The displacement of hydrogen by carbon monoxide on the (100) face of tungsten: A photoemission and thermal desorption study

Photoelectron spectra (hv = 21.22 eV) and thermal desorption data were obtained for CO and H coadsorbed on W(100) at 80 K. When the clean surface is exposed to a saturation dose of H₂, subsequent exposure to CO results in the formation of a state whose emission spectrum is similar to that of molecular α-CO. Upon heating to ~280 K, a structural rearrangement occurs in which most of the adsorbed CO is converted to the strongly bound β form as the hydrogen is simultaneously desorbed. These data plus the observation that H₂ cannot be adsorbed to any significant degree on a saturated layer of β-CO suggest that adsorbed β-CO and H occupy the same atomic sites on the W(100) surface. The distinction between long and short range repulsive COH interactions is discussed. For CO adsorbed on clean W(100), the range of activation energies for vigin to β conversion is calculated from the UPS data to be 45–62 kJ/mol.     

The ground state of molecular hydrogen

The v = 0?0 quadrupole spectrum of H₂ has been recorded using a 0.005-cm^?1 resolution Fourier transform spectrometer. The rotational lines S(1) through S(5) are observable in the spectra, in the region 587 to 1447 cm^?1. The spectral position for S(0) was also obtained from its v = 1-0 ground-state combination difference. The high accuracy of the H₂ measurements has permitted a determination of four rotational constants. These are (in cm^?1) B₀ = 59.33455(6); D₀ = 0.045682(4); H₀ = 4.854(12) × 10^?5; L₀ = ?5.41(12) × 10^?8. The hydrogen line positions will facilitate studies of structure and dynamics in astrophysical objects exhibiting infrared H₂ spectra. The absolute accuracy of frequency calibration over wide spectral ranges was verified using 10-μm CO₂ and 3.39-μm CH₄ laser frequencies. Standard frequencies for 5-μm CO were found to be high by 12 MHz (3.9 × 10^?4 cm^?1).     

Experimental microwave collision diameters in the rotational spectrum of H2CO

Collision diameters for some select transitions in the rotational spectrum of H₂CO have been determined using pressure broadening of the spectral lines. Transitions of the type ΔJ = 0, K_?1 = 1, and ΔK₊₁ = 1 with 1 ≤ J ≤ 5 were investigated for both self-broadening and foreign gas broadening (He and H₂) of the spectral lines. Pressure ranges from 0.001 Torr to 0.1 Torr were explored in obtaining the line width parameters Δν_p for each transition. Collision diameters were found to be very nearly constant (14 Å) over the J states studied for H₂COH₂CO interaction, 2.5–5.8 Å for H₂COH₂ interaction and 2.7–3.5 Å for H₂COHe interaction.     

Spectra-Structure Correlations in 2,2′-Bipyridine Mercury(II) Saccharinate: Comparison With Mercury(II) Saccharinate And Chloromercury(II) Saccharinate∗

Abstract

The infrared spectrum of 1:1 complex of mercury(II) saccharinate with 2,2′-bipyridine, [Hg(C₇H₄NO₃S)₂(C₁₀H₈N₂)], was studied in the CO and SO₂stretching regions. The appearance of the spectrum in the region of the carbonyl stretching modes was correlated with the number of non-equivalent CO groups in the structure. The spectral and structural characteristics of the CO groups in the title compound were compared with the corresponding ones in the mercury(II) saccharinate and chloromercury(II) saccharinate. It was found that the frequency of the CO stretchings in the spectrum of Hg(bpy) (sac)₂ is significantly lower (1630 and 1615 cm^?1) than the frequency of the corresponding modes in the spectra of covalently bonded mercury(II) saccharinate (1705 and 1680 cm^?1) and chloromercury saccharinate (1694 cm^?1). An attempt was also made to assign the bands which are mainly due to the symmetric and the antisymmetric SO₂ stretching vibrations.

     

Chemical effects in the N7 VV Auger spectra from W(100) and W(110) surfaces

Fine structure in the N₇ VV Auger spectra from clean W(100) and W(110) surfaces has been interpreted by Lander's band model for the doubly ionized final state. It is shown that the energies of the prominent emissions in the spectra are similar for the two surfaces and furthermore are consistent with the self convolution of a bulk density of states for tungsten. An additional feature in the spectrum from the W(100) surface has been attributed to emission from an intrinsic surface state at ?0.4 eV. The localization of this state at the surface was confirmed by its sensitivity to adsorbates (H₂, CO, O₂ and I₂). During the interaction of these gases with the surface the Auger spectra always retained the features attributed to the bulk density of states which were modified only by a shift in the background intensity profile. New emission features in this part of the spectra were not seen except for the example of hydrogen adsorption when a single new emission could be seen on each of the two tungsten surfaces. However, each adsorbate produce either one (H₂) or two (CO, O₂ and I₂) new emissions at lower energies which were attributed to emissions from new adsorbate derived levels which reside at energies below the prominent features of the tungsten valence band. The location of these new adsorbate levels is compared and contrasted with the equivalent ultraviolet photoelectron spectroscopic determinations.     

