Papers

AVS1996 Session SS-WeP: Surface Science Poster Session III

Wednesday, October 16, 1996 5:00 PM in Ballroom A

Wednesday Afternoon

Time Period WeP Sessions | Topic SS Sessions | Time Periods | Topics | AVS1996 Schedule

SS-WeP-1 Carbon-Carbon Bond Formation on Nickel Surfaces from Coadsorbed Methanethiol and Benzenethiol S. Kane (University of Michigan); D. Huntley (Oak Ridge National Laboratory); J. Gland (University of Michigan) Carbon-carbon bond formation has been demonstrated for coadsorbed thiols on the Ni(111) and Ni(100) surfaces. Toluene formation has been observed from both surfaces for the thermal reaction of adsorbed methanethiol and benzenethiol. Increasing the surface concentration of phenylthiolate and methylthiolate, the surface intemediates, increases the amount of toluene produced. In addition to toluene, hydrogen gas and the hydrogenolysis products methane and benzene are observed. On adsorption, dissociation of the sulfur-hydrogen bond in both thiols results in the formation of a stoichiometric amount of adsorbed thiolate and adsorbed hydrogen. The competition between condensation and hydrogenolysis has been probed by increasing and decreasing the availability of surface hydrogen. Addition of external hydrogen decreases toluene production by up to 25 % from the untreated surface yield on the Ni(111) surface and up to 10 % on the Ni(100) surface, despite the presence of hydrogen from initial thiol dissociation. Oxygen pretreatment on the nickel surfaces results in increased toluene production for a wide range of thiolate coverages. Water formation below the toluene formation temperature decreases surface hydrogen causing the toluene yield to increase substantially compared to methane and benzene yield. The toluene yield doubled for high coadsorbed thiolate coverages with predosed oxygen on the Ni(100) surface, while increases up to a factor of twenty were observed on the Ni(111) surface. Together these results clearly indicate that competition between hydrogen addition and alkylation controls toluene formation.
SS-WeP-2 Antimony Adsorption on the Si(111) Surface with the 7x7 and \sr\3x\sr\3-Ga Structures Studied with Scanning Tunneling Microscopy Y. Kusumi, K. Fujita, M. Ichikawa (Joint Research Center for Atom Technology, Japan) The chemical reactivity investigation of a surface with co-existing different reconstructions is important because of not only scientific but also technological interests, such as doping control on the nanometer scale. In this study, we have studied the initial stage of Sb adsorption on Si(111) surfaces with both 7x7 and \sr\3x\sr\3-Ga structures using scanning tunneling microscopy. The STM observations were performed at room temperature, after Sb deposition at 430\super o\C with nominally 0.01ML coverage. We have found that Sb atoms were preferentially adsorbed on the 7x7 regions. Within the 7x7 regions, Sb atoms were uniformly distributed, which was similar to the case of the Sb deposition at room temperature. This suggests that impinging Sb atoms hardly diffuse on the 7x7 surface below 430\super o\C. In contrast, Sb clusters whose average intervals was 32nm, were formed on the \sr\3x\sr\3-Ga regions although Sb atoms were preferentially adsorbed on the boundaries of \sr\3x\sr\3-Ga domains. This indicates that the diffusivity of Sb is enhanced on \sr\3x\sr\3-Ga surfaces, owing to the termination of Si dangling bonds by Ga adatoms. It can be expected that the selective adsorption of Sb on the 7x7 regions may occur when the size of the \sr\3x\sr\3-Ga regions is smaller than the diffusion length of Sb on the \sr\3x\sr\3-Ga surfaces. We have successfully formed the stripe pattern with the Sb-adsorbed 56nm-wide 7x7 regions and 35nm-wide \sr\3x\sr\3-Ga regions using vicinal Si(111) surfaces. This work was partly supported by NEDO.
