Covalent binding of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7

The adsorption reactions and binding configurations of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7 were studied using high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and DFT calculation. The covalent attachments of these unsaturated hydrocarbons to Si(1 1 1)-7 × 7 through the formation of Si–C linkages are clearly demonstrated by the observation of the Si–C stretching mode at 450–500 cm⁻¹ in their HREELS spectra. For chemisorbed cyclohexene, the involvement of π_C=C in binding is further supported by the absence of C=C stretching modes and the disappearance of the π_C=C photoemission. The chemisorption of both 1,3-cyclohexadiene and 1,4-cyclohexadiene leads to the formation of cyclohexene-like intermediates through di-σ bonding. The existence of one π_C=C bond in their chemisorbed states is confirmed by the observation of the C=C and (sp²)C---H stretching modes and the UPS and XPS results. DFT calculations show that [4 + 2]-like cycloaddition is thermodynamically preferred for 1,3-cyclohexadiene on Si(1 1 1)-7 × 7, but a [2 + 2]-like reaction mechanism is proposed for the covalent attachment of cyclohexene and 1,4-cyclohexadiene.     

High resolution XPS studies on hexadecafluoro-copper-phthalocyanine deposited onto Si(1 1 1)7×7 surface

High resolution X-ray photoelectron spectroscopy measurements have been performed onto ultrathin films of hexadecafluoro copper phthalocyanine deposited, at room temperature and in ultrahigh vacuum conditions, onto clean Si(1 1 1)7×7 substrate (silicon, Si). The measurements, performed at various film thicknesses, show a strong interaction between the molecule and the Si substrate. All the core level peaks present strong modifications induced by the substrate interaction. In particular the fluorine (F) spectrum clearly presents the effect of the interaction of some F atoms of the molecule with the substrate, which determines the formation of F–Si bonds while the copper spectrum indicates a charge transfer from the Si substrate. The changes observed in the other core level spectra have been attributed to a different charge distribution in the molecule, after the formation of F–Si bonds. We suggest a planar growth of these molecules on the Si substrate starting from the first layer.     

Metastable (1 × 2) and (1 × 3) reconstructions of Pd(110)

By adsorption and subsequent reduction of oxygen on Pd(110), metastable (1 × 2) and (1 × 3) reconstructed surfaces have been produced. Oxygen was not present after the reduction but a small amount of residual hydrogen (< 0.15 monolayers) remained. However this is not the origin of the reconstruction as adsorption of this amount of hydrogen on the clean surface did not cause reconstruction. The structures were stable up to ˜ 370 K, and at higher temperatures they reverted to (1 × 1). These results are compared with Rh(110) where similar reconstructions have been found.     

A novel (8 × 4) superstructure as precursor to the c(4 × 4) phase during Sb/Si(0 0 1) desorption

The role of kinetics in the superstructure formation of the Sb/Si(0 0 1) system is studied using in situ surface sensitive techniques such as low energy electron diffraction, Auger electron spectroscopy and electron energy loss spectroscopy. Sb adsorbs epitaxially at room-temperature on a double-domain (DD) 2 × 1 reconstructed Si(0 0 1) surface at a flux rate of 0.06 ML/min. During desorption, multilayer Sb agglomerates on a stable Sb monolayer (ML) in a DD (2 × 1) phase before desorbing. The stable monolayer desorbs in the 600–850 °C temperature range, yielding DD (2 × 1), (8 × 4), c(4 × 4), DD (2 × 1) phases before retrieving the clean Si(0 0 1)-DD (2 × 1) surface. The stable 0.6-ML (8 × 4) phase here is a precursor phase to the recently reported 0.25-ML c(4 × 4) surface phase, and is reported for the first time.     

