Ulrik Gelius

Molecular Spectroscopy by Means of ESCA II. Sulfur compounds. Correlation of electron binding energy with structure

Electron spectra from an extensive series of sulfur compounds have been studied. A correlation has been established between the observed position of inner electron lines of sulfur and structure. The influence of structure on the electron binding energies is discussed in terms of a calculated atomic charge, based on the concepts of electronegativity and partial ionic character of bonds. The results are useful for the study of bonding and structure in sulfur chemistry, and are applied particularly to the discussion of the sulfur-oxygen bond (S=O).

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.

Recent progress in ESCA studies of gases

Abstract The development during the most recent years of ESCA applied to free molecules is reviewed. The advantages of studying, when possible, a substance in t

A new ESCA instrument with improved surface sensitivity, fast imaging properties and excellent energy resolution

Results from the first experiments with a new ESCA instrument with monochromatic X-ray excitation are presented. The measurements were selected to assess the performance of the instrument in terms of energy and spatial resolution, information rate and surface sensitivity, and to probe the new dimension added to XPS analysis with the introduction of an imaging system. Novel design features described in the paper include a high-power monochromatic X-ray source and an electrostatic lens system permitting either large luminosity or high spatial resolution. A new geometry allows excellent access to the sample area and gives maximum surface sensitivity at glancing angles. The best energy resolution achieved so far with the instrument has been evaluated from the Fermi edge of silver to 0.27 eV (FWHM). The FWHM of the Ag 3d52 line at this resolution is 0.44 eV. At an instrument energy resolution of 0.37 eV (FWHM) a peak intensity of the Ag 3d52 line corresponding to a single-channel detector count rate of 1.9*106 cps has been measured. At this resolution, high quality spectra from surface elements of 30 μ × 30 μm can be recorded in a few minutes. The spatial resolution (20%–80%) has been measured to 23 μm. The enhanced surface signal at glancing angles is demonstrated in a series of measurements of the angular dependence of the XPS spectrum from a silicon surface. Preliminary experiments using an image integrating system show the potential of the combination of high spatial resolution in XPS with such techniques to give new information on the distribution of chemical properties or electrostatic potential variations over a surface. Future developments are discussed, including parallel recording of spectra from a large number of small spots, advanced handling of information from an imaging XPS system and further improvements of the spatial resolution.

Binding Energies and Chemical Shifts in ESCA

A comprehensive account is presented of the theory of electron binding energies, chemical shifts, and related phenomena in electron spectroscopy. The potential model at various levels of approximation is examined and a few other models for the chemical shift in ESCA, such as the thermodynamic model based on the equivalent core approximation, are discussed. Other phenomena dealt with in this survey are, the reorganization energy following core ionization, multiplett splittings in paramagnetic substances, and electron shake-up and shake-off. Finally, the theoretical conditions for correlating chemical shifts in ESCA with chemical shifts in NMR and Mossbauer spectroscopy are considered.

Vibrational and lifetime line broadenings in ESCA

Abstract The line profile of the narrow, symmetric 1s line from neon, recorded with the new ESCA instrument with X-ray monochromatization, is analyzed. The natural linewidth of this line is found to be 0.23 ± 0.02 eV, in good agreement with theoretical calculations of the oscillator strengths for Auger transitions and X-ray emission. Spectra from molecules show frequently asymmetric core electron lines under high resolution. This rules out previous explanations based on a chemical influence on the natural lifetime. Contrary to earlier assumptions, vibrational excitations are shown to be important in core electron spectra. For methane, the vibrational energy spacing is large enough to allow the vibrational lines to be partly resolved. Recent results from accurate PNO CI calculations on methane agree well with the experimental findings. The Franck-Condon transitions in the C1s and N1s lines from CO and N2 are shown to be well described in the harmonic approximation and approximating the potential curves of the highly excited core hole states with the potential curve for the ground state of NO+, X1 Σ+. Knowledge of vibrational excitations in core electron spectra is shown to be valuable in the analysis of high resolution X-ray emission spectra of free molecules.

The ESCA Spectra of Benzene and the Iso-electronic Series, Thiophene, Pyrrole and Furan

The ESCA spectra of C6H6, C4H4S, C4H5N and C4H4O have been studied using MgKα radiation, 1253.6 eV. The study included both the core orbitals and the valence orbitals. The C1s chemical shifts have been compared with the shifts predicted by the potential model, using both CNDO and ab initio gross atomic charges. We have obtained the binding energies of the deeper lying valence orbitals which cannot be reached by ultraviolet radiation. Further, the valence orbital spectra have been analysed using line intensities predicted from the atomic population of the molecular orbitals.

Valence Bands and Core Levels of the Isoelectronic Series LiF, BeO, BN, and Graphite Studied by ESCA

The core lines and valence bands of LiF, BeO, BN and graphite have been studied by the ESCA technique. The energy differences between inner levels and valence bands are compared with X-ray transition energies. The changes in binding energy for the Bels level when going from metal to oxide and fluoride are compared with X-ray spectroscopic data and with a study of the disintegration constant in electron capture of 7Be in the same compounds.

Theory of band shape formation in Auger and autoionization spectra of molecules. Numerical applications and new high‐resolution spectra for CO

Theory of band shape formation in Auger and autoionization decay in molecules is presented and numerically applied to spectra of the CO molecule. In particular, the role of lifetime vibrational interference in the various vibronic decay channels is elucidated. New high resolution Auger and autoionization spectra are presented. The calculated band shapes are compared with the experimental recordings. The vibronic spectra of the intermediate states in the Auger and autoionization decay are analyzed and compared with available ESCA and EELS spectra, respectively. The role of the electron correlation on the formation of the core state potential surfaces is investigated in that connection. The assignment problem of the outer part of the CO Auger spectrum is reinvestigated and the effect of interference due to hole mixing in the wave functions is pointed out. The time dependent Franck–Condon (FC) formalism used to derive the lifetime vibrational interference is shown to have a bearing on the analogy of the sudd...

ESCA studies of CO2, CS2 and COS

Abstract The core level electron spectra of CO 2 , CS 2 and COS excited by Mg Kα radiation have been studied to identify shake-up satellite lines associated with ionization from these levels. A number of such lines have been seen and possible assignments have been suggested using the excited states of the molecule as a guide. The valence spectra have also been recorded and they too were found to be rich in shake-up structure. The observed variation of the valence line intensities is discussed and compared with predictions made from an intensity model. The validity of distinguishing between π and σ symmetries in linear molecules in applying the intensity model is confirmed.