SS-WeP-3 Interaction of S and Pt in Bimetallic Surfaces: Reaction of S\sub 2\ with Pt-Al Alloys J. Rodriguez, M. Kuhn (Brookhaven National Laboratory) Bimetallic surfaces that contain Pt are very sensitive to sulfur poisoning. Recently, we have investigated the reaction of S\sub 2\ with Pt-Al surface alloys prepared by depositing Al on Pt(111) or Pt films. The Pt-Al surface alloys display a Pt 5d band that appears at much higher binding energy (1.8-2.1 eV) than in metallic Pt. This is accompanied by positive shifts in the Pt 4f (1.0-1.3 eV) and Al 2s (0.2-0.3 eV) levels. Results of ab initio SCF calculations indicate that the Pt-Al bond is complex, involving an Al(3s,3p) --> Pt(6s,6p) charge transfer and a Pt(5d) --> Pt(6s,6p) rehybridization that localize electrons in the region between the two metal centers. After exposing Pt-Al surface alloys to S\sub 2\, aluminum is fully sulfidized, while Pt remains in a metallic state. The chemisorption of sulfur induces the segregation of Al toward the surface. AlS\sub x\ films formed on top of Pt(111) decompose at temperatures above 1100 K. The behavior observed for the Al/Pt(111) surfaces in the presence of sulfur is different from that seen for Cu/Pt(111) and Ag/Pt(111) surfaces, where the admetals promote Pt-S interactions and the formation of several layers of PtS\sub x\. Changes in the nature of the bimetallic bond and in the relative stability of the metal sulfides are responsible for this difference in behavior. ** This work was supported by the US Department of Energy (DE-AC02-76CH00016)
SS-WeP-4 Reactions of Alcohols on Co, Co-Oxygen and Co-Sulfur Overlayers on Mo(110) D. Chen, C. Friend (Harvard University) This work addresses issues important to understanding the bonding and reactivity of alcohols in bimetallic systems. Ultrathin metal films are known to exhibit different electronic and chemical properties from those of the constituent bulk metals. The Co on Mo(110) system is of particular interest since Co acts as a promoter for the commercial Mo-based catalysts used in deoxygenation and desulfurization processes. Temperature programmed reaction experiments show that 2-propanol, ethanol and methanol yield products that are characteristic of both the Co and Mo(110) surfaces. Reactions occuring on Co and Mo can be distinguished since alcohols undergo C-O bond scission on Mo(110) while the C-O bond is retained on Co due to the greater Mo-O bond strength. For example, in the case of 2-propanol, C-O bond retention products (acetone,CO) decrease with increasing Co coverage whereas C-O bond cleavage products (propene, propane) decrease. Notably, alcohol reaction on the Co overlayers do not exhibit a syngergistic effect. Studies of methanol on small Co clusters on a highly oxidized Mo surface demonstrate that the continuous Co film and Co clusters and have different reactivities; methanol reaction produces CO and hydrogen on the continous Co overlayers whereas only methanol desorption at ~250 K is observed on the Co clusters. The formation of d\sub 4\- acetaldehyde from d\sub 6\-ethanol reaction on Co-sulfur overlayers provides evidence for possible synergistic reactivity since d\sub 4\acetaldehyde is not produced on pure Co or sulfur overlayers or on Mo. The reactions of gaseous methyl radicals on the oxygen and Co-oxygen overlayers are currently under investigation in order to develop a better understanding of the mechanisms for the partial oxidation of methane to formaldehyde. Gaseous methyl radicals are generated from the pyrolysis of azomethane, and electron energy loss spectroscopy is used to distinguish oxygen at different binding sites.
SS-WeP-5 Collision Induced Chemistry of N \sub 2\ on Ru(001) L. Romm, M. Asscher (The Hebrew University, Israel) It was shown earlier that translational energy plays an important role in increasing the rate of N \sub 2\ dissociation over Fe,W and Re. Initial dissociative sticking probability, S \sub 0\, over Ru(001) rises more than 4 orders of magnitude by increasing the translational energy from 0.15 eV to above 4 eV at normal incidence. In contrast to the cases of the translationally enhanced dissociation of N \sub 2\ over Fe(111) S \sub 0\ does not depend upon crystal temperature, indicating direct dissociation mechanism. Unusually strong coupling between translational and vibrational energy was experimentally found. Vibrational energy of N \sub 2\ was found to be 5 to 10 times more efficacious than the equivalent energy in translation. It implies a late activation barrier for dissociation in the N \sub 2\ - Ru(001) potential energy surface. These experimental observations are in good agreement with the results of quantum dynamics calculations involving a mechanism of N \sub 2\tunneling through an activation barrier of 2.2 eV reflecting transition between molecular and atomic nonadiabatic PES's. Collision induced desorption (CIDE) of nitrogen molecules, physisorbed on Ru(001) at T \sub s\ = 90K by energetic inert atoms (Ar,Kr) has a threshold energy of 0.8 eV at very low coverages. CIDE of N \sub 2\ has a strong angular dependence. At the same total energy of the collider a cross section for CIDE rises with the angle of incidence. These experimental results are in good agreement with classical MD calculations, which involve realistic Morse potential for N \sub 2\ - Ru(001) interaction and Lennard-Jones potential for inert gas atom interaction with surface and with the adsorbate
SS-WeP-6 The Reactivity of Various Forms of Adsorbed NO Toward Methyl Radicals on Mo(110) K. Queeney, C. Friend (Harvard University) The adsorption and reaction of NO on clean and oxygen-covered Mo(110), as well as its reaction with gas-phase methyl radicals, are studied to address the mechanism of NO reduction by methane. The three-way catalytic converter currently used in automobiles has not proven as effective at eliminating NO\sub x\ emissions as it is at controlling hydrocarbon and CO emissions, prompting a search for more effective NO\sub x\ reduction catalysts. Previous studies have shown effective coupling of CH\sub 4\ oxidation with NO reduction over various oxide catalysts and proposed that methyl radicals enhance the reduction process by scavenging surface oxygen left behind by NO dissociation. Our study focuses on the interactions between methyl radicals and different forms of adsorbed NO and its dissociation products in order to gain a clearer understanding of the possible reduction mechanism. On clean Mo(110) most NO dissociates to form adsorbed nitrogen and oxygen by 600 K. Electron energy loss spectroscopy identifies the atomic oxygen thus formed as distinct from that arising from the dissociation of O\sub 2\. Specifically, a new loss feature at \t \980 cm\sup -1\ is attributed to doubly bonded atomic oxygen on the nitrogen-covered surface. A small amount of molecularly adsorbed NO reacts to form gas-phase N\sub 2\O below 300 K; surface infrared spectroscopy of isotopically mixed overlayers probes the existence of surface NO dimers as precursors to N\sub 2\O. Precovering Mo(110) with oxygen inhibits the dissociation pathway for NO, such that at the highest oxygen coverage studied, virtually no gaseous N\sub 2\ is formed by the recombination of surface N atoms. The reactions of NO and its dissociation products with gas phase methyl radicals formed from the pyrolysis of azomethane are studied over a variety of oxygen coverages to explore the reactivity of molecular versus dissociated NO by analysis of surface intermediates and gas-phase products thus formed.