Synthesis of N-deficient GaN nanoparticles and its enhanced dielectric response

In this paper, GaN nanoparticles were firstly synthesized through a facile solid-state reaction using an organic reagent cyanamide (CN₂H₂) and Ga₂O₃ as precursors. The structural properties were investigated in detail. It is found that these nanoparticles having average size of 40 nm were N-deficient with the N vacancies reaching as high as 12%. The Raman scattering spectrum of these nanoparticles presented some interesting features. The room-temperature frequency spectrum of the relative dielectric constant ?_r was measured and indicated that these nanoparticles exhibited sharp enhancement at low frequency range comparing with GaN nanomaterials and N-deficient microparticles. It is thought both the rotation direction polarization (RDP) and the space charge polarization (SCP) process should be responsible for the enhancement of ?_r in these N-deficient GaN nanoparticles.     

Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

Infrared vibration spectroscopy of molecular adsorbates on tungsten using reflection inelastic electron scattering

The observation of adsorbate vibrational energies in the range, 30 ?, hv_vib ? 1000 meV, by electron-energy-loss spectroscopy, provides detailed information on the geometry of atomic and molecular complexes. The “surface normal dipole selection rule”, is discussed and illustrated with results obtained for CO and C₂H₂ adsorption on the principal low-index faces of tungsten, viz.: W(100), W(110) and W(111) using a high-resolution electron reflection spectrometer. Specifically, the behaviour of chemisorbedd diatomic carbon monoxide and polyatomic acetylene is compared as a function of coverage and surface crystallography. Comparison is made with the spectral information obtained by reflection infrared spectroscopy and recent ultraviolet photoelectron spectroscopy studies of the chemisorption binding energies. The energy loss spectra are discussed in terms of current adsorbate models and the possible formation of “distorted rehybridized surface molecular complexes” based on molecular orbital theories of organometallic compounds.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Neutron inelastic spectroscopy of benzene chemisorbed on Raney platinum

The neutron inelastic scattering spectrum of benzene adsorbed at 300 K on Raney platinum has been measured between 350 and 2250 cm^?1. No deshydrogenation of the molecules is observed so that the benzene ring must be adsorbed parallel to the surface. Slight modifications of the force field of the model molecule (C₆H₆)Cr(CO)₃ were introduced to account for the vibrational frequency shifts. The benzene molecule is found less perturbed on platinum than on nickel. The calculated frequencies of adsorbed C₆H₆ and C₆D₆ are used to reassign some modes previously observed by electron loss spectroscopy.     

Beam maser measurement of deuterium quadrupole coupling in CD2F2

Hyperfine structure on the 2₂₀ → 3₁₃ rotational transition of CD₂F₂ was measured using a molecular beam maser spectrometer with 6 kHz linewidth (f.w.h.m.). The measured deuterium quadrupole coupling strength along the CD bond direction is eq_zzQ = 186 ± 10 kHz. The best fit asymmetry parameter is η = ?0.15 ± 0.05. Spin-rotation interaction strengths obtained from the spectrum are related to similar terms for CH₂F₂. The center frequency for the observed transition is 22 727 863 ± 2 kHz.     

UPS measurements of molecular energy level of condensed gases

There have been numerous attempts to use ultraviolet photoemission spcctroscopy (UPS) to monitor the chemical states of adsorbed gas molecules on metal surfaces. To interpret the data correctly, one has to determine the effect of photoemission on the measured energy levels of the molecule. We have measured the UPS spectra of seven gases (C₆H₆, C₅H₅N, CH₃OH, C₂H₅OH, H₂CO, H₂O, NH₃) condensed on a LN₂ cooled MoS₂ substrate at hv = 21.2 eV. The inertness of the MoS₂ substrate assures that no strong chemical bonding exists between the substrate and adsorbed molecules. For each gas, the spectrum of the condensed phase is similar to the corresponding spectrum of the gas phase except all the energy levels are shifted up by the same amount. This shift ranges from 1 to 1.65 eV for the gases studied. The energy shift is attributed to the dielectric screening of the hole produced during the optical excitation.     

Thermodynamic test of defect formation energies calculated by simulation techniques for nonstoichiometric oxides. application to CeO2−x, UO2 ± x, TiO2−x, Mn1−xO,Fe1−xO

A relation has been derived between (i) ΔH(O₂), the partial molar enthalpy of mixing of oxygen in a nonstoichiometric oxide, which may be directly measured by high temperature microcalorimetry, and (ii) the defect formation energies which may be calculated by simulation techniques. This relation permits the testing of the consistency between the experimental ΔH(O₂) and the theoretical defect formation energies. This thermodynamic test, though necessary, is not sufficient since it does not take entropy terms into account. As yet this test can be rigorously applied only for small deviations from stoichiometry, but may be however very useful for large deviations. Applications have been developed for the oxides UO_2+x, UO_2?x, TiO_2?x, Mn_1?xO, Fe_1?xO and CeO_2?x. For the first five oxides, it concluded that the theoretical formation energies for the majority defects derived from simulation calculations are not compatible with experimental ΔH(O₂). A good agreement is found for CeO_2?x.     

