Sapphire (0 0 0 1) surface modifications induced by long-pulse 1054 nm laser irradiation

We have investigated modifications of sapphire (0 0 0 1) surface with and without coating, induced by a single laser pulse with a 1054 nm wavelength, 2.2 s duration, 7.75 mm spot and energy of 20-110 J. A holographic optical element was used for smoothing the drive beam spatially, but it induced small hotspots which initiated damage on the uncoated and coated surfaces. The individual damage effects of hotspots became less pronounced at high fluences. Due to high temperature and elevated non-hydrostatic stresses upon laser irradiation, damage occurred as fracture, spallation, basal and rhombohedral twinning, melting, vitrification, the formation of nanocrystalline phases, and solid-solid phase transition. The extent of damage increased with laser fluences. The formation of regular linear patterns with three-fold symmetry ( directions) upon fracture was due to rhombohedral twinning. Nanocrystalline -Al₂O₃ formed possibly from vapor deposition on the coated surface and manifested linear, triangular and spiral growth patterns. Glass and minor amounts of -Al₂O₃ also formed from rapid quenching of the melt on this side. The - to -Al₂O₃ transition was observed on the uncoated surface in some partially spalled alumina, presumably caused by shearing. The nominal threshold for laser-induced damage is about 47 J cm⁻² for these laser pulses, and it is about 94 J cm⁻² at the hotspots.     

Shell model calculations of the adsorption and diffusion of K, Cl, and KCl on KCl(0 0 1)

Equilibrium adsorption positions and diffusion pathways of the ions K⁺ and Cl⁻ as well as of the molecule KCl on the terrace of the (0 0 1) surface of KCl were determined by shell model calculations allowing relaxations of the crystal lattice in the vicinity of the adsorbed species. For the ions each one adsorption position was found, in which the ions are located above the hollow site at the center of a slightly distorted square formed by two cations and two anions of the uppermost surface layer of the KCl crystal. Adsorption energies of −1.52 eV for K⁺ and −1.61 eV for Cl⁻ were calculated. Jumps of the ions occur from these positions to adjacent hollow positions in the ±[1 0 0] and ±[0 1 0] directions with a jump distance of a₀/2. The activation energies for the jumps result as 0.142 for K⁺ and 0.152 eV for Cl⁻ and the mean diffusion lengths as and . For the KCl molecule four distinct adsorption minima with energies between −0.932 and −0.825 eV were found. Because of the smaller lattice relaxation caused by the molecule the adsorption energies are considerably lower than for the single ions. In the position with the largest adsorption energy the ions of the admolecule are again placed above adjacent hollow sites. In two more adsorption positions only one ion is at the hollow site and the other one in a top position above an oppositely charged ion of the surface. In the fourth position with the smallest adsorption energy both ions are in top positions. Jumps between the different adsorption positions proceed by rotations of the molecule, in which one of its ions remains essentially attached to a local minimum position. The diffusion and desorption of a KCl molecule was studied by a Monte Carlo method, resulting in a mean diffusion length x_s (nm) = 0.39 exp[0.84 (eV)/2kT], which agrees rather well with an experimental value of . Values for the mean stay time as well as for the surface diffusion coefficient are derived.     

Aromatic interactions in the close packing of phenyl-imides at Cu(1 1 0) surfaces

The oxidation of aniline at Cu(1 1 0) surfaces at 290 K has been studied by XPS and STM. A single chemisorbed product, assigned to a phenyl imide (C₆H₅N(a)), is formed together with water which desorbs. Reaction with preadsorbed oxygen results in a maximum surface concentration of phenyl imide of 2.8 × 10¹⁴ mol cm⁻² and a surface dominated by domains of three structures described by , and unit meshes. However, concentrations of phenyl imide of up to 3.3 × 10¹⁴ mol cm⁻² were obtained from the coadsorption of aniline and dioxygen (300:1 mixture) resulting in a highly ordered biphasic structure with and domains. Comparison of the STM and XPS data shows that only half the phenyl imides at the surface are imaged. Pi-stacking of the phenyl rings is proposed to account for this observation.     