Adsorption of thiophene on Al(1 1 1) and deposition of aluminum on condensed thiophene

The adsorption of thiophene on clean Al(1 1 1) at 90 and 130 K has been studied with X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and work function measurements. Relatively weak chemisorption compared to adsorption on transition metals is indicated by minor changes in the valence spectrum in progressing from monolayer to multilayer thiophene, a modest work function change of −0.50 eV due to saturation dosing, and return of the work function and valence spectrum to that of clean Al(1 1 1) upon annealing at 210 K. The complementary experiment in which aluminum is thermally deposited on multilayer thiophene condensed on gold at 130 K has also been performed. XPS peak area analysis shows that metal doses less than 14×10¹⁵ atoms/cm² result in penetration through the physisorbed thiophene, but higher doses lead to the gradual build up of metal throughout the organic layer. Persistence of the thiophene UPS valence features for metal doses of 50×10¹⁵ atoms/cm² is consistent with penetration and aluminum island formation. For aluminum deposition on thiophene, charge transfer from aluminum is evidenced by metal-induced low binding energy components in the C 1s and S 2p spectra at 282.6 and 162.5 eV, respectively, and a shift in the Al 2p spectrum of 0.5 eV to higher binding energy compared to metallic aluminum. UPS also indicates progression of the frontier orbital toward the Fermi level as aluminum is deposited.     

ESDIAD studies of the structure of TiO2(110)(1 × 1) and (1 × 2) surfaces and interfaces in conjunction with LEED and STM

The TiO₂(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti₂O₃ row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O⁺ ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H⁺ ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H⁺ desorption with a weaker off-normal contribution consistent with the Ti₂O₃ model of the reconstruction.     

Catalytic dehydrogenation and thermal chemistry of cyclohexene and 1-methyl-1,4-cyclohexadiene on Si(111)7 × 7

Recently, we reported a thermal desorption study on the evolution of an intense mass 78 profile for the room-temperature exposure of cyclohexene to Si(111)7 × 7 surface, which was believed to give rise to the formation of benzene by a surface dehydrogenation reaction. Because mass 78 was also found to be the base ion in the gas-phase cracking patterns of both 1,3- and 1,4-cyclohexadiene, the dehydrogenation of cyclohexene on clean, sputtered and oxidized Si(111)7 × 7 surfaces has been re-examined in order to determine the origin of the intense mass 78 desorption profile; i.e. whether it was in fact due to the evolution of benzene or cyclohexadiene, or both. Moreover, a similar dehydrogenation reaction giving rise to toluene desorption between 350 and 600 K has been observed for the room-temperature exposure of 1-methyl-1,4-cyclohexadiene to clean and sputtered Si(111)7 × 7 surfaces. The effects of methyl substitution on the reactivity of these cyclic olefins towards Si(111)7 × 7 can be inferred from these studies. Furthermore, the catalytic activity of Si(111)7 × 7 was found to be enhanced significantly by extending the thermal desorption cycles to a higher temperature of 925 K. The dehydrogenation of these olefins on Si(111)7 × 7 also gave rise to a unique 7 × 1 low energy electron diffraction pattern. Possible factors that may play a role in any proposed model for the dehydrogenation reaction are discussed. Finally, evidence of other surface reactions including cyclohexene hydrogénation to cyclohexane will also be presented.     

Adsorption mode change from adlayer- to restructuring-type with increasing coverage, evidenced by structural determination of c(2 × 2)→(4 × 4)→(5 × 5) sequence formed on Ni(001) by Li deposition

We determined surface structures in a structural sequence c(2 × 2)→(4 × 4)→(5 × 5) formed on Ni(001) at 370 K with increasing Li coverage by a dynamical low-energy electron diffraction analysis. The (4 × 4) and (5 × 5) are complex surface-structures involving restructuring of substrate surface atoms, and are analogous to the previously determined (3 × 3) and (4 × 4) structures formed for Li/Cu(001). The c(2 × 2) at low coverages is a Li adlayer, so a change of the adsorption mode from adlayer- to restructuring-type is evidenced in the course of increasing coverage within a monolayer range.     