SS-WeP-7 Coadsorption of CO\sub 2\ and H\sub 2\O on TiO\sub 2\(110): Possible Formation of Bicarbonate at Oxygen Vacancy Sites M. Henderson (Pacific Northwest National Laboratory) The coadsorption of CO\sub 2\ and H\sub 2\O was examined on the clean and vacuum-annealed (> 800 K) surfaces of TiO\sub 2\(110). Annealed TiO\sub 2\(110) possesses thermally-induced defects, designated as oxygen vacancy sites [1], that are active for H\sub 2\O dissociation [1,2]. TPD of CO\sub 2\ adsorbed on clean TiO\sub 2\(110) yielded a single desorption state at 135 K. An additional state was present at 165 K on the annealed surface. The initial sticking coefficient of CO\sub 2\ on both surfaces was close to unity. HREELS indicates that the \nu\(OCO) frequency of adsorbed CO\sub 2\ was similar to the gas phase value suggesting the adsorption state of CO\sub 2\ was linear. Losses from CO\ sub 2\ at the defect sites were not detected in HREELS. Coadsorption of CO\sub 2\ and H\sub 2\O on the annealed surface resulted in competition for both defect and non-defect sites. Coadsorption also resulted in a new CO\sub 2\ TPD state at 215 K that was only observed on the preannealed surface. Preadsorption of H\ sub 2\O into the defect sites prevented formation of the 215 K CO\sub 2\ TPD state suggesting that CO\sub 2\ must be adsorbed at the defect first in order for the 215 K CO\sub 2\ state to form. Although the 215 K CO\sub 2\ TPD state is tentatively assigned to a bicarbonate species HOCO\sub 2\, HREELS was unable to detect losses that might confirm or refute this assignment. SSIMS analysis (not yet performed at the date of abstract submission) may better address the chemical identity of the 215 K CO\sub 2\ TPD state.1. M. B. Hugenschmidt, L. Gamble and C. T. Campbell, Surf. Sci. 302 (1994) 329. 2. M. A. Henderson, Langmuir, submitted.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Material Science. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.
SS-WeP-8 Heterocycle Chemistry on Pd(111): Thiophene, Furan, and Pyrrole Decomposition Studied by LITD/FTMS T. Caldwell, I. Abdelrehim, D. Hunka, D. Land (University of California, Davis) The chemistry of sulfur-, oxygen-, and nitrogen-containing compounds on transition metal surfaces is of great interest due to the necessity of their removal from petroleum and biomass hydrocarbon resources. Compounds such as thiophene, furan, and pyrrole are models for understanding the interactions between heterocompounds and catalyst surfaces. In particular, late transition metals have been shown to be useful promoters in hydrotreating catalysis, however, very few UHV studies of heterocompounds on late transition metals have been performed. Recent results from this group using TDS and laser-induced thermal desorption with FT mass spectrometry indicate that the low-temperature decomposition mechanisms of these three compounds on clean Pd(111) differ significantly, despite the similarity in their structures. Thiophene decomposition involves cleavage between the C-S bonds, resulting in the deposition of surface sulfur and the formation of a tightly bound C\sub 4\ species on the surface which undergoes hydrogenation to form a significant amount of 1,3-butadiene. Furan, on the other hand, decomposes via elimination of \alpha\-H (desorbing as H\sub 2\) and CO, leaving a C\sub 3\H\sub 3\ species on the surface. Heating to 320K causes dimerization of some of the C\sub 3\ species, forming a small amount of benzene. Deuterium labeling studies will be presented to further elucidate the mechanism. Preliminary results involving pyrrole indicate that HCN is the major decomposition product, the formation of which seems analogous to that of CO production from furan decomposition.