Reactivity to sulphur of clean and pre-oxidised Cu(1 1 1) surfaces

The reactivity of clean and pre-oxidised Cu(1 1 1) surfaces exposed to sulphur (H₂S) has been studied at room temperature by Auger electron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. On the clean surface, the sulphur-saturated surface structure is dominated by the or so-called “zigzag” superstructure. It is shown that a single orientation domain is favoured by the slight misorientation (∼2°) of the surface with respect to the (1 1 1) plane. Scanning tunneling microscopy measurements also revealed two minority structures. Pre-oxidation was performed by exposure to 1.5 × 10⁴ L of O₂ at 300 °C. Under exposure to H₂S (1 × 10⁻⁷ mbar) at room temperature, the oxygen is totally substituted by sulphur. Once initiated, sulphur adsorption seems to propagate to cover the whole surface on the O-covered surface faster than on the clean Cu(1 1 1). At saturation by adsorbed sulphur, the surface is completely covered by the superstructure of highest coverage. This enhanced uptake of sulphur is assigned to the surface reconstruction of the copper surface induced by the pre-oxidation, causing a stronger reactivity of the Cu atoms released by the decomposition of the oxide.     

Laser excitation spectrum of C3 in the region 26 000-30 700 cm

The vibrational structure of the electronic state of C₃ in the region 26 000-30 775 cm⁻¹ has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born-Oppenheimer components of the state, and by the effects of the double minimum potential in the Q₃ coordinate, recognized by Izuha and Yamanouchi [16]. The present work shows that there is also strong anharmonic resonance between the overtones of the ν₁ and ν₃ vibrations. For instance, the levels 2 1⁺ 1 and 0 1 ⁺ 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm⁻¹ apart, centered about the expected position of the 2 1⁺ 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm⁻¹ (a vibrational energy of over 5000 cm⁻¹) can now be assigned. A vibronic level at 30181.4 cm⁻¹, which has a much lower B′ rotational constant than nearby levels of the state, possibly represents the onset of vibronic perturbations by the electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36].     

Rotational levels of the (0 0 0) and (0 1 0) states of D2O from hot emission spectra in the 320-860 cm region

The far-infrared emission spectra of deuterated water vapour were measured at different temperatures (1370, 1520, and 1950 K) in the range 320-860 cm⁻¹ at a resolution of 0.0055 cm⁻¹. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 1150 new measured lines for the D₂¹⁶O molecule corresponding to transitions between highly excited rotational levels of the (0 0 0) and (0 1 0) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J_max=26 and for the (0 0 0) ← (0 0 0) band, J_max=25 and for the (0 1 0) ← (0 1 0) band, and J_max=26 and for the (0 1 0) ← (0 0 0) band. The estimated accuracy of the measured line positions is 0.0005 cm⁻¹. To our knowledge no experimentally measured rotational transitions for D₂¹⁶O within an excited vibrational state have been available in the literature so far. An extended set of experimental rotational energy levels for (0 0 0) and (0 1 0) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.0012 cm⁻¹ for 692 rotational levels of the (0 0 0) state and 0.0010 cm⁻¹ for 639 rotational levels of the (0 1 0) vibrational state. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surface [J. Chem. Phys. 106 (1997) 4618] for the (0 0 0) and (0 1 0) states is discussed.     

A novel approach to measure the step line tension and the step dipole moment on vicinal Au(0 0 1) electrodes

In this paper we introduce a new experimental approach to determine the potential dependence of the step line tension on metal electrodes in contact with an electrolyte: (0 0 1) and (1 1 n) surfaces of single crystal gold electrodes were investigated by impedance spectroscopy in solutions containing weakly adsorbing anions, such as , F⁻ and . Within the limits of error the shift in the potential of zero charge is proportional to the step density of the vicinal surfaces indicative of a well-defined dipole moment per step length. The dipole moments per step atom are 6.8 ± 0.8, 5.2 ± 0.4, 5.8 ± 0.5 × 10⁻³ eÅ for , , and F⁻ containing electrolytes, respectively. Using the values for the pzc and the potential dependence of the capacitance curves, the potential dependence of the surface tension of the vicinal surfaces is determined. The line tension of the steps is then calculated from the difference between the surface tension of the stepped and the step free surface. Our results represent the first experimental confirmation of a recent theoretical model proposing that in absence of specifically adsorbed ions the step line tension should decrease (roughly linear) with potential.     

