Ruthenium dioxide, a fascinating material for atomic scale surface chemistry

Over the past few years, RuO₂ has developed into one of the best-characterized late transition metal oxides in surface science, revealing unique and promising redox properties. The CO oxidation reaction over RuO₂ (110) was intensively studied by low-energy electron diffraction, scanning tunneling microscopy, high resolution core level spectroscopy, and density functional theory calculations, connecting structural and electronic properties with chemical properties. On the atomic scale the presence of one-fold coordinatively unsaturated Ru sites (1f-cus Ru) is the primary reason for the high activity of stoichiometric RuO₂ (110) towards the oxidation of CO and other small alcohols. On the stoichiometric RuO₂ (110) surface, CO molecules adsorb strongly (adsorption energy exceeding 1.2 eV) on top of the 1f-cus Ru atoms, from where the actual oxidation reaction step takes place via recombination with under-coordinated lattice oxygen to form CO₂ (the so-called Mars–van Krevelen mechanism); the conversion probability of this process is as high as 80%. This mechanism leads to a (partial) reduction of the RuO₂ (110) surface, producing two-fold coordinatively unsaturated Ru sites (2f-cus Ru) via the removal of bridging O atoms. Therefore, equally important for being a good catalyst is the facile re-oxidation of the mildly reduced RuO₂ (110) surface by oxygen supply from the gas phase. A weakly held oxygen species was found to adsorb on top of the 1f-cus Ru atoms and to actuate the restoration of the reduced RuO₂ (110) surface. On the reduced RuO₂ (110) surface, CO molecules adsorb in bridge sites above the 2f-cus Ru atoms by 1.85 eV, while the CO bond strength over 1f-cus Ru atoms is 1.61 eV. Received: 27 March 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Chemical adsorption of C60 on diamond (100)(2×1) surfaces

Molecular-dynamics simulations (MDSs) and ab initiocalculations are used to investigate the adsorption behavior of C₆₀ molecules on a clean dimer-reconstructed (100)(2×1) diamond surface. C₆₀ molecules have some probability to be adsorbed on the diamond surface at low incident energy (6∼45 eV). Electron-density contours show strong chemical interaction between C₆₀ molecules and the substrate surface. The adsorption property depends strongly on the incident energy and the impacting point. An incident energy of 18 eV may be an appropriate energy to grow a sub-monolayer or monolayer C₆₀ film on a clean C(100)(2×1) surface at room temperature. Received: 5 July 2000 / Accepted: 17 October 2000 / Published online: 28 February 2001     

Laser gain measurements at 193 nm in a small discharge cell containing ArF excimer laser gas mixtures

Spatial and temporal gain profiles as well as the peak net gain at 193 nm have been measured in X-ray preionized discharges excited by a single pulse electrical system working in the charge transfer mode. Ar- and F₂-containing laser gas mixtures with He or Ne as a buffer gas have been used. With a pumping pulse duration of ∼ 100 ns (FWHM) and a specific peak power deposition of ∼ 1 MW cm^-3 bar^-1 in a gas mixture containing F₂ : Ar : He (0.1%:5%:94.9%), at 2 bar total pressure, a very high peak net gain coefficient of ∼ 30% cm^-1 was measured in the gas discharge. The FWHM of the gain waveform was ∼ 60 ns. PACS 42.55.Lt; 42.60.Lh; 52.80.-s     

Laser-induced fluorescence study of a xenon Hall thruster

2 ⁰→6p[3/2]₂(³P₂-¹D₂) transition at 823.2 nm and the xenon-ion 5d[3]_7/2→6p[2]_5/2 ⁰(⁴D_7/2-⁴P_5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s^-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar⁺-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s^-1 to 400 m s^-1 and ion velocities as high as 12 km s^-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm^-1. Received: 20 January 1997/Revised version: 12 May 1997     

Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF₃– C₆H₄–N=N–C₆H₄–O–(CH₂) _n –SH on (111)-oriented poly-crystalline gold films on mica were examined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The spectra are analyzed with the help of density-functional-theory calculations of the isolated molecule. Only one doublet is detected in the sulphur 2p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C 1s XP spectra and the corresponding XAS π ^* resonance exhibit a rich structure which is assigned to the carbon atoms in the different chemical surroundings. Comparing XPS binding energies of the azobenzene moiety and calculated initial-state shifts reveals comparable screening of all C 1s core holes. While the carbon 1s XPS binding energy lies below the π ^*-resonance excitation-energy, the reversed order is found comparing core ionization and neutral core excitation of the nitrogen 1s core-hole of the azo group. This surprising difference in core-hole binding energies is interpreted as site-dependent polarization screening and charge transfer among the densely packed aromatic moieties. We propose that a quenching of the optical excitation within the molecular layer is thus one major reason for the low trans to cis photo-isomerization rate of azobenzene in aromatic-aliphatic SAMs.     

Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers

We present an application of continuous-wave (cw) cavity-enhanced absorption spectroscopy (CEAS) with off-axis alignment geometry of the cavity and with time integration of the cavity output intensity for detection of narrow-band and broadband absorbers using single-mode red diode lasers at λ=687.1 nm and λ=662 nm, respectively. Off-axis cw CEAS was applied to kinetic studies of the nitrate radical using a broadband absorption line at λ=662 nm. A rate constant for the reaction between the nitrate radical and E-but-2-eneof (3.78±0.17)×10^-13 cm³ molecule^-1 s^-1 was measured using a discharge-flow system. A nitrate-radical noise-equivalent (1σ≡ root-mean-square variation of the signal) detection sensitivity of 5.5×10⁹ molecule cm^-3 was achieved in a flow tube with a diameter of 4 cm and for a mirror reflectivity of ∼99.9% and a lock-in amplifier time constant of 3 s. In this case, a noise-equivalent fractional absorption per one optical pass of 1.6×10^-6 was demonstrated at a detection bandwidth of 1 Hz. A wavelength-modulation technique (modulation frequency of 10 kHz) in conjunction with off-axis cw CEAS has also been used for recording 1f- and 2f-harmonic spectra of the ^RR(15) absorption of the b¹Σ_g ⁺-X³Σ_g ^- (1,0) band of molecular oxygen at =14553.947 cm^-1. Noise-equivalent fractional absorptions per one optical pass of 1.35×10^-5, 6.9×10^-7 and 1.9×10^-6 were obtained for direct detection of the time-integrated cavity output intensity, 1f- and 2f-harmonic detection, respectively, with a mirror reflectivity of ∼99.8%, a cavity length of 0.22 m and a detection bandwidth of 1 Hz. Received: 24 June 2002 / Revised version: 12 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: vlk@physchem.ox.ac.uk     

Doping and photoelectric properties of C60 films prepared by ionized cluster beam deposition

60 films by means of ionized cluster beam (ICB) deposition. X-ray diffraction (XRD) measurement showed the C₆₀ films to be polycrystalline. The films show negative resistance–temperature coefficients, and their room-temperature resistivity is greater than 10² Ω cm. The films were implanted with 80-keV phosphorus, BBr₃, Ar, and He ions, under doses ranging up to 10¹⁶ cm^-2. The resistivity of the implanted films decreases with increasing doses. n-type electrical conduction was observed for phosphorus-implanted C₆₀ films. The interaction of impinging ions with C₆₀ clusters was found to force the C₆₀ molecules to disintegrate and the films to amorphize. p-type conduction was observed for the C₆₀ films doped with aluminum by simultaneously sputtering aluminum during deposition. C₆₀/Si structures show heterojunction characteristics that can be influenced by light illumination. The photoelectric properties of the films were found to be improved by doping with aluminum. Received: 12 January 1998/Accepted: 24 March 1998     

Photoluminescence in fullerene-doped porous glasses

We report enhanced and broad-band photoluminescence (PL) behavior of C₆₀ doped in porous silica glasses. By carrying out photoluminescence and photoluminescence-excitation spectroscopy of C₆₀ doped in surface-unpassivated and -passivated porous glasses, we show that the interaction of C₆₀ with the silica surface is quite strong and that the modified C₆₀ molecules contribute only in the orange-red region (∼1.9 eV) of the observed PL spectrum. The PL intensity in the blue-green region (∼2.3 eV) observed in doped glasses is found to originate in the porous glass itself. Received: 13 July 2000 / Revised version: 24 January 2001 / Published online: 9 May 2001     

Determination of nonlinear σ-ω-ρ model parameters in the relativistic mean-field theory by nuclear-matter properties