SS-WeP-9 Resonance Enhanced Multiphoton Ionization Detection of GeO Desorbing in Reactive Scattering of O\sub 2\ with Ge(100) Surface I. Kamioka (National Research Institute for Metals and University of Tsukuba, Japan); M. Kitajima (National Research Institute for Metals, Japan); T. Kawabe (University of Tsukuba, Japan); K. Nakamura (National Research Institute for Metals, Japan) Reactive scattering of pulse O\sub 2\ molecular beam with a Ge(100) surface has been studied at substrate temperature between 870 and 1020 K by using resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. GeO and isotopic analogues have been observed as a reaction product. 2+1 REMPI measurements were performed on the \super 74\GeO at a wavelength around 297 nm (F-X transition). The GeO desorbing rate has been determined by analyzing waveform change of desorbing GeO from an incident O\sub 2\ waveform using a transfer function, and has been estimated to be k=10\super 17.6\+-\0.4\ exp(-2.4 \+-\0.1eV/k\sub B\T)[1/s]. The desorption angular distribution has been also studied. On a virgin surface the angular dependence of the \super 74\GeO signal intensity showed a desorption of GeO with cosine spatial distribution. On a highly etched surface, the angular distribution became sharper and slanted from the surface normal. This results are due to a facet made by etching reaction. The vibrationally resolved REMPI spectrum have been observed at a substrate temperature of 970 K through the F-X transition. The ratio of FX(1,1) vs FX(0,0) is estimated to be 0.24 \+-\ 0.07 which is comparable to the vibrational distribution ratio of GeO equilibrated at the surface temperature. From reaction rate the mean residence time of O atom on the surface is calculated to be 13 \+-\ 4 \mu\s and then the GeO stays on the Ge(100) surface with long time which is sufficient for GeO to be vibrationally equilibrated.
SS-WeP-10 Adsorption of C\sub 2\H\sub 2\ on Clean and Potassium Covered Co(0001) J. Lahtinen, J. Vaari, T. Vaara, P. Hautoj\um a\rvi (Helsinki University of Technology, Finland) We have studied the adsorption of acetylene on clean and K-covered Co(0001) by XPS, TDS and XPD. Acetylene is adsorbed molecularly at room temperature on clean Co(0001) with its molecular axis lying in the plane of the surface. Heating to 430 K causes a change in the adsorption geometry, resulting in most likely to vinylidene species with molecular axis perpendicular to the surface. Further heating results in the decomposition of the molecule to "graphitic" and "carbidic" carbon via the desorption of hydrogen. Potassium induces an additional acetylene adsorption state which is filled after the one found on clean Co(0001). The desorption temperature of potassium is increased due to acetylene when compared to the desorption of pure potassium monolayer. The decomposition of acetylene leads again to graphitic and carbidic carbon, but the relative amount of graphitic carbon is decreased due to the presence of potassium.
SS-WeP-11 Effect of Preadsorbed Phosphorous on Hydrogen Desorption from Si (100) M. Jacobson, M. Chiu, J. Crowell (University of California, San Diego) In-situ doping of silicon to obtain uniform dopant profiles is an area of interest in semiconductor processing. The rate limiting step in the codeposition of silicon and phosphorous is the removal of hydrogen to free up dangling bond sites for subsequent growth. Adatoms such as phosphorous are known to significantly modify surface properties. This study uses Temperature Programmed Desorption (TPD) to determine the effect of preadsorbed phosphorous on hydrogen desorption. The Si (100) surface is exposed to phosphine at room temperature and heated to remove hydrogen. Atomic deuterium is subsequently adsorbed on the phosphorous dosed surface and TPD experiments are performed. This process is repeated to build up the phosphorous coverage. Phosphorous can be present on the Si (100) surface in the form of islands consisting of both P-P dimers and P-Si heterodimers. Surface phosphorous contains a lone pair of electrons rather than a dangling bond. These sites are thus inaccessible to hydrogen and serve to hinder its diffusion. Results show an increase in the deuterium desorption temperature with phosphorous coverage. In correlation with results obtained by Ning et al. (Surface Science 295 (1993) 79-98) of germanium on silicon we reason that the increase in desorption temperature is due to an increase in activation energy caused by local charge transfer effects rather than an increase in the diffusion length caused by the presence of surface phosphorous.