One-dimensional defects in iodine adlayers on Pt(1 0 0)

One-dimensional defect structures of closed-packed adlayers of iodine on Pt(1 0 0) were studied with scanning tunneling microscopy (STM). On the terraces of the Pt(1 0 0) surface we observed rotational domains with line defects running in [0 1 0] directions, in coexistence with nearly defect-free domains. In addition to these prevailing line defects (A-defects) with a local coverage lower than that of a defect-free surface, we report on much less frequently observed line defects with higher local coverages (B- and C-defects). The strong dependence of the concentration of these defects on the adsorption temperature is governed by the decrease of the overall iodine coverage with increasing temperature. Iodine adsorption at ∼1100 K leads to self-organization of A-defects in quasi-periodic arrangements. The relevance of these defects as important structural elements of commensurate superstructures of iodine on Pt(1 0 0) is stressed.     

Morphology of Ni ultrathin films on Mo(1 1 0) and W(1 0 0) studied by LEED and STM

The morphology of ultrathin Ni films on Mo(1 1 0) and W(1 0 0) has been studied by low-energy electron diffraction and scanning tunneling microscopy. Ni films grow pseudomorphically on Mo(1 1 0) at 300 K for a coverage of 0.15 ML. A (8 × 1) structure is found at 0.4 ML, which develops into a (7 × 1) structure by 0.8 ML. The film undergoes a structural change to fcc Ni(1 1 1) at 6 ML. The growth mode switches from layer-by-layer to Stranski-Krastanov between 4 ML and 6 ML. Annealing at around 850 K results in alloying of submonolayer films with the substrate, while for higher coverages the Ni agglomerates into nanowedge islands. Ni films grow pseudomorphically on W(1 0 0) up to a coverage of around 2 ML at 300 K, above which there is a structural change from bcc to hcp Ni with the epitaxial relationship . This is accompanied by the formation of orthogonal domains of uniaxial strain-relieving dislocations from the third layer of the film. For coverages up to 1 ML the growth proceeds by formation of two-dimensional islands, but shifts to three-dimensional growth by 2 ML with rectangular islands aligned along the 〈0 1 1〉 substrate directions. Annealing at around 550 K results in agglomeration of Ni into larger islands and increasing film roughness.     

Oxygen induced surface structure of Mo(1 1 0) studied by scanning tunneling microscopy

The oxygen induced surface structures formed on Mo(1 1 0) by oxygen exposure at 1300 K in UHV has been studied by scanning tunneling microscopy (STM). Two kinds of oxygen-adsorbed surface structures are observed. One consists of one-dimensional rows running along or directions at substrate molybdenum lattices, and another shows more complex structure including discrete arrangement of large protrusions and zig-zag alignments of small protrusions. This complex structure is probably a further oxygen-adsorbed structure than the well-known p(2 × 2) structure of 0.3 ML coverage. On the basis of STM image, an atomic model is proposed, where adsorbed oxygen atoms occupy both long-bridge and the quasi-threefold sites of molybdenum lattice (0.4 ML coverage). This structure is presumed to be a transient state during site-conversion with increase of oxygen exposure.     