The parameters of the σ-ω-ρ model in the relativistic mean-field theory with nonlinear σ-meson self-interaction are determined by nuclear-matter properties, which are taken as those extracted by fits to data based on nonrelativistic nuclear models. The values of the relevant parameters are C _σ ²∼ 94, C _ω ²∼ 32, C _ρ ²∼ 26, b∼ - 0.09, c∼ 1, and the σ-meson mass m _σ∼ 370 MeV, while the value of the calculated nuclear- surface thickness is t∼ 1.4 fm. The field system is shown to be stable, since the σ-meson self-interaction energy is a lower bound in this whole parameter region with positive c. On the other hand, the effective nucleon mass M^* is larger than 0.73M, if the symmetry incompressibility K_s is assumed to be negative and the nuclear-matter incompressibility K₀ is kept less than 300 MeV. Received: 27 June 2001 / Accepted: 5 October 2001     

Synthesis and Fluorescence Properties of Lanthanide (III) Perchlorate Complexes with Naphthyl-Naphthalinesulphonylpropyl Sulfoxide

A ligand with double sulfinyl groups, naphthyl-naphthalinesulphonylpropyl sulfoxide(dinaphthyl disulfoxide, L), was synthesized by a new method and its several lanthanide (III) complexes were synthesized and characterized by element analysis, molar conductivity, coordination titration analysis, IR, TG-DTA, ¹HNMR and UV spectra. The composition of these complexes, were RE₂(ClO₄)₆·(L)₅·nH₂O (RE = La, Nd, Eu, Tb, Yb, n = 2 ∼ 6, L = C₁₀H₇SOC₃H₆SOC₁₀H₇). The fluorescent spectra illustrated that the Eu (III) complex had an excellent luminescence. It was supposed that the ligand was benefited for transferring the energy from ligand to the excitation state energy level (⁵D₀) of Eu (III). The Tb (III) complex displayed weak luminescence, which attributed to low energy transferring efficiency between the average triplet state energy level of ligand and the excited state (⁵D₄) of Tb (III). So the Eu (III) complex displayed a good antenna effect for luminescence. The phosphorescence spectra and the relationship between fluorescence lifetime and fluorescence intensity were also discussed.     

Low voltage operating OFETs based on solution-processed metal phthalocyanines

The class of sodium salts of sulphonated metal phthalocyanines (MePCS _x , S = SO₃Na, x=1–4) was investigated as a p-type channel component in organic field-effect transistors (OFETs). The solubility of these materials appears to be enhanced compared to their non-sulphonated counterparts (MePCs). We fabricated transistors based on MePCS _x varying the central metal atom (Me = Ni, Co, Zn, Al) and we evaluated the dependence of transistor performance on the nature of the central atom and the degree of sulphonation. The best results were obtained in the case of Ni and low sulphur content. In this case the mobility value is μ=1.08 cm² V⁻¹ s⁻¹ and the on/off current ratio ∼10³. The degree of sulphonation affects the electric field inside the active film in a way analogous to the case of polyelectrolyte-gated OFETs. The Na⁺ counter ions present in the channel contribute to the device characteristics but their concentration should be controlled in order to optimize device performance.     

Vibrational spectrum of perfluoroethane

We have obtained IR absorption spectra of a C₂F₆ gas and a C₂F₆ cryosolution in Xe (T = 163 K) in the fundamental and overtone ranges. We have interpreted 28 bands of ¹²C¹²CF₆ and three bands of ¹³C¹²CF₆. In the spectral ranges that correspond to vibrations that are combinations with ν₁, ν₇, and ν₅, we observe multiplets, which we attribute to interactions of the type of Fermi resonances between the states ν₁(A _1g ) ∼ ν₆(A _1g ), ν₇(E _g ) ∼ ν₆ + ν₁₁(E _g ) ∼ 2ν₈(E _g ), ν₅(A _2u ) ∼ ν₈ + ν ₁₁(A _2u ). We reveal an anomalous intensity distribution in the spectrum of an asymmetric isotopologue. For the basic and isotopic configurations of perfluoroethane, we calculate the coefficients of shapes of vibrations and the intensities of absorption bands. We reveal that the behavior of the groups ¹²CF₃ and ¹³CF₃ is indifferent to the excitation of doubly degenerate stretching vibrations ν₇(E _g ) and ν₁₀(Eu).     