SS-WeP-12 Electron Stimulated Desorption from PF/sub 3/ Adsorbed on Pt M. Akbulut, T. Madey (Rutgers University); L. Parenteau, L. Sanche (Universite de Sherbrooke, Canada) We have studied electron stimulated desorption (ESD) of negative and positive ions from PF/sub 3/ adsorbed on a Pt substrate over a wide electron energy range (0-175 eV). The goal is to determine the electronic excitations that cause desorption of ions from the surface. Electron bombardment of adsorbed PF/sub 3/ gives rise to F/super -/ (predominantly), F/sub 2/super -/, P/super -/ negative ion signals, and F/super +/ (predominantly), P/super +/, PF/super +/, PF/sub 2/super +/ positive ion signals. The threshold for F/super -/ desorption is at ~7.5 eV, with a sharp peak (resonance) at 11.5 eV; there is a second threshold for F/super -/ at ~15 eV. The threshold for F/super +/ desorption from the 1-ML PF/sub 3/ is at a higher energy, ~27 eV. The P/super +/ appearance potential is ~23 eV. We identify the the F/super +/ threshold for adsorbed PF/sub 3/ as being due to excitation of the F 2s level, while the near threshold P/super +/ formation corresponds to excitation of the 6a/sub 1/ level. These results indicate that the desorption of positive ions from adsorbed PF/sub 3/ results from core level excitations, while the formation of negative ions from adsorbed PF/sub 3/ is due to dissociative attachment for electron energies <15 eV and dipolar dissociation for electron energies >15 eV.
SS-WeP-13 XPS Study of Oxidation and Reduction Behavior of Rh/Al\sub 2\O\sub 3\ Catalyst K. Dohmae, H. Yoshiharu, M. Kimura (Toyota Central Research and Development Laboratories, Inc., Japan) The behavior of Rh on alumina after high-temperature oxidation and reduction treatment was investigated by X-ray Diffraction(XRD), X-ray photoelectron spectroscopy(XPS), and transmission electron microscope(TEM). \gamma\, \theta\, \alpha\-phase alumina were used for supports. The 1wt% Rh was impregnated with Rh(NO\sub 3\)\sub 3\, dried at 383K in air and reduced in 10%H\sub 2\ in N\sub 2\ at 773K for 2h. The samples were oxidized in air at 873K, 1173K, 1473K for 10h. The reduction treatment was achieved in the infrared-furnace connected to XPS chamber in 0.1 atm of 10%H\sub 2\ in N\sub 2\ at 1073K. XRD measurements of Rh/\gamma\-alumina sample oxidized in air at 1473K for 10h, shows that the alumina had transformed to alpha phase, and the Rh had fully dissolved into alumina. The Rh3d\sub 5/2\ appeared at about 310.2eV which is 1.8eV higher binding energy respect to bulk Rh\sub 2\O\sub 3\ by XPS, therefore we have been able to distinguish the chamical states of Rh dissolved into alumina and oxidized on alumina. XPS revealed that a part of Rh was dissolved into any phase alumina by oxidation at 873K. After 1173K aging, almost all Rh were dissolved into \gamma\ and \theta\-alumina, and about 80% of detected Rh was dissolved into \alpha\-alumina. By the XPS results from reduced samples after oxidation at 1473K, it was shown that a part of Rh was reduced and the Rh3d\sub 5/2\ was shifted about 3eV lower binding energy from dissolving state. TEM observation found clusters smaller than 2nm in diameter on the support. It is considered that Rh was reduced to metal and clustered on the alumina.
SS-WeP-14 XPS and IRAS Studies of Formate Synthesis on Zn/Cu(111) T. Fujitani (National Institute for Resources and Environment, Japan); I. Nakamura, H. Nishimura, T. Uchijima, J. Nakamura (University of Tsukuba, Japan) We have previously shown that Zn-deposited Cu(111) surface is a good model of Cu/ZnO catalysts for methanol synthesis by the hydrogenation of CO\sub 2\, where surface formate species was suggested to be an important intermediate relating to the rate-determining step of methanol synthesis. Here, we report the role of Zn in both the synthesis and decomposition of formate species over clean Cu(111) and Zn-deposited Cu(111) surfaces using XPS and IRAS. The formate synthesis experiments were performed using a gas mixture of H\sub 2\/CO\sub 2\=1, reaction temperature 323 ~ 353 K, and a total pressure of 1 atm. The initial rate of formate synthesis at 353 K over clean Cu(111) was measured to be approximately 6.6 x 10\super -4\ formate molecules per copper atom per second. The rate of formate synthesis over Zn-deposited Cu(111) was the same as that on clean Cu(111), indicating that the formate species is synthesized on the copper surface and Zn has no promotional effect on formate synthesis. On the other hand, the surface formate species on Zn-deposited Cu(111) decomposed at a temperature higher than that on clean Cu(111) by ~50 K, suggesting that Zn deposited on Cu(111) creates special sites stabilizing formate species. It is considered that the formate species formed initially on Cu surfaces subsequently migrate onto the special site which can be regard as the active site of methanol synthesis.