A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

Development of a 1K × 1K, 8–12 μm QWIP array

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a 1,024 × 1,024 (1K × 1K), 8–12  μm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using an L3/Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). We have integrated the 1K × 1K array into an SE-IR based imaging camera system and performed tests over the 50–80 K temperature range achieving BLIP performance at an operating temperature of 57 K. The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, well-established GaAs processing capabilities. One of the advantages of GaAs QWIP technology is the ability to fabricate arrays in a fashion similar to and compatible with silicon IC technology. The designer’s ability to easily select the spectral response of the material from 3 μm to beyond 15 μm is the result of the success of band-gap engineering and the Army Research Lab is a leader in this area. In this paper we will present the first results of our 1K × 1K QWIP array development including fabrication methodology, test data and imaging capabilities.     

Sodium adsorption on a Ge(100)-(2 × 1) surface

The adsorption of Na on a Ge(100)-(2 × 1) surface has been studied by means of AES, LEED, EELS, TPD and work-function measurements. In the submonolayer coverage region the coverage dependencies of the desorption activation energy E(Θ) and desorption frequency v(Θ) have been determined using the threshold TPD method. Our experimental data show that after the completion of the first Na layer, 3D crystallites develop on the Na/Ge(100) surface (Stranski-Krastanov growth mode). For Θ > 1 ML, formation, followed by decomposition of a certain Na---Ge surface compound occurs in the temperature range 410–550 K.     

Decomposition mechanism of triethylindium (TEI) on a GaP(0 0 1)-(2×1) surface studied by LEED, AES, TPD and HREELS

Adsorption and decomposition of triethylindium (TEI: (C₂H₅)₃In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C₂H₅ to form P–C₂H₅ bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface.     

Adsorption and dissociation of Si2H6 on Ge(001)2 × 1

Adsorption and thermally-induced dissociation of disilane (Si₂H₆) on clean Ge(001)2 × 1 surfaces have been investigated using a combination of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), reflection high-energy electron diffraction (RHEED), and scanning tunneling microscopy (STM). With initial Si₂H₆ exposure at room temperature, the Si surface coverage increased monotonically, the EELS surface dangling bond peak intensities continuously decreased, and the intensity of half-order RHEED diffraction rods decreased. The low-coverage Si₂H₆ sticking probability at 300 K on Ge(001) was found to be 0.5 while the saturation coverage was 0.5 ML. A new EELS feature, GSH, involving Si-H and Ge-H bond states was observed at Si₂H₆ exposures φ 3.4 × 10¹³ cm⁻². In contrast to Si₂H₆ -saturated Si(001), the saturated Ge(001) surface significant fraction of dimerized bonds. Adsorbed overlayers were highly disordered with the primary species on saturated surfaces being SiH₂, GeH, and undissociated SiH₃· Si₂H₆-saturated Ge(001)2 × 1 substrates were annealed for l min at temperatures T_a between 425 and 825 K. Admolecules were mobile at T_a = 545 K giving rise to significant ordering in one-dimensional chains. By T_a = 605 K, essentially all of the admolecules were captured into coarsened islands. Dangling-bond EELS peaks reappeared by 625 K and the intensities of the half-order RHEED diffraction rods increased. Ge segregation to the surface, which began at T_a 625 K, occurred rapidly at T_a 675 K. All H was desorbed by 725 K.     

Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.     

The atomic geometries of the ordered (2 × 2) and (√3 × √3)R30° phases of K, Rb, and Cs on Rh(111) in comparison with Ru(0001)

The local adsorption geometries of K, Rb and Cs in the (√3 × √3)R30° and (2 × 2) phases on a Rh(111) surface at coverages of 0.33 and 0.25 ML, respectively, are determined by analyzing LEED intensity data. For all (√3 × √3)R30° phases investigated, the three-fold hcp site is found. For the (2 × 2) overlayer, K remains in the hcp position, while Cs favors the on-top position. For the case of Rb-(2 × 2), LEED analysis suggests occupation of the unusual two-fold bridge site. Since LEED analysis of the Rb-(2 × 2) phase is not completely conclusive, additional experimental evidence is necessary to firmly establish this adsorption geometry.     

