LEED crystallographic studies of the (311) surface of nickel: I. Structural analysis

An intensity analysis with low-energy electron diffraction is reported for the stepped (311) surface of nickel. Intensity versus energy curves were measured for 14 diffracted beams at normal incidence, and comparisons made with I(E) curves calculated with the layer-doubling method. The latter showed some numerical instabilities at particular energies and topmost spacings. Evidence is presented for detecting such problems by plotting, at fixed energy, emergent beam intensities as a function of topmost spacing. Calculated and experimental intensity curves were assessed with reliability indices proposed both by Zanazzi and Jona and by Pendry. Good correspondences were obtained with an unreconstructed surface model in which the topmost spacing is contracted by about 14% from the bulk value.     

A re-interpretation of the LEED structures formed by iodine on W(l10)

The early work done by Avery on the adsorption of I₂ on W(110) has been re-interpreted using adatom models originally developed for the Cl₂/Br₂/I₂/Fe (100) systems. The experimental coverages and LEED patterns are described precisely using variable, non-coincident nets of halogen atoms. It is shown that the movement of spots within the diffraction pattern arises from the movements of iodine atoms along simple crystallographic directions. The model assumes repulsive lateral interactions between iodine adatoms which is consistent with the desorption behaviour. The reasons for structural changes within the adlayer are discussed using the model, and the internuclear spacings and geometry of the adlayer are shown to be consistent with previous work on Fe(100) and W(100).     

LEED crystallographic studies of the (311) surface of nickel: II. Methodology and uncertainties

The reliability indices proposed for LEED structural analyses by Zanazzi and Jona and by Pendry have been examined with data for the (311) surface of nickel. Consideration has been given to how these indices are affected by various operations and procedures including the smoothing of experimental data, the averaging of I(E) curves which are closely equivalent, the correcting of measured intensities for differences in grid transparencies, and including energy-dependent forms of V_or the multiple-scattering calculations. Simultaneously, assessments have been made of three potential procedures for estimating uncertainties in the determined surface geometrical parameters. In this regard a new measure of uncertainty (S₃) appears useful. It provides a compromise between being based entirely on the single-beam reliability index values, for the condition of minimum in the overall R, and being based entirely on geometries that minimise values of the single-beam indices.     

Esdiad from CO on W(100): Evidence for non-vertically adsorbed CO

Mass-discriminating measurements of the angular distributions of electron-stimulated ion desorption (ESDIAD) from CO adsorbed on W(100) and coadsorbed with C and O have been performed. The O⁺ beams indicate normal and off-normal (by 5 to 13°) desorption which is interpreted as due to CO molecules bound in symmetric and in two types of unsymmetric bridges. Preadsorption of C suppresses the vertical state, while oxidation of the surface suppresses the off-normal states.     

An investigation of an alternative method of LEED structural analysis

Tests have been made for the alternative method of LEED structural analysis, proposed recently by Clarke, wherein diffracted beam intensities from experiment and from multiple-scattering calculations are compared at particular energies. This approach appears unreliable in general, although it successfully indicates the topmost interlayer spacing for Ni(311) at individal energies greater than 130 eV. By contrast this method seems less reliable for assessing the top layer registry of Zr(0001), and it is unable to yield the structures of either of the adsorption systems Rh(100)?(2 × 2)-S or Rh(110)?c(2 × 2)-S.     

The He(I) photoelectron spectra of some γ-substituted Co(III) acetylacetonate complexes

The He(I) photoelectron spectra of the complexes of Co(III) with acetylacetone and several derivatives are reported. The ionization potentials of metal-localized d-electrons and ligand π-orbitals are used to probe the extent of metal-to-ligand π-bonding and possible aromatic character in the chelate ring.     

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.     

Microwave determination of the conformation and dipole moment of 1,3-difluoroacetone

The microwave spectra of the ground state and several low-lying vibrational modes of 1,3-difluoroacetone have been assigned and analyzed. The assigned form has a molecular conformation in which one fluorine atom lies cis and the other trans to the oxygen atom. The rotational constants of the ground state species were determined using a centrifugal distortion analysis: A = 6024.843 ± 0.006 MHz, B = 2454.414 ± 0.001 MHz, C = 1783.897 ± 0.001 MHz. The molecular dipole moment components of the ground state species lie along the a and b principal axes with μ_a = 2.38 ± 0.03 D, μ_b = 0.89 ± 0.03 D, and μ_T = 2.54 ± 0.03 D. Comparative intensity measurements with OCS microwave lines indicate that the assigned form constitutes only 20% to 30% of the total gas mixture, the remainder presumably consisting of one or more other conformers, perhaps the gauche-gauche form. The lowest vibrational frequency (82 ± 12 cm^?1) is attributed to the trans-CH₂F torsion, while the next-higher vibrational frequency (127 ± 15 cm^?1) is believed to be the cis-torsion. A low-frequency in-plane bending motion is found at 285 ± 25 cm^?1.     

Design of a cylindrical mirror analyzer for use with synchrotron radiation to investigate autoionization phenomena and angular distributions of photoemitted electrons

A first-order cylindrical mirror analyzer was designed and constructed to take advantage of the continuum nature and polarization of synchrotron radiation to study autoionization and angular distributions of electrons. Autoionization in the region 14.20–16.35 eV was found to contribute intensity to the lowest ionic state of ethylene. Seven Rydberg series converging to the seven vibrational levels of the fourth ionic state are reported. The applicability for measuring gas-phase angular distributions and the expected intensity variations from this type of experiment are discussed.     

