Brian E. Bent

Benzene adsorption on Cu(111): Formation of a stable bilayer

The structure of benzene deposited on a Cu(111) surface has been investigated by a combination of temperature‐programmed desorption (TPD), high‐resolution electron energy loss spectroscopy (HREELS), and near‐edge x‐ray absorption fine structure (NEXAFS) measurements. The results indicate that benzene forms a stable bilayer on Cu(111) at 110 K prior to multilayer formation. The TPD studies show that the second layer benzene desorbs with a peak temperature 5 K higher than that for benzene multilayers. HREELS and NEXAFS results indicate that benzene in the first layer bonds with its π ring parallel to the surface. With increasing coverage, benzene forms a second layer with its π ring significantly tilted away from the surface. The results are consistent with an approximately perpendicular configuration between the first and second layer benzene molecules, which is analogous to the structure of crystalline benzene. Isotope labeling experiments indicate there is almost complete mixing between molecules in diff...

Iodobenzene on Cu(111): formation and coupling of adsorbed phenyl groups

Abstract The chemistry of iodobenzene on a Cu(111) surface has been studied by Auger electron spectroscopy (AES), high resolution electron energy loss spectroscopy (HREELS), temperature-programmed reaction (TPR) spectroscopy, and work function change measurements. The results from these techniques and from H atom titration experiments show that iodobenzene dissociates at 175 K to form phenyl groups and iodine atoms and that the phenyl groups are thermally stable to above 300 K. The HREELS spectral intensities indicate that these phenyl groups bond with their π-rings approximately parallel to the surface plane. A possible reason for this unexpected adsorption geometry is the formation of phenyl anions. Biphenyl, formed by coupling of the phenyl groups, is evolved at temperatures above 300 K, while the iodine remains adsorbed on the surface until temperatures above 900 K. Comparison of the biphenyl evolution rate after iodobenzene adsorption with the rate after biphenyl adsorption indicates that phenyl coupling as opposed to biphenyl desorption is the rate-determining step in yielding biphenyl from adsorbed phenyl groups. Factors affecting the coupling rate are discussed.

A COMPARISON OF GAS PHASE AND ELECTRO-CHEMICAL HYDROGENATION OF ETHYLENE AT PLATINUM SURFACES

Rates of hydrogenation of ethylene to ethane under gas-solid (G-S) and liquid-solid (L-S) (i.e. electrochemical) conditions at well-defined Pt(lll) and smooth polycrystalline Pt surfaces are reported. The activation energies are 5.9 kcal/mole for the L-S reaction and 10.8 kcal/mole for the G-S reaction. Comparison of the rate laws under appropriate conditions shows that the hydrogenation proceeds by different reaction ~echanisms at the two different interfaces. We have used surface science techniques (low-energy electron diffraction, Auger electron spectroscopy, highresolution electron energy loss spectroscopy, temperature programmed desorption), and electrochemistry (a combination of solution and ultra-high vacuum procedures) to characterize the adsorbed species formed under G-S and L-S reaction conditions and to gain insight into the·reaction mechanisms. We propose that in hydrogenation at the L-S interface, ethylene is reduced on the Pt surface by adsorbed H atoms, while during hydrogenation at the G-S interface, H atoms must be transferred from the Pt surface through a layer of irreversibly adsorbed ethylene to ethylene that is adsorbed on top of this layer.

Methyl radical adsorption on Cu(111): Bonding, reactivity, and the effect of coadsorbed iodine

Abstract Methyl groups have been generated on a Cu(111) surface by thermal dissociation of CH 3 I and by adsorption of methyl radicals produced by pyrolyzing azomethane (CH 3 N 2 CH 3 ). High-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed reaction (TPR) have been used to study the bonding and chemistry of adsorbed CH 3 . Both in the presence and absence of coadsorbed iodine, methyl groups decompose by rate-determining CH bond scission above 400 K to produce methane and ethylene; ethane is also formed by rate-determining methyl coupling at high coverage. The product evolution temperatures are ∼ 15 K higher when iodine is coadsorbed on the surface. In addition, the maximum methyl coverage attained with CH 3 I is a factor of 3 to 4 less than that achieved by adsorbing methyl radicals. Surface vibrational spectra with and without coadsorbed iodine are quite similar. These results suggest that the predominate effect of iodine is to block surface sites.

A VIBRATIONAL STUDY OF ETHANOL ADSORPTION ON SI(100)

The adsorption of ethanol-d0, -d3, and -d6 on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range (<1450 cm−1) is made possible by using specially prepared Si(100) wafers which have a buried metallic CoSi2 layer that acts as an internal mirror. We find that ethanol dissociatively adsorbs across the Si(100) dimers near room temperature to form surface bound hydrogen and ethoxy groups. Furthermore, the ethoxy groups are oriented such that the C3v axis of the methyl group is nearly perpendicular to the surface, unlike the case for ethoxy groups bound to metal surfaces. This adsorption geometry is deduced on the basis of the surface dipole selection rule, which applies to these Si(100) samples with a buried CoSi2 layer. Ab initio cluster calculations using gradient-corrected density functional methods confirm the proposed adsorption geometry for ethoxy on Si(100) and accurately reproduce the obs...

