P. A. Agron

Use of the multiple‐scattering method for calculating the asymmetry parameter in the angle‐resolved photoelectron spectroscopy of N2, CO, CO2, COS, and CS2

Calculations based on the multiple‐scattering method have been performed for the asymmetry parameter (β) as a function of photon energy (from the ionization threshold to 2 Ry above threshold) for the gaseous molecules: N2, CO, CO2, COS, and CS2. For the diatomic molecules N2 and CO we have investigated the use of an overlapping spheres transition state model. The calculations were critically compared with experimental data in the literature. In addition, we have carried out angle‐resolved photoelectron spectroscopy on the five molecules using a polarized source of He(i) radiation. Although the calculations show in general good predictive capability, there are also several instances where these calculations seem inadequate and suggestions are made for future work.

Xenon Difluoride and the Nature of the Xenon-Fluorine Bond

Xenon reacts with fluorine to form XeF2 which can be isolated before it reacts with fluorine to form XeF4. The linear configuration of XeF2 with the 2.00-A bond length and the vibrational force constants support the assignment of 10 electrons to the valence shell of xenon. Similar arguments support the assignment of 12 and 14 valence electrons respectively to xenon in XeF4 and XeF6.

X‐RAY AND NEUTRON DIFFRACTION STUDIES OF MOLTEN ALKALI HALIDES

Eight molten alkali halide salts were studied by x-raydiffraction, and 3 of them also by neutron diffraction. Radial distribution functions clearly show the first 2 coordination shells, although not completely resolved. The first shell is a band of about 0.5 A half-width, with the most frequent interatomic distance being smaller than that in the solid at room temperature. It corresponds to an average coordination of about 4 to 5 neighboring ions. Neutron patterns confirm that the first coordination shell involves unlike ions. The second shell, which corresponds to 7 or more ion neighbors, is less well defined. This information, together with the density decrease on melting, indicates that the melts have quite open structures with a wide variety of individual ion coordinations. (auth)

XENON TETRAFLUORIDE MOLECULE AND ITS THERMAL MOTION: A NEUTRON DIFFRACTION STUDY

A neutron-diffraction analysis of the xenon tetrafluoride crystal structure has confirmed the square-planar symmetry of the molecule; from the data the average Xe to F bond length is 1.953 � (σ = .002) and the F to Xe to F angle is 90.0� (σ = 0.1). A description of the thermal displacements in the crystal is given.

Photoelectron spectroscopy of methyl, ethyl and isopropyl alcohol adsorbed on the (110) surface of Cu

Abstract Methyl, ethyl and isopropyl alcohol were allowed to adsorb on a clean (110) face of a copper crystal at room temperature, and photoelectron spectra were obtained using He(I) (21.2 eV) radiation. The spectra of the adsorbed species differed sufficiently from those of the free molecules to indicate a chemical change on adsorption. In accordance with studies in the literature using flash-desorption and energy-loss spectroscopy, it is suggested that the observed spectra are those of the corresponding alkoxy radicals formed as a result of dehydrogenation. In the case of the methoxy radical, calculations were carried out for a cluster of nine copper atoms and the results shown to be consistent with the experimental data. Results were also obtained on the rates of adsorption and desorption and on the dependence of the spectra on the polar angle between the normal to the surface and the photoelectron direction. Finally, results were obtained on the extent of adsorption by means of X-ray photoelectron spectroscopy.

X‐Ray Diffractometry with Slightly Absorbing Samples

A method for measuring x-ray diffraction intensities of samples which are deeply penetrated, such as liquid and nitreous materials, is described. The sensitive angle of the detector is limited by a scatter slit which is focused on exactly the same area of the sample surface as is illuminated by the primary beam. This scatter slit is placed between the receiving slit and the sample. The absorption correction for the arrangement is given. (C.J.G.)

X‐Ray Diffraction Study of Bismuth Polymer in Aqueous Perchlorate Solution

X‐ray diffraction study of an aqueous solution of bismuth perchlorate has established the presence of a polymeric species containing six bismuth atoms in octahedral arrangement. The bismuth‐bismuth distances are 3.70 and 5.23 A.

Structure of Silico‐Tungstic Acid in Aqueous Solution

The structure of 12‐silico‐tungstic acid, H4SiW12O40, in aqueous solution has been investigated by x‐ray diffraction. The results indicate that 12 tungsten atoms are contained in a monodisperse species with arrangement similar to that found in the crystalline acid. Each tungsten atom has two near tungsten neighbors at each of the distances 3.36 A and 3.71 A.

Angle-resolved polarization-dependent photoemission spectra of benzenethiol adsorbed on Cu(110)

Abstract Angle-resolved photoelectron spectra of benzenethiol chemisorbed on the Cu(110) face have been obtained with p - and s -polarized HeI (21.2 eV) radiation at room temperature. Comparison with the gas-phase spectrum and molecular-orbital correlation diagrams indicates that the benzenethiol bonds to the copper by way of the sulfur, rather than phenyl ring π-orbitals, and that the adsorption is dissociative, yielding a phenyl sulfide (mercaptide) surface species. An analysis of the polarization/angular dependence of the band intensities and comparison to symmetry-allowed transitions confirms that the molecule has the plane of the phenyl ring perpendicular to the surface. The assumed C 2v symmetry together with packing considerations further allows the orientation of the molecule and nature of the bonding to be suggested.

Photoelectron spectroscopy of sulfur heterocycles adsorbed onto a Cu(110) surface

Abstract Photoelectron spectra of Cu(110) after exposure to thiacyclobutane, thiacyclopentane, and thiacyclohexane, as well as of the clean Cu(110) surface, have been obtained using He(I) radiation. Comparison of the spectra of the absorbates with gas-phase spectra suggests a strong sulfur-to-copper dative bond. Sulfur lone-pair orbital stabilization energies were obtained for thiacyclobutane (1.4 eV), thiacyclopentane (1.1 eV), and thiacyclohexane (0.6 eV), and these stabilization energies are correlated with the dative sulfur-to-surface bond energies. Ring-strain changes are suggested as the reason for additional small shifts of absorbate-bond energies in the thiacyclobutane spectrum. Relaxation energies were found to decrease in the order 4- > 5- > 6-membered ring. An empirical linear relation was found between relaxation shifts and lone-pair orbital stabilization energies for monoheteroatom heterocycles bonded to Cu(110) by a lone-pair dative bond. All adsorbates studied showed marked angular dependence, suggesting nonrandom orientation on the surface. Al K α X-ray data were used to calculate percentage surface coverages. Other sulfur-containing adsorbates are discussed, including thiophene, allyl mercaptan, and thiacyclopropane. Thiacyclopropane was found to decompose upon exposure to copper.