J. Hedman

Molecular Spectroscopy by Means of ESCA III. Carbon compounds

Carbon 1s energies are measured by ESCA for a series of aliphatic saturated compounds, carbonyl compounds, and some aromatic compounds. For convenient use in chemical structure analysis the binding energy shifts are correlated with a charge parameter obtained from electronegativity considerations. The shifts are also analyzed in terms of group shifts from which group electronegativities are derived. A comparison is made between the shifts in solid and gaseous samples and it is shown that solid state effects are small for non-ionic compounds. The observed shifts are then compared with results of semi-empirical and ab initio molecular orbital calculations on free molecules. The theoretical calculations are simplified by use of an electrostatic potential model.

Calibration of electron spectra

Abstract Energy calibration procedures, which have been used in ESCA, are reviewed. Binding energies of calibration lines suitable for solid and for gaseous samples have been determined. Charging effects of nonconducting samples have been investigated for different X-radiation, sample thickness and material. A calibration relative to lines from some insulating standard compound mixed into the sample was shown to be unreliable, while experiments indicated that in calibrating relative to a surface layer, the carbon line from the hydrocarbon contamination layer can be used as well as a line from a thick layer of a noble metal on a part of the sample.

Band Structure of Transition Metals Studied by ESCA

The position and shape of the energy bands of the following transition metals have been studied by ESCA: Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au. The Fermi levels of the metals with unfilled d-bands are found in the high-energy flanks of the valence band spectra. For the noble metals the Fermi level is shifted toward higher energies. These observations are in general accordance with the overlap of d- and sp-bands in transition metals. An increase in band width is noted between corresponding elements in each series of transition metals. A comparison is drawn between band widths obtained in the present study and those deduced from cohesive energy data. An observed splitting in some of the bands seems to be much too large to be attributable to spin-orbit interaction. Core electron lines are recorded for the purpose of obtaining an energy calibration, estimating the contribution to the observed band spectra from inelastically scattered electrons, and checking the chemical state of the sample. The photoexcitation process and the energy losses of the electrons due to single-particle and plasmon excitations are discussed.

Escape Depths of X-ray Excited Electrons

The escape depth of X-ray induced photoelectrons and Auger electrons has been measured for energies between 1 keV and 4 keV in metallic gold and aluminum oxide. The escape depth was found to be proportional to the square root of the electron energy and ranged from 19 ? (at 0.9 keV) to 37 ? (at 3.2 keV) in gold and from 13 ? (at 1.4 keV) to 22 ? (at 3.9 keV) in aluminum oxide.

Electron escape depth in silicon

Abstract The ESCA electron escape depth in silicon is determined from the peak areas in the electron spectra from evaporated thin films. For electron energies in the region 320 eV to 3.6 keV values from 13 to 83 A are obtained. The escape depth in silicon dioxide is determined for the energies 1.6 and 3.6 keV. Binding energies and Auger energies are determined in silicon and silicon oxides.

Determination of the electron escape depth in gold by means of ESCA

Abstract The intensity of core electrons photoexcited by soft X-ray has been determined as a function of the thickness of deposited metal films. An escape depth of (22 ± 4) A is obtained for 1.2 keV electrons in gold. The possible impact of this result on band structure studies is discussed.

Gallium Nitride Studied by Electron Spectroscopy

The electron structure of hexagonal GaN has been investigated by means of ESCA. In particular the valence region has been studied. Comparisons with existing pseudopotential band structures are made. The value of an ionicity parameter for GaN is derived from the electron spectrum.

Ethene and the Chloroethenes Studied by ESCA

ESCA studies have been made of core and valence electrons in ethene and all the chloroethenes, using an instrument incorporating X-ray monochromatization. The orbitals are assigned according to calculated intensities and Extended Huckel type (EHT) calculations, as well as previously available data. Intensities are calculated from a model based on net atomic EHT populations and are found to agree well with experimental results. Also, CNDO calculations were performed.

The Electronic Structure of Some Palladium Alloys Studied by ESCA and X-ray Spectroscopy

The alloys Cu.60Pd.40, Rh.60Pd.40, Ag.71Pd.29 and Au.45Pd.55 have been studied with ESCA and soft X-ray spectroscopy. ESCA spectra from the energy bands and some core levels in the alloys were recorded and compared with spectra from the pure elements. X-ray emission bands have been recorded for the components of the alloys and compared with ESCA valence band spectra. Core electrons of copper in the alloy Cu.60Pd.40 were found to have 0.8 eV lower binding energy than in copper metal. The core levels of the other elements had either very small or nondetectable shifts. No differences between the spectra from f.c.c. and b.c.c. Cu.60Pd.40 were found.

Energy splitting of core electron levels in paramagnetic molecules

Abstract ESCA spectra of nitrogen oxides and oxygen have shown a splitting of core electron levels due to interaction between the emitted core electron and the unpaired valence electrons. No such effect is observed in the nitrogen molecule which is diamagnetic.