Absorption spectra of Cu2+ in azurite

EPR and optical absorption studies in azurite have been carried out at room and low temperatures. The EPR spectrum reveals that the ground state for Cu²⁺ ion is ²B₁. Peak to peak linewidth of EPR spectrum is calculated (ΔH_p = 76 G) and found to be close with the observed value. The Cu²⁺ ion situated in D_4h symmetry with spin-orbit interaction exhibits bands at 11,806, 16,484, 17,952 and 19,793 cm^?1. The tetragonal field parameters are calculated to be Ds = ? 3364 cm^?1 and Dt = ? 604 cm^?1. The crystal field splitting parameter is found to be Dq = ? 1175 cm^?1.     

Microwave spectrum of 2-chloropyridine

The microwave spectrum of 2-chloropyridine, C₅H₄NCl, has been studied in the frequency range from 26.5–40.0 GHz. The spectrum is characterized by strong parallel type transitions of a near-prolate asymmetric top. The assigned transitions have been used to evaluate the ground state rotational constants of the two chlorine isotopes. The rotational constants are (in MHz): A = 5872.52, B = 1637.83, C = 1280.48 for the ³⁵Cl isotopic species and A = 5872.16, B = 1591.76, C = 1252.17 for the ³⁷Cl isotopic species. The small inertial defect indicates the molecule is planar. In addition an excited vibrational state of C₅H₄N³⁵Cl has been observed and analyzed. The chlorine quadrupolar coupling constants were determined for the ground state and are: χ_aa = ?71.9 MHz for ³⁵Cl and χ_aa = ?54.9 MHz for ³⁷Cl. By assuming the pyridine ring structure the CCl bond length is found to be 1.72 Å.     

Diode-side-pumped Nd:GGG laser at 1,105?nm and frequency-doubled laser at 552?nm

A high-power diode-side-pumped 1,105?nm Nd:GGG laser and a laser at 552?nm based on intracavity frequency doubling of 1,105?nm laser are demonstrated for the first time. A 26.8-W 1,105?nm laser continuous wave output was achieved under the incident pump power of 170?W. A LiB₃O₅ crystal is used for second harmonic generation of 1,105?nm laser. When the pump power was 170?W, the average output power at 552?nm of 7.3?W was obtained, corresponding to the optical conversion efficiency of 4.3?%. And the minimum pulse width is 181?ns with the pulse repetition rate of 10?kHz. The M ² factors are measured to be 19.8 and 17.6 in the horizontal and vertical directions, respectively.     

An estimation of surface vibration excitation cross sections of CO,O and H adsorbed on tungsten

A simple model is described for the evaluation of adatom loss spectra of dipole vibrations perpendicular to the substrate. It considers the inelastic electron scattering before and after specular reflection on the substrate. By comparing the loss to elastic peak area the total cross section σ of the inelastic electron scattering can be estimated. The model was used for evaluating experimental results published in the literature by Ibach, Froitzheim, Adnot, Backx and Barnes on the systems WCO, WO and WH. The main results are: for the CO 258 meV loss peak, σ = (11.6?18.3)×10^?18cm²; for the O 78 meV loss peak, σ = (8.4?16.2) × 10^?18cm²; and for the H 130 and 155 meV loss peaks, σ =(0.73?2.2) × 10^?18cm². They are close to the theoretical values. A reasonable agreement was found between σ values determined on WO and WβCO (dissociated) systems.     

Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001): II. Ordered CO overlayer

Electron spin polarization and intensity profiles have been measured in low electron diffraction (LEED) for the (00) beam at θ = 13° and ø = 0° from a W(001) surface exposed to CO and annealed to obtain an ordered c(2 × 2) CO overlayer. The annealed surface with additional CO adsorbed was also studied. The polarization was found to be sensitive to the surface condition and the very distinct P?V profile corresponding to the c(2 × 2) overlayer is believed to be a very sensitive indicator of CO in the β₃ phase. The properties of the annealed surface exposed to further CO suggest the use of this surface as a low energy electron spin polarization analyzer.     

The use of SCF-MO calculations and experimental data in evaluating general quadratic force fields of light atom molecules

The semiempirical MOCIC method, in which SCF-MO calculations are combined with limited frequency data in evaluating general quadratic force fields, has been tested on several light atom molecules (F₂CO, H₂CO, FNO, HCCH, NCCN, and H₂CCCH₂). Both MNDO and available ab initio treatments have been employed to establish the MOCIC constraints. The MOCIC potential functions are found to be remarkably close to potential functions determined uniquely from a plethora of experimental data. In comparison with potential constants evaluated using SCF-MO methods, the MOCIC functions are in much better agreement with experimentally determined counterparts. In addition to improving the estimates of primary potentials, the MOCIC approach gives statistically significant improvement in the estimate of interaction constants for these multiply bonded molecules where stretch-stretch interactions are large. The MOCIC constraint procedure is shown to work extremely well for fluorinated molecules.