The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

Electronic structure and surface reconstruction of adsorbed oxygen on copper(0 0 1)

The electronic structure of the c(2 × 2) and the missing row phases of chemisorbed O on Cu(0 0 1) at a coverage of 0.5 monolayers has been calculated using a full-potential semi-infinite embedding technique. Calculations are made over a range of Cu-O layer spacings, and from the change in the work function, the effective charge on O is obtained. The effective charge is the same for both phases of the O/Cu(0 0 1) surface with a value of −0.3|e| on O, and consequently too small to drive any surface instability. The angular momentum-resolved density of states and energy-resolved charge densities are used to describe the binding and the spatial electronic overlap at the surfaces. In the reconstructed phase the O and the surface Cu atoms undergo displacements, which optimises the bonding.     

Segregation of Sn and Sb in a ternary Cu(1 0 0)SnSb alloy

Surface segregation studies of Sn and Sb in Cu(1 0 0)-0.14 at.% Sn-0.12 at.% Sb ternary alloy, have been done by making use of Auger Electron Spectroscopy. The method of Linear Temperature Ramp (LTR) was employed, whereby the sample was heated and cooled linearly at a constant rate. The positive heating rate showed both a kinetic segregation profile, as well as a narrow equilibrium segregation region, at higher temperatures. The equilibrium segregation profile was extended by cooling the sample. Sn was first to segregate to the surface due to its higher diffusion coefficient, mainly from a smaller activation energy E_Sn. Sb, due to its higher segregation energy, eventually replaced Sn from the surface. The modified Darken model was used to simulate the profile yielding the following segregation parameters: D_o(Sn) = 6.3 × 10⁻⁶ m²/s, D_o(Sb) = 2.8 × 10⁻⁵ m²/s; E_Sn = 175.4 kJ/mol, E_Sb = 186.3 kJ/mol; , ; Ω_Cu-Sn = 3.4 kJ/mol, Ω_Cu-Sb = 15.9 kJ/mol and Ω_Sn-Sb = −5.4 kJ/mol.     

Photoemission study of the Li/Ge(1 1 1)-3 × 1 reconstruction

In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)-3 × 1 reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1 reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the and directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the direction. In the direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)-3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge 3d core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge 3d bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)-3 × 1 case, no double bond between Ge atoms in the top layer was found.     

Growth of well-aligned Bi nanowire on Ag(1 1 1)

We report the fabrication of one-dimensional (1D) Bi nanowires grown on Ag(1 1 1) with average lateral width from 9 to 20 nm by physical vapor deposition in ultra high vacuum conditions. In situ low-temperature scanning tunneling microscopy analyses reveal that the preferred growth of 1D Bi nanostructures is driven by the highly anisotropic bonding in the crystallographic structure of the Bi(1 1 0) plane. The Bi nanowires grow along direction and align with the directions on Ag(1 1 1). The growth of the Bi nanowires proceeds in a bilayer growth mode resulting from the layer pairing in Bi(1 1 0) which saturates the dangling bonds and lowers the total energy.     

Dissociative adsorption and thermal evolution of dichloroethylenes on Ni(1 0 0)

The room temperature (RT) adsorption and thermal evolution of cis- and trans-dichloroethylene (DCE) and their structural isomer, iso-DCE, on Ni(1 0 0) have been studied by vibrational electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS). For RT adsorption, both cis- and trans-DCE exhibit very similar EELS features that are different from those found for iso-DCE. These differences indicate the formation of different fragments upon RT adsorption. In particular, the primary adspecies for cis- and trans-DCE are ethane-1,1,2,2-tetrayl () and acetylide-like () adspecies along with a small amount of chlorovinyl adspecies, while ethylylidyne () is the more plausible adspecies for iso-DCE. The differences in the adstructures upon dissociative adsorption at RT underline the important isomeric effects. Furthermore, both AES and TDS results for all three DCE isomers show that most of the Cl atoms produced by dechlorination remain on the surface and its surface concentration remains unchanged upon annealing the samples above 500 K. Upon further annealing to 550 K, the EELS spectra of all three isomers exhibit a broad feature near 1600 cm⁻¹, which suggests the formation of carbon clusters on the surface. The presence of surface Cl atoms therefore appears to prevent the CC bond cleavage during thermal evolution of the adspecies on Ni(1 0 0).