reflectance anisotropy spectroscopy (RAS) signal derived from an intramolecular electronic transition. The magnitude of the molecular signal on the p(2×1)O/Cu(110) surface is 4–5 times larger than on the clean surface. We present a complete vibrational assignment of adsorbed 9-AC based on Fourier transform infrared and on- and off-specular high resolution electron energy loss spectroscopy (HREELS) and ab initio calculations. Correlation of the off-specular HREELS on p(2×1)O/Cu(110) with the RAS results demonstrates that the magnitude of the RAS signal depends on the degree of azimuthal orientation.