Microwave spectrum and substitution structure of methylene chloride

The microwave spectrum of methylene chloride has been reinvestigated in order to obtain a complete substitution (r_s) structure of well-defined precision. Measurements on the ¹³CH₂Cl₂ species have yielded the following rigid-rotor rotational constants: A = 30746.20 ± 0.10 MHz, B = 3320.63 ± 0.11 MHz, and C = 3053.44 ± 0.10 MHz. These data, combined with revised values reported earlier for other isotopic species, yields the following r_s structural parameters: CCl = 1.767 ± 0.002 Å, CH = 1.085 ± 0.002 Å, ∠HCH = 112.1 ± 0.2°, and ∠ClCCl = 112.2 ± 0.1°.     

Microwave spectrum,conformation, and quadrupole coupling constants of cyclopentyl chloride

The microwave spectra of the normal and two isotopic species of cyclopentyl chloride have been observed and analyzed. For the normal isotopic species the rotational constants (in MHz) are A = 4547.77 ± 0.01, B = 2290.22 ± 0.01, and C = 2073.34 ± 0.01. From the rotational constant data, it has been shown that the stable molecular conformation is the bent axial form. Quadrupole coupling constants have been measured for the ³⁵Cl nucleus, the values being (in MHz) χ_aa = ?23.70 ± 0.10, χ_bb = 32.33 ± 0.36, and χ_cc = ?8.63 ± 0.37. When transformed to the CCl bond axis system, the coupling constants confirm the axial structure. Extensive vibrational satellite structure, presumably arising from the pseudorotational ring mode with a fundamental frequency of 52 ± 5 cm^?1, has been observed and assigned. No spectral evidence has been observed for a second stable molecular conformer.     

Microwave spectrum,structure, and chemical bonding of trimethylbromosilane

The microwave spectrum of trimethylbromosilane (trimethylsilylbromide) has been investigated in the 26.5–40.0-GHz region. Spectra of several isotopic species have been observed, including the previously unreported spectrum of the ¹H¹²C³⁰Si⁸¹Br species, which yielded a B rotational constant of 1440.92 ± 0.02 MHz. From the experimental data it has been possible to derive the following structural parameters: SiBr = 2.235 ± 0.002 Å, SiC = 1.856 ± 0.010 Å, and ∠CSiBr = 107.5 ± 0.1°. The results have been compared to other relevant data and discussed in terms of prevailing chemical bonding principles.     

The infrared spectra of HCF3 and DCF3

Experimental measurements of the infrared spectra of HCF₃ and DCF₃ are reported with emphasis on the overtone and combination bands and on the numerous resonances which occur. These include Fermi, l-type, and Coriolis x-y resonances.     

XPS and AES studies of anodic passive films grown on chromium electrodes in sulphuric acid baths

The surface composition and the thickness of anodic passive films formed on chromium electrodes in sulphuric acid baths have been studied by XPS and AES using Ar⁺—ion sputtering. The results are consistent with a 13–16-Å thick oxide layer of composition close to Cr₂O₃. Some evidence is also given concerning hydrated species located in the outermost part of the passive film.     

Vibration-rotation spectra of CH3CN: The ν7 band and its hot bands near 1040 cm−1

Infrared spectra of CH₃CN were measured in the range 170–600 cm^?1 with a Fourier transform spectrometer (0.06 cm^?1 resolution) and several small portions in the range 1020–1065 cm^?1 with a tunable Pb_1?xSn_xTe diode laser spectrometer (0.001 cm^?1 resolution). The ν₇ band was analyzed by taking account of local Fermi resonance with 3ν₈¹, and the following parameters were determined: ν₇ = 1041.8446(15) cm^?1; 3ν₈¹ =1077.88(5) cm^?1; and ∥k₇₈₈₈∥ = 1.98(1) cm^?1. Two hot bands in the ν₇ band region, i.e., (ν₇ + ν₈)² ? ν₈¹ and ν₆¹ ? ν₈¹, were also analyzed, and ν₈ = 365.05(5) cm^?1 and ζ₈ = 0.874(1) were determined by use of the observed transitions of the ν₇ + ν₈ and ν₆ bands.     

The ν1 bands of 32S18O2 and 34S18O2 from infrared-microwave double resonance

Effective band constants for the ν₁ bands of both ³²S¹⁸O₂ and ³⁴S¹⁸O₂ have been determined from infrared-microwave double resonance spectroscopy. These results allow conclusions to be drawn over the processes involved in previously reported experiments with CO₂ lasers and oxygen-18 samples of SO₂.     

The infrared spectrum of HC13F3

The infrared spectrum of HC¹³F₃ is reported for the first time. Fundamental frequency shifts due to carbon-13 substitution have been measured and rotational analyses of several of the fundamental and combination bands are given. Coriolis x-y and Fermi perturbations are evident in several of the bands. Table VI summarizes the data obtained in this study.     

Interactions between coadsorbed species on the Rh(111) surface as a function of the separation distance between them

The interaction energies as a function of the separation distance between two molecules of CO and between CO and H or O coadsorbed on the Rh(111) surface have been calculated by the Extended Hückel Method. Weak interactions of the order of a few hundredths of eV occur between the adsorbates via the metal lattice. The interaction energy is found to change its signs as the distance between the adsorbates increases. For first neighbor occupation sites the interaction energies are positive. Second and third neighbor interaction energies between CO and CO are, however, negative, the latter being stronger than the former. The interaction energies between CO and H or O are positive for first and second order neighbors and, in general, negative for third order neighbors. The computed interaction energies allow one to explain the observed LEED patterns of adsorbed CO at low temperatures. The compression of the p(2 × 2) LEED structure of CO to the (√3 × √3)R30° structure, induced by the subsequent exposure to hydrogen or oxygen, is explained on the basis of the more favorable interaction energies when CO and H, or CO and 0 are segregated.