Linewidth studies on Cd+ 636.0 nm and 537.8 nm lines excited in a He-Cd hollow cathode discharge

Hollow cathode (HC) lasers usually operate in a single axial mode without any optical selection. This is attributed to the large homogeneous linewidth of the gain curve due to the relatively high filling pressure of these lasers. Collisional and Doppler broadening (Δν_C and Δν_D) of the Cd⁺ 636 nm and Cd⁺ 538 nm lines (laser transitions of the HeCd⁺ laser) excited in a HC discharge tube were determined using a Fabry–Perot interferometer technique. It was found that in the pressure range 7–25 mbar Δν_D was nearly constant, while, as expected, Δν_C increased linearly with pressure. The broadening constants were α(636 nm)= (47±2) MHz/mbarand α(538 nm)=(11.8±0.5) MHz/mbar. The first constant is large enough to explain single-mode operation of the red HeCd⁺ laser; but in the case of the green laser, the exact reason for the single-mode operation remained unclear. Received: 23 November 2000 / Revised version: 30 March 2001 / Published online: 7 June 2001     

IR spectral shifts and adsorption potentials of CO and N2 adsorbed on LiF and LiCl

Following our recent work on IR spectra of molecules adsorbed on C₆₀ embedded in LiF and LiCl films, adsorption potentials of CO and N₂ adsorbed on LiF (100) and LiCl (100) were calculated. For CO on LiF, a value of 2.0 kcal mol⁻¹ was obtained, close to that calculated for CO adsorbed on a single C₆₀ molecule. The calculated value for CO on LiCl is much higher, 6.8 kcal mol⁻¹. It is therefore concluded that in the case of CO adsorbed on mixed LiF/C₆₀ films, the adsorbed CO molecules are distributed almost evenly on the LiF and C₆₀ single molecules, whereas in the case of CO adsorbed on mixed LiCl/C₆₀ films the salt is greatly preferred as the adsorption site. Adsorption potential calculations for a similar system, N₂ on LiF and LiCl, gave values of 1.5 and 4.4 kcal mol⁻¹, respectively. In this case, a much too large value was found for the adsorption potential on the LiCl surface. IR spectra of CO on the two substrates showed two strong absorptions for each of them. With N₂ induced spectra were obtained. Spectral shifts have been calculated for the above systems and were all toward higher frequencies, in agreement with experimental findings.     

Comparative Raman study of the Ti complex Cp2Ti(η2-C60) · C6H5CH3 and TixC60 films

Raman spectra from the first Ti fullerene complex Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ are presented. Compared to spectra of pure C₆₀, the spectra of the Ti complex exhibit a number of new peaks due to the symmetry lowering for C₆₀. The A _g(2) mode is downshifted by 12 cm⁻¹ compared to C₆₀, which corresponds to a charge transfer of one electron per Ti-C₆₀ bond. This value (6 cm⁻¹ for one transferred electron) is identical to the downshift of the A _g(2) mode in alkali metal fullerides with ionic bonding. The spectra of Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ were compared to the spectra of evaporated Ti_xC₆₀ films. The A _g(2) mode in Ti₄C₆₀ showed a downshift of about 25 cm⁻¹ compared to pure C₆₀, which corresponds to a charge transfer of one electron per Ti atom; this is similar to the ionic alkali metal fullerides and different from η²-C₆₀-type bonding. From Fizika Tverdogo Tela, Vol. 44, No. 3, 2002, pp. 483–485. Original English Text Copyright ? 2002 by Talyzin, Jansson, Usatov, Burlakov, Shur, Novikov. This article was submitted by the authors in English.     

Electron multiplicity in slow collisions of Ar ions with C

Electron capture by Ar⁸⁺ in collisions with C₆₀ fullerene has been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of multicharged Cr+ ₆₀ recoil ions and their fragments Ci+ m and the final charge state of outgoing projectiles Ar(8-s)+ (). The number of captured electrons r is the sum of the numbers of stabilized and emitted electrons: r = n + s. The ratio n / s decreases by a factor three with s increasing from 1 to 7 showing that the multiply excited states populated by capture of a large number of electrons are rather stable against auto-ionisation. Each kinetic energy spectrum of Ar⁺ and Ar²⁺ projectiles is composed of two peaks which we attribute to collisions “inside” and “outside” the C₆₀ cage. The measured energy shift of the projectile keV is consistent with the corresponding energy loss keV in a carbon foil with an equivalent thickness. Inside collisions are characterized by a strong dissociation of recoil ions into light monocharged fragments and by a high multiplicity of ejected electrons. Received: 25 March 1998 / Received in final form and Accepted: 9 June 1998     