Reconstruction of the Ir(110) surface: A mixed faceted (1 × 3) and (1 × 1) structure

Time-of-flight scattering and recoiling spectrometry (TOF-SARS) has been used to show that the reconstructed Ir(110) surface, following annealing to 1400 ° C, consists primarily of domains of faceted (1 × 3) structures (with two missing first-layer rows and one missing second-layer row); the data are consistent with secondary domains of (1 × 1) structures (with no missing rows). This structure is determined from scans of (i) backscattering (BS) versus incidence angle , (ii) forwardscattering (FS) versus , and (iii) FS versus scattering angle Θ.     

Selective β-elimination in the reaction of 2-propanol to acetone on Rh(111)-p(2 × 1)-O

The formation of acetone from 2-propanol and Rh(111)-p(2 × 1)-O has been investigated by temperature programmed reaction and X-ray photoelectron spectroscopies and isotopic labeling experiments under ultrahigh vacuum conditions. Some 2-propanol forms 2-propoxide on Rh(111)-p(2× 1)-O below 250 K and selective β (with respect to the metal in 2-propoxide) C-H bond breaking at 270 K is the primary path for acetone evolution. A minor amount of reversible C-H bond activation is also observed. β-carbon-hydrogen bond breaking is proposed to be the rate-limiting step for the initial acetone evolution from 2-propanol on Rh(111)-p(2× 1)-O at high coverage based on kinetic isotope effects. The rate of acetone evolution is in part rate-limited by desorption, however, for low 2-propanol exposures. In addition, there is some oxygen exchange between the surface and the acetone at 320 K. Combustion to H₂O, CO and CO₂is a competing pathway. Irreversible γ-C-H bond breaking primarily leads to combustion. The reactivity of 2-propanol on the (2 × 1)-O surface is dramatically different from that on clean Rh(111), where nonselective decomposition to CO and H₂ is induced. The inhibition of extensive, nonselective C-H and C-C bond breaking is a crucial factor in determining the selectivity for β-dehydrogenation to produce acetone.     

Photon-stimulated desorption of methanol on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds

Photostimulated desorption experiments have been performed on deuterated methanol adsorbed on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds. D⁺ and the masses of the series CD⁺_x are produced in the photofragmentation process in both energy ranges. A comparison has been made with the photofragmentation spectra of methanol in the gas phase and two different desorption mechanisms have been hypothesized for the desorption of D⁺ and higher masses from the silicon surfaces at the C 1s threshold.     

A new solution of d = 11 supergravity with internal isometry group SU(3) × SU(2) × U(1)

A new solution of the bosonic sector of D = 11 supergravity is presented in which the internal space has the topology CP² × S² × S¹ and the internal isometry group is SU(3) × SU(2) × U(1).     

Baryon number generation in a flipped SU(5) × U(1) model

We consider the possibilities for generating a baryon asymmetry in the early universe in a flipped SU(5) × U(1) model inspired by the superstring. Depending on the temperature of the radiation background after inflation we can distinguish between two scenarios for baryogenesis: (1) after reheating the original SU(5) × U(1) symmetry is restored, or there was no inflation at all; (2) reheating after inflation is rather weak and SU(5) × U(1) is broken. In either case the asymmetry is generated by the out-of-equilibrium decays of a massive SU(3) × SU(2) × U(1) singlet field φ_m. In the flipped SU(5) × U(1) model, gauge symmetry breaking is triggered by strong coupling phenomena, and is in general accompanied by the production of entropy. We examine constraints on the reheating temperature and the strong coupling scale in each of the scenarios.