SS-WeP-15 Effect of Herringbone Reconstruction on Interference of Two-dimensional Electron Gas on Au(111) Surface D. Fujita, T. Yakabe, H. Nejoh (National Research Institute for Metals, Japan); T. Sato, M. Iwatsuki (JEOL Ltd., Japan) Close_packed surfaces of noble metals (Au,Ag,Cu) are well-known to have Shockley-type surface states, whose energy bands show very simple free-electron-like dispersions in the gaps of the projected bulk states. Those surface-state electrons behave like a two-dimensional(2D) nearly free electron Fermi-gas and may be scattered by potential barriers such as mono-atomic steps, atomic-defects and adatoms. The interference between the incident and reflected electron-waves around the scattering sites results in the formation of standing-waves and the modulation of the local-density-of-states. Among these surfaces, Au(111) surface is especially interesting since it is the only face-centered-cubic metal whose close-packed (111) surface reconstructs. The reconstruction of Au(111) surface consists of long-range herringbone structure resulting from the formation of stress domains of about 15 nm wide. Here, we have studied on the standing-wave formation on the Au(111) reconstructed surface in ultrahigh vacuum at 30 K using a UHV-STM. We have succeeded in the observation of the significant effects of the herringbone reconstruction on the formation of the standing-waves of the 2D electrons at the Fermi-level using extremely low tunneling-bias voltage. The observed constant-current STM images clearly show the important roll of the herringbone structure on the propagation and interference of surface-state electrons.
SS-WeP-16 Physisorption and Chemisorption of Alkanethiols and Disulfides on Au(111) as a Function of Chain Length D. Lavrich, S. Wetterer, T. Cummings, S. Bernasek, G. Scoles (Princeton University) Helium beam scattering has been used to study the physisorption and chemisorption energies of alkanethiols and disulfides on Au(111) in an effort to advance the understanding of self-assembled monolayers of alkanethiols on Au(111). First a study of the simpler alkane system, which only physisorbs, was conducted to develop the experimental techniques. Adsorbates were deposited on Au(111) at a fixed temperature in the range -173K to 573K followed by measurement of the desorption rate via monitoring the He beam specularity in real time. Rates at several substrate temperatures yielded Arrhenius plots from which enthalpies of adsorption were determined. Comparison of alkane desorption energies as a function of chain length indicate about 0.6eV binding energy per methylene group. The alkanethiol chain length trend is similar, but is shifted upwards by about 0.2eV with respect to the corresponding trend for the alkanes. We also used Temperature Programmed Desorption (TPD) which was implemented by dosing the Au(111) surface at low temperature followed by TPD measurements obtained by monitoring the specularity of the surface to the He beam. Alkanes showed a single desorption peak consistent with the binding energy obtained from the Arrhenius plots. Alkanethiols showed at least two desorption peaks; a lower energy peak dependent on the chain length and a higher energy peak (1.4eV) independent of chain length. The dialkyldisulfide adsorbate showed only one peak around 1.4ev.
SS-WeP-17 Identification of the Iminium Species formed by the Reaction of Cyanogen (C\sub 2\N\sub 2\) and Hydrogen on Pt(111) P. Mills, D. Jentz, M. Trenary (University of Illinois, Chicago) Reflection absorption infrared spectroscopy (RAIRS) and normal mode calculations were used to obtain and analyze the spectra of six hydrogenated and deuterated cyanogen (C\sub 2\N\sub 2\) isotopomers, formed by the reactions of \super 12\C\sub 2\\super 14\N\sub 2\, \super 12\C\sub 2\\super 15\N\sub 2\ and \super 13\C\sub 2\\super 14\N\sub 2\ with H\sub 2\ and D\sub 2\, on Pt(111) at 300 K. RAIRS spectra obtained with the post-adsorption of H\sub 2\ identify aminomethylidyne (CNH\sub 2\) and a species with the formula (CNH\sub 2\)\sub 2\ as the respective hydrogenation products of CN and C\sub 2\N\sub 2\ on Pt(111). The pre-adsorption of H\sub 2\, however, results in the exclusive formation of the previously unidentified (CNH\sub 2\)\sub 2\ adsorbate. The small number of observed bands and their respective frequencies show that the (CNH\sub 2\)\sub 2\ adsorbate possess two iminium (C=NH\sub 2\) like functionalities and a high degree of symmetry. Normal mode calculations identify \mu\\sub 2\, \eta\\super 2\-di-aminoethylene (H\sub 2\NC=CNH\sub 2\), bonding to the surface with a parallel di-metalated olefin geometry, as the most likely hydrogenation product of cyanogen on Pt(111). The similarities between \mu\\sub 2\, \eta\\super 2\-di-aminoethylene and the previously identified aminomethylidyne (CNH\sub 2\) species show that a new class of surface intermediates containing iminium (C=NH\sub 2\) like functionalities are formed by hydrogenation of the surface nitrile groups (C\sub 2\N\sub 2\ and CN) on Pt(111). Iminium intermediates also appear to play an important role in the catalytically important hydrogenation / dehydrogenation chemistry of CN containing species on Group VIII transition metal surfaces, as aminomethylidyne has also been observed during the thermal decomposition of hydrogen cyanide (HCN), azomethane (CH\sub 3\N\sub 2\CH\sub 3\) and methylamine (CH\sub 3\NH\sub 2\).