Hydroxyl tagging velocimetry (HTV) in experimental air flows

 The new nonintrusive instantaneous molecular flow tagging method, hydroxyl tagging velocimetry (HTV), previously demonstrated only for high-temperature reacting flows, is now demonstrated in low-temperature (300 K) ambient air flowfields. Single-photon photodissociation of ground-state H₂O by a ∼193-nm ArF excimer laser ‘writes’ very long grid lines (>50 mm) of superequilibrium OH and H photoproducts in a room air flowfield due to the presence of ambient H₂O vapor. After displacement, the positions of the OH tag lines are revealed through fluorescence caused by A²Σ⁺ (ν^′=0)?X²Π_i (ν^′′=0) OH excitation using a pulsed frequency-doubled dye laser with an operating output wavelength of ∼308 nm. The dye ‘read’ laser accesses the strong Q₁(1) line, compensating for the relatively weak 193-nm absorption of room-temperature H₂O. The weak absorption of ground vibrational state H₂O has previously precluded the use of HTV at low temperatures, since previous HTV systems relied on a KrF excimer ‘read’ laser that could only access a weak (3?0) OH transition. The instantaneous velocity field is determined by time-of-flight analysis. HTV tag lifetime comparisons between experimental results and theoretical predictions are discussed. Multiple-line tag grids are shown displaced due to an experimental air flowfield, thus providing 2-D multipoint velocity information. Due to the instantaneous nature of the HTV tag formation, HTV is particularly suitable for, but not limited to, a variety of fast flowfield applications including nonreacting base flows for high-speed projectiles and low-temperature hypersonic external or internal flows. Received: 3 July 2001 / Revised version: 6 November 2001 / Published online: 17 January 2002     

X-ray photoelectron spectroscopy and near-edge X-ray-absorption fine structure of C60 polymer films

We report core-level and valence-band X-ray photoelectron spectroscopy (XPS) and carbon [ _]K near-edge X-ray-absorptionfine structure spectroscopy (NEXAFS) results of plasma-polymerized C₆₀. In comparison with evaporated C₆₀ the C 1s peak is broader and asymmetric for the C₆₀ polymer and its shake-up satellites diminished. Furthermore, the features of the valence-band as well as the features of the π^* antibonding orbitals of the C₆₀ polymer are broader and reduced in intensity. Changes in the electronic structure are attributed to the polymerization of C₆₀, the post-plasma functionalization of the surface by oxygen after exposure to atmosphere, and the occurrence of amorphous carbon. Received: 28 May 1999 / Accepted: 31 August 1999 / Published online: 8 March 2000     

Image-potential states on stepped Cu (001) surfaces

Image-potential states on Cu (117) and Cu (119) surfaces were studied by means of two-photon photoelectron spectroscopy. The regular array of steps generates a lateral potential on the vicinal surfaces, which modifies the surface-electronic structure. Compared to Cu (001), the band bottom of the n=1 image-potential states shifts by 40 meV to lower binding energy. The periodicity of the step-induced superlattice manifests itself as back-folding of the n=1 and 2 dispersion bands. At the surface Brillouin zone boundary a mini-gap opens with a width of 135 meV for the first image-potential state on Cu (117). On the vicinal surfaces the lifetime of the image-potential states is reduced by a factor of three as compared to Cu (001). This is attributed to a narrowing of the surface-projected bulk-band gap when projected along the [11n] direction. While the dephasing rate of the first image-potential state is close to the decay rate, higher members of the Rydberg-like series show negligible dephasing. Received: 16 October 2001 / Revised version: 9 April 2002 / Published online: 6 June 2002     

Effect of growth rate and Mg content on dendrite tip characteristics of Al–Cu–Mg ternary alloys

An experimental analysis is presented to correlate the secondary dendrite arm spacing λ ₂ and dendrite tip radius R with growth rate V and Mg content C _0-Mg of Al–Cu–Mg ternary alloys. Under constant temperature gradient G (4.84±0.13 K mm⁻¹), a series of directional solidification experiments were performed at five different growth rates V (16.7–83.3 μm/s) and five different Mg contents C _0-Mg in Al–5 wt.% Cu–(0.5–5) wt.% Mg alloys. Solid–liquid interface was investigated from the longitudinal sections of the quenched samples, and λ ₂ and R were measured on the dendrite tips. The dependencies of λ ₂ and R on V and C _0-Mg were determined. The experimental results showed that the values of λ ₂ and R decrease as V and C _0-Mg increase at a constant G. The present exponent values related to V are found to be slightly lower than the values of the theoretical models and previous experimental works; however, C _0-Mg exponent values are found to be much lower than the theoretical models and previous experimental works. The ratio of the secondary dendrite arm spacing to the dendrite tip radius is 2.09±0.15, in good agreement with the scaling law. At a constant C _0-Mg, the values of VR ² were found to slightly increase with the ascending V. However, as C _0-Mg increases, the values of VR ² decrease.