NEAR EDGE CORE EXCITATION SPECTRA OF COMPLEX MOLECULES

Recent results illustrating current themes in the interpretation of the inner shell excitation spectra (NEXAFS) of complex molecules are presented. Results discussed include : A) the Cls, Ols and Fls spectra of bis (trifluoromethyl) peroxide - illustrating orbital mapping ; B) the Cls spectra of cyclopentene in gas, solid and monolayer on Pt(111) phases - illustrating the synergy of interphase comparison ; C) the Cls spectra of gaseous cyclopropane, propane, cyclobutane and butane - illustrating effects of ring strain ; and D) the Cls, S2p and Sls spectra of thiophene and thiolane, examining the bond length sensitivity of σ*(C-S) resonances

CARBON AND OXYGEN K-SHELL EXAFS OF GASES STUDIED BY ELECTRON ENERGY LOSS SPECTROSCOPY

EXAFS in the soft X-ray range from intramolecular, low Z backscattering has been investigated in the inner shell spectra of gaseous CO, CO2, CF4, c-C6H12, C6H6 and C6F6, recorded by inelastic electron scattering in the dipole regime. The 01s continua of CO and CO2 do not exhibit the oscillations recently reported by Yang et al (Phys. Lett A110 (1985) 301). The Cls spectrum of CF4 contains extended continuum features similar to those reported previously. The Cls continua of c-C6H12, C6H6 and C6F6 exhibit weak extended continuum features which appear to be EXAFS although the analysis procedures used so far are not able to reproducibly extract accurate bond lengths

ORIENTATION AND TEMPERATURE DEPENDENCE OF THE Cd K-SHELL EXAFS OF SINGLE CRYSTAL Cd METAL : FIRST EXAFS OBSERVATION OF R2

The K-shell X-ray absorption spectrum of a Cd single crystal has been measured at temperatures from 4 to 300K with the crystal in two orientations - one optimizing EXAFS from the first Cd distance (R1 = 2.97Å), the other optimizing EXAFS from the second Cd distance (R2 = 3.29Å). Clear evidence is obtained for R2 although its visibility varies greatly with temperature on account of the large anisotropy in the thermal motion. The Debye-Waller factors for the thermal motion along R1 and R2 are derived and compared to theory

Wind loading codes: International users' perspectives

Most buildings are designed using wind loading codes rather than wind tunnel testing. This paper discusses some of the experiences of using a range of international codes in an international design practice. A comparison of wind loads predicted by a range of wind loading codes for two building types, a city centre tall building and an isolated low-rise building with a long-span roof, are then presented

Fluorination Effects on the Inner-Shell Spectra of Unsaturated Molecules

The applicability of the perfluoro effect to the X-ray spectra (300–800 eV) of unsaturated organic molecules is explored. The C1s and F1s (and Ols where appropriate) oscillator strength spectra of five fluoroethylenes, octafluorocyclopentene, formyi fluoride, carbonyl fluoride, hexa-fluorobutadiene, trifluoroacetic acid, hexafluorobutyne-2, hexafluoroacetone, and octafluorona-phthalene were derived from electron impact energy loss spectra recorded under electric-dipole scattering conditions. These spectra are analyzed and compared with those of their perhydro analogs, several of which (naphthalene, acetic acid, butyne-2) are reported for the first time. In unsaturated systems in which all the atoms lie in the molecular plane, such as ethylene, formaldehyde, benzene, etc., perfluorination results in approximately 10 eV shifts of the inner-shell energy loss spectra to higher energies, yet the term values for the C1s→1π* excitations are shifted by only 1 eV and often less. In direct contrast, the term values for the equivalent C1s→1π* excitations in unsaturated systems having atoms out of the molecular plane, such as butene-2 and acetone, are shifted upward by up to 3 eV upon perfluorination. These different spectral behaviors of planar and nonplanar systems on fluorination quantitatively parallel those which were observed earlier for valence-level ionization potentials (10–20 eV) and attributed to the perfluoro effect. It is observed for the first time that the C1s→1π* excitation energies in planar hydrocarbons are only very weakly dependent on the spatial extent of the π-electron system. An explanation involving a localized C1s hole is proposed to rationalize this behavior. The perfluoro effect also predicts that excitations to σ* MO's will become relatively low-lying in highly fluorinated planar systems. Such low-lying inner-shell excitations induced by fluorination are observed in the fluoroethylene series and in the fluorocarbonyls. When the negative-ion spectra of the fluoroethylenes are assigned in a self-consistent manner, a σ* MO is found to drop into the vicinity of 1π* upon fluorination. A similar intrusion of the lowest σ MO among the π* MO's is also observed upon fluorinating benzene, while evidence for this in the case of naphthalene is less clear, on account of the complex pattern of multiple C1s→nπ* transitions in this molecule. Inner-shell oscillator strength distributions are reported for all the spectra considered herein. In general, perfluorination increases the oscillator strengths of C1s→1π* transitions by up to a factor of two. Variation of the C1s→1π* and O1s→17π* oscillator strengths in the series H2CO, HFCO, F2CO shows clearly how the 17π* MO becomes more polarized toward C as fluorination proceeds. In some cases, C1s→σ* (C-F) oscillator strengths exceed those for C1s→1π* transitions

Absolute Oscillator Strengths From the K-Shell Spectra of Fluoroethenes and 1,3-perfluorobutadiene

Absolute oscillator strengths in the region of carbon and fluorine K-shell excitation have been derived for CH2CH2, CH2CHF, cis-CHFCHF, CH2CF2, CHFCF2, CF2CF2, 1,3-C4H6, and 1,3-C4F6 from electron-energy-loss spectra recorded under dipole-dominated conditions. The methods used to derive absolute oscillator strengths from relative energy-loss intensities are discussed in detail. The accuracy of the procedures is tested through comparisons with literature results for N2, CO, and CO2. The total C 1s→π* and C 1s→σ*(C—F) intensities increase systematically as the degree of fluorination increases. The spectra are discussed in terms of bond-length correlation and potential barrier concepts

Structures of selected boranes and carboranes

Nido-pentaborane(9) (B5H9), 1, nido-2,3-diethyl-2,3-dicarbahexaborane(8)[( C2H5)2C2B4H6], 2, nido-decaborane (14) (B10H14), 3, closo- 1,2-dicarbadodecaborane(12) (H2C2B10H10), 4, can be used as possible source compounds for boron and boron carbide thin film deposition. Inner shell electron energy-loss spectroscopy (ISEELS) studies of the boron Is and carbon Is core excitations of gas phase species have been undertaken so as to characterize these molecular precursors at solid surfaces. The near edge structure of ISEELS provides a good fingerprint for the identification of these molecular species. A comparison is made between calculated [modified neglect of differential overlap (MNDO)] bond lengths for molecular clusters and the x-ray or electron diffraction bond lengths, bond lengths determined from extended energy-loss fine structure