NEXAFS studies of halobenzenes and phenyl groups on Cu(111)

Abstract The orientations of halobenzenes (C 6 H 5 Cl and C 6 H 5 I) and phenyl (C 6 H 5 ) groups adsorbed on a Cu(111) surface have been studied by the near edge X-ray absorption fine structure (NEXAFS) method. For near-monolayer coverages, both chlorobenzene (C 6 H 5 Cl) and iodobenzene (C 6 H 5 I) bond with the plane of their aromatic π-rings tilted away from the surface plane by 45 ± 5°. Phenyl groups, generated by thermal dissociation of iodobenzene on the surface at T > 180 K, bond with their π-rings tilted away from the surface plane by an average angle of 43 ± 5° at near-saturation coverages. This geometry is analogous to the η 1 structure found for pyridine adsorption on metal surfaces.

Evidence for an Eley–Rideal mechanism in the addition of hydrogen atoms to unsaturated hydrocarbons on Cu(111)

The addition of hydrogen atoms to ethylene and benzene on a Cu(111) surface has been studied by temperature‐programmed desorption and integrated desorption mass spectrometry. The results show that adsorbed ethylene and benzene react with atomic hydrogen from the gas phase at temperatures as low as 110 K. The reaction intermediates, ethyl groups and partially hydrogenated benzene, can be isolated on the surface at this low temperature. When the surface is heated to above 150 K, hydrogen elimination reactions occur to produce ethylene, benzene, cyclohexadiene, and cyclohexene. Complete hydrogenation to alkanes also occurs for larger H‐atom exposures. The absence of these addition reactions when H atoms are adsorbed onto the surface before ethylene or benzene suggests Eley–Rideal mechanisms for these processes.

Vibrational study of silicon oxidation : H2O on Si(100)

Abstract The vibrational spectrum of water dissociatively adsorbed on Si(100) surfaces is obtained with surface infrared absorption spectroscopy. Low frequency spectra ( −1 are acquired using a buried CoSi 2 layer as an internal mirror to perform external reflection spectroscopy. On clean Si(100), water dissociates into H and OH surface species as evidenced by EELS results [1] in the literature which show a SiH stretching vibration (2082 cm −1 ), and SiOH vibrations (OH stretch at 3660 cm −1 and the SiOH bend and SiO stretch of the hydroxyl group centered around 820 cm −1 ). In this paper, infrared (IR) measurements are presented which confirm and resolve the issue of a puzzling isotopic shift for the SiO mode of the surface hydroxyl group, namely, that the SiO stretch of the OH surface species formed upon H 2 O exposure occurs at 825 cm −1 , while the SiO stretch of the OD surface species formed upon D 2 O exposure shifts to 840 cm −1 , contrary to what is expected for simple reduced mass arguments. The higher resolution of IR measurements versus typical EELS measurements makes it possible to identify a new mode at 898 cm −1 , which is an important piece of evidence in understanding the anomalous frequency shift. By comparing the results of measurements for adsorption of H 16 2 O, H 18 2 O and D 2 O with the results from recently performed first-principles calculations, it can be shown that a strong vibrational interaction between the SiO stretching and SiOH bending functional group vibrations of the hydroxyl group accounts for the observed isotopic shifts.

Disproportionation and coupling reactions of alkyl iodides on a Au(111) surface

Abstract The chemistry of linear (C2-C4) alkyl iodides on a Au(111) surface has been studied by temperature programmed reaction (TPR) spectroscopy, surface work function change (ΔΦ) measurements, Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). Alkyl iodides adsorbed at 110 K remain molecularly intact on the surface. Thermal dissociation of the C-I bond occurs above 200 K to give surface bound alkyl groups which then undergo coupling and disproportionation reactions. Isotope labelling experiments suggest that the disproportionation reaction is initiated by rate-limiting β-hydride elimination to give the corresponding alkenes. The surface hydrogen atoms produced by these reactions undergo rapid reductive elimination with unreacted alkyl groups to evolve alkanes. The net result is a strict disproportionation reaction with no detectable molecular H2 desorption from the surface. Photodissociation studies of alkyl bromides show that the reaction pathways and kinetics are unaffected by whether the coadsorbed species is Br or I. An interesting observation is that, unlike alkyl reactions on other metals where either the coupling reaction (Ag) or the decomposition reaction (Pt, Cu) is predominant, on gold, both of these reactions occur and give appreciable yields at all exposures. It appears to be coincidental that both alkyl coupling and disproportionation occur at ~265 K on Au(111). Factors influencing the relative rates of these processes are also discussed.

Dry etching of GaAs with Cl2: correlation between the surface Cl coverage and the etching rate at steady state

While dry etching of GaAs with chlorine is technologically important for manufacturing semiconductor devices, little is known conclusively about the surface chemical reactivity responsible for this etching process. In this work, modulated molecular beam scattering (MMBS) has been combined with temperature-programmed reaction (TPR) and Auger electron spectroscopy (AES) to study the reaction of molecular chlorine with GaAs. The MMBS and AES results indicate that the surface coverage of chlorine during steady state etching over the temperature range of 350–650 K is in the monolayer regime. Above 700 K the surface is chlorine free. A direct correlation is observed between the number of vacant surface sites and the Cl2 reaction probability. This result suggests a Langmuir adsorption model for the surface reaction, and it is shown that such a model combined with the product evolution kinetics determined from TPR studies successfully simulates the temperature dependence of the chlorine evolved from the surface during modulated molecular beam scattering.