K. Siegbahn

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.

Electron Spectroscopy of Open‐Shell Systems: Spectra of Ni(C5H5)2, Fe(C5H5)2, Mn(C5H5)2, and Cr(C5H5)2

The high resolution HeI electron spectra of Ni(C5H5)2, Fe(C5H5)2, Mn(C5H5)2, and Cr(C5H5)2 have been recorded and analyzed in terms of a molecular orbital description of the electronic structure. The ground state electronic configurations have been assigned by considering the feasible ground state configurations, determining the number and type of ionic states obtained from ionization of these configurations, and then comparing the predicted transitions with those observed experimentally. The ground state configuration and adiabatic first ionization potential of these molecules are: Cr(C5H5)2, ··· (e2g)3(a1g)1, 3E2g, I.P.=5.50 eV; Mn(C5H5)2, ··· (e2g)4 (a1g)1, 2A1g, I.P.=6.55 eV; Fe(C5H5)2, ··· (a1g)2 (e2g)4, 1A1g, I.P.=6.72; Ni(C5H5)2, ··· (a1g)2 (e2g)4 (e1g)2, 3A2g, I.P.=6.2 eV. Vibrational structure has been observed in the spectrum of ferrocene and is assigned to progressions in ν4, the symmetric ring‐metal stretching mode.

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.

Vibrational and Vibronic Structure in the Valence Electron Spectra of CH3X Molecules (X=F, Cl, Br, I, OH)

Valence electron spectra of the methyl halides and of methyl alcohol have been induced by means of the HeI resonance radiation at 21.22 eV. A large number of new vibrational bands are reported. The observation of these was facilitated by a high instrumental resolving power and fast data acquisition. In most cases the new bands are observed in 1A1?2E transitions but also the electron band of the 1A1?2A1 transition of CH3I exhibits extensive progressions of closely spaced vibrational bands not previously reported. The vibrational structure of the 1A1?2E transitions is affected by vibronic coupling. The structure of the 2e electron band of CH3Cl probably reflects the operation of the Ham effect. Calculations of the band structure including spin-orbit coupling are performed according to a model which is briefly described.

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.

ESCA applied to liquids

Abstract The conditions for applying ESCA on liquid samples are discussed. A “liquid beam” technique is developed which meets the various requirements. The first spectrum of a liquid is presented, namely from formamide. It is shown that one can adjust the liquid beam so that a complete separation between the ESCA signals from the liquid and the vapour is achieved. The binding energies of the core levels of the elements in the molecule differ from each other in the liquid and the vapour phase by about 1.6 eV. One can also obtain electron spectra from dissolved chemical species. This is illustrated by potassium iodide dissolved in formamide.

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.

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.

Analysis of vibrational structure and Jahn‐Teller effects in the electron spectrum of ammonia

The electron spectra of the 2A1←1A1 and the 2E←1A1 transitions of ammonia have been measured with sufficient resolution to observe considerable fine structure. The first transition, 2A1←1A1, contains two vibrational progressions which are assigned to the ν2 bending mode. The second transition 2E←1A1, consists of two overlapping electron bands due to Jahn‐Teller splitting of the 2E state. The vibronic structure accompanying this transition has been partially resolved and compared to the results of model calculations.