SS-WeP-18 Substrate-Mediated Interactions and Their Role in Nanoscale Processes M. Kamna, T. Graham (Pennsylvania State University); S. Stranick (National Institute of Standards & Technology); P. Weiss (Pennsylvania State University) Surface features and adsorbates perturb the electronic properties of the surrounding substrate surface. This modifies the adsorption, dynamics, and chemistry of nearby atoms and molecules. Using scanning tunneling microscopy we have imaged these perturbations at low temperature on Cu[111] and have observed their effects on the ordering and dynamics of adsorbed molecules. We discuss the important implications that substrate-mediated interactions can have on the atomic-scale mechanisms of film growth and heterogeneous selective catalysis. We speculate on how such interactions can be exploited in the design and creation of atomically precise self-assembling structures at the nanometer scale and beyond.
SS-WeP-19 Molecular Contrast within Organic Films with Scanning Tunneling Microscopy J. Arnold, L. Bumm, M. Cygan, P. Weiss, D. Allara (Pennsylvania State University); J. Tour (University of South Carolina) Scanning tunneling microscopy (STM) has become a valuable tool in the study of organic thin films. Although widely studied, the question of how these molecules contribute to tunneling or conduction remains unanswered. We have grown and modified multi-component alkanethiols on Au(111) by a combination of deposition and processing techniques and have studied them using high-resolution STM. We have been able to show STM can distinguish between molecules of varying chain length and composition. By analyzing images and local spectra of isolated and aggregated alkanethiols and/or conjugated thiols, we have been able to obtain insight into the mechanism by which these films can be imaged and the extent to which organic molecules conduct.
SS-WeP-20 The Dynamics of Benzene Adsorption on Ni(110) J. Ferris, J. Kushmerick, J. Johnson, P. Weiss (Pennsylvania State University) The low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) offers the unique opportunity to study single adsorbates and surface structures of small numbers of adsorbates. We use a low-temperature UHV STM to study the dynamics of adsorbates on single crystal metal surfaces in the dilute coverage regime. We dose a cold Ni(110) crystal with benzene molecules from the room temperature UHV chamber above the STM. At 4K diffusion is sufficiently slow to allow imaging of isolated molecules. For lateral motion, i.e. transient mobility, induced by surface processes such as adsorption we are able to analyze the final positions of the adsorbates in order to measure the distances covered and to elucidate the means by which energy is accommodated to the surface. When dosing, we align the crystal on a goniometer in a cooled chamber so that molecules adsorbing from the gas phase have surface plane projections of their momentum vectors pointing toward surface step risers at an oblique angle, thus increasing the effective terrace width. Finite amounts of adsorbed hydrogen induce the formation of line defects on the Ni(110) surface. These line defects form comb-like structures originating at step edges. We use these to investigate two-dimensional scattering and the probability with which molecules stick at defects.
SS-WeP-21 Hot Electron Dynamics at GaAs (100) Surfaces S. Diol, C. Miller, Y. Gao (University of Rochester); R. Miller (University of Toronto, Canada) The research of carrier dynamics at surfaces and interfaces is driven by a host of technological applications. There have been proposals to utilize hot electrons at the semiconductor-liquid interface in solar cells to double the theoretical maximum energy conversion efficiencies. The issue is whether or not hot electrons from the semiconductor can be stored through electron transfer to discrete molecular states i.e. can interfacial charge transfer processes occur competitively with thermalization dynamics in the solid state? Investigation of this interface also serves as a model for charge transfer at electrodes. We have performed surface sensitive time resolved two photon photoemission measurements of electron relaxation at MBE grown GaAs(100) surface quantum wells in which the electron distribution is directly determined with 0.1-0.2eV resolution. The relaxation dynamics were studied in the 1eV excess energy range corresponding to the relevant energetics for surface photochemistry. The electron lifetime spans 20fs to 600fs depending on QW doping, QW thickness and excitation density. Electron transfer to outer- sphere liquids has been probed in situ where the electron lifetime at a semiconductor-liquid interface is measured using time correlated single photon counting. From the concentration dependence and other studies, the molecular acceptors appear to have electron capture cross-sections comparable to molecular cross-sections. This is the optimal condition for hot electron capture. This work provides parameters to make the hot electron transfer channel competitive with electron relaxation enabling a future generation of solar cells exploiting hot electrons.
SS-WeP-22 Absorption Studies of Co and H on S covered Pt V. Thomas, J. Schwank, J. Gland (University of Michigan) Adsorption studies of carbon monoxide and hydrogen have been performed on hydrogen sulfide generated sulfur overlayers on a platinum foil. Temperature Programmed Desorption (TPD) is used to analyze chemisorption of these simple molecules. There are significant modifications in the chemisorptive behavior of these molecules on the sulfur populated platinum foil. Changes in the TPD spectra for these molecules on sulfur overlayers as compared to equivalent coverages on a clean Pt foil include: attenuation of peak intensities, shift of peak desorption temperatures, and modified peak desorption shapes. Hydrogen adsorption on a sulfur saturated platinum surface behaves first order and is governed by the energetics of clean platinum. TPD stack plots for a given coverage of hydrogen on surfaces having decreasing amounts of sulfur starting from a sulfur saturated surface have superimposed leading edges. This suggests that these lower binding energy sites are filling first due to an attractive potential imposed by sulfur. The modified chemisorptive behavior of carbon monoxide on sulfur overlayers is more complicated. The energetics of chemisorption for carbon monoxide is more effected by sulfur evident by the appearance of new peaks in the TPD spectra, a mobile precursor on densely poplulated sulfur surfaces, and greater than 150K shifts in peak desorption temperatures for a given coverage on sulfur overlayers having decreasing amounts of sulfur.
SS-WeP-23 Electron Stimulated Desorption of Protons and Their Transmission through Ultrathin Xe Films Z. Ma, M. Akbulut, T. Madey (Rutgers University) In order to identify a reproducible source of protons (H/super +/) for measurements of ion transmission through ultrathin layers of various materials, we have investigated the ESD (electron stimulated desorption) of H/super +/ from H/sub 2/O ice films 1 to 10 ML (monolayer) thick formed on a Ru(0001) surface at ~30K. H/super +/ ions with a peak energy of ~4 eV are generated using a focused electron beam (~300 eV), and their yield, energy and angular distributions are measured with a digital ESDIAD ( ESD ion angular distribution) detector. The angular distributions of H/super +/ desorbing from 10 ML ice are centered on the surface normal with a full width at half maximum (FWHM) ~20/super o/, while the angular distributions from 1 ML ice are much broader (FWHM~40/super o/). We use a 10 ML ice film condensed on Ru(0001) at 30 K as a source of protons in studies of H/super +/ transmission through ultrathin Xe films. We find that the H/super +/ yield is attenuated to 1% by ~3 ML Xe. The thickness dependence of H/super +/ attenuation by thin Xe films (< 2 ML) is considerably less steep than for thicker films ( > 2 ML); we attribute this to a structural effect in H/super +/ transmission through fcc Xe films. We discuss the attenuation of H/super + /in terms of elastic scattering and charge transfer.
SS-WeP-24 Absolute Surface Coverage Determination by Quantitative Thermal Desorption Spectroscopy D. Dwyer, R. Jackson (University of Maine) A highly precise gas delivery system has been built which permits both calibration of sample dosing and direct calibration of the mass spectrometer used in thermal desorption spectroscopy. The gas delivery system consists of a spinning rotor gauge mounted on a small UHV chamber of known volume. The outlet of the gas delivery system is a crimped capillary positive shutoff valve which can be opened to either a micro-channel array gas doser or opened into the cone surrounding the mass spectrometer. The relationship of gas flux versus pressure in the small UHV chamber is determined by the pressure versus time curve. The novel aspect of the system is that direct calibration of the mass spectrometer used in TDS measurements can be made. The system allows for precise calibration of surface coverages of a variety of adsobates. Data from a detailed study of oxygen on Pd(110) will be presented.
SS-WeP-25 The Interaction of Carbon Monoxide with a Platinum\sub 0.25\ - Rhodium\sub 0.75\ (111) Single Crystal Surface F. Rutten, B. Nieuwenhuys (University of Leiden, The Netherlands); M. McCoustra, M. Chesters (University of Nottingham, United Kingdom); P. Hollins (University of Reading, United Kingdom) Carbon monoxide (CO) is an important and undesirable product in the exhaust gases of combustion engines. The common so called "Three Way" catalyst to remove CO and other contaminants, notably hydrocarbons and NO species, consists of a numner of materials with Pt and Rh the main active ingredients. These metals are known to form metal alloy particles in the working catalyst. Hence the need to study the surface phusico-chemical properties of the alloy in addition to these of the pure metal components. We will report studies of the interaction of CO with the substrate using reflection absorption infrared spectroscopy (RAIRS), thermal desorption (TPD) and work function change measurements as a means of elucidating adsorption mechanism, adsorption site and heat of adsorption.
SS-WeP-26 Photochemical Processing of CO\sub 2\ and CO\sub 2\-Water Ices Studied using X-ray Photoelectron Spectroscopy T. Dillingham, D. Cornelison, W. Ruppart, S. Tegler, B. Lutz (Northern Arizona University) The investigation of ices and the chemical changes that occur during the photoprocessing of ices have important applications in atmospheric physics and planetary astronomy. In this investigation we present results concerning CO\sub 2\ and CO\sub 2\-water ices formed at liquid nitrogen temperature in the UHV environment of the XPS chamber. The ices have been bombarded continuously for up to ten hours with high intensity Mg K\sub alpha\ radiation. Significant changes are observed in the XPS core-level lineshapes associated with these ices. During the photoprocessing of these ices, a quadrupole mass spectrometer was also used to monitor the species evolving from the surface. The XPS and mass spectrometer results are correlated and the possible mechanisms associated with the photochemical processing of the ices are presented. Supported by the NAU Organized Research Program and the NASA Origins of Solar Systems Program.

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