C.-M. Liegener

Doubly charged valence states of formaldehyde, acetaldehyde, acetone, and formamide studied by means of photon excited Auger electron spectroscopy and ab initio calculations

The doubly ionized states in formaldehyde, acetaldehyde, acetone, and formamide have been studied by means of x‐ray excited core (C1s, N1s, O1s)–valence–valence Auger electron spectroscopy. Assignments of the spectra have been made using ab initio Hartree–Fock, Green’s function, and configuration interaction (CI) calculations. A molecular orbital analysis has been carried out for the high kinetic energy part of the spectra. The breakdown of the single particle picture is found to be important over a large energy interval in the spectra. The results obtained illustrate the usefulness of Auger electron spectroscopy in characterizing the doubly ionized states even in the case of large molecular systems. The first double ionization energies for the four molecules presented have been determined to be 33.8, 30.3, 28.0, and 30 eV, respectively.

Effects of topology on the electronic properties of fused-ring polymers

Abstract Large differences in the band gap and other electronic properties between polyacene, polyphenanthrene and polybenzanthracene are rationalized by topological arguments. Ab initio crystal orbital calculations are reported for the three systems.

Spectral density of the one-particle green's function for polymers

Abstract The spectral density of the one-particle Greens function in the valence-band region of trans-polyacetylene and polyethylene has been calculated. Semiquantitative agreement with experimental photoelectron spectral lineshapes can be achieved by second-order Moller-Plesset perturbation theory.

Ab initio cluster and band structure calculations on systems modeling La2CuO4. Effects of charge transfer between the different planes, Madelung potentials, doping and correlations

Abstract Based on an initio Hartree-Fock (HF) calculations both on the cluster and aand structure level, the electronic structure of La 2 CuO 4 (2-1-4) was investigated. As model systems [CuO 6 ] n − (cluster case) and [CuO 4 ] m − units (band structure case) were chosen, i.e. the “apical” oxygens were explicitly included. Surrounding ions were treated as point charges (cluster case) and via partially self-consistent Madelung potentials (band structure case), respectively. Using the HF approximation we found a too low density of states (DOS) around the Fermi level, while the charge distribution in the ground state seems to be reasonable. To obtain DOS curves comparable with the experimental ones, we had to go beyond the one-particle description by using Moller-Plesset many-body perturbation theory. A comparison with density functional (DF) calculations in the local density approximation (LDA) is given. Finally, we studied the effects of doping on the HF level. Here we found that upon doping holes of ω symmetry and predominant oxygen character are formed within the CuO 2 planes of 2-1-4. At the same time increasing covalent bonding between in-plane Cu and O was observed.

Theoretical design of polymers from topological arguments: electronic properties of polyisophenanthrene

Abstract Topological rules are discussed which describe the behaviour of the electronic properties of different fused-ring polymers. A new polymer, polyisophenanthrene (poly[3-4,4-3]phenanthrene), is suggested which should have a smaller gap than polyphenanthrene (poly[1-2,2-1]phenanthrene). Ab initio crystal orbital calculations are presented on polyisophenanthrene and related fused-ring polymers in order to check the topological predictions and to judge on topologically unrelated systems.

Electron spectroscopy of adsorbates via autoionization of core-tobound excited states: experiment and theory

Abstract We use autoionization Spectroscopy after core-to-bound excitation of adsorbed molecules to get information about the spectral distribution of valence electrons in adsorbates and about the nature of shake up satellite states. Examples are presented for a physisorbed system (CO 2 /Ni(110)), a weakly chemisorbed system (N 2 /Ni(110)) and a strongly chemisorbed system (CO/Ni(110)). It is shown that in the first case detailed calculations of the autolonization rates of the isolated molecule can basically explain the experimental findings. For N2/Ni(110) autoionization spectroscopy is shown to verify an assignment put forward for the photoemission spectrum, i.e. that satellites contribute heavily to the valence ionization of many systems. In the third example, the angular dependences of the autoionization lines are used to deduce Information on the symmetries of ion states.

Quasi-particle bands of a graphite monolayer

Abstract The quasi-particle band structure of a graphite monolayer has been calculated by a Greens function method in second order of Moller-Plesset perturbation theory. Rules for the use of symmetry in correlation calculations are given. The XPS spectrum has been calculated within the Gelius model and compared to the experimental spectrum of graphite.

Auger electron spectroscopic study of the CN anion: A study of the NaCN(001) surface in comparison with CO and N2

Abstract We have recorded the carbon and nitrogen KVV Auger spectra of NaCN, excited with X-rays. The spectra are interpreted on the basis of ab initio many-body calculations of the double hole state energies and intensities of CN−. The nitrogen (KVV) Auger spectrum of NaCN is more similar to the N(KVV) spectrum of N2 than the carbon (KVV) Auger spectrum of NaCN is to the C(KVV) spectrum of CO. This observation is well reproduced by the calculations. We conclude that the strong influence of the localization of the orbitals in CO on the Auger spectra has disappeared in CN−, although the delocalization is not as pronounced as in N2. On the basis of the calculations it is possible to assign satellite structure in the N(KVV) Auger spectrum of CN−, in comparison to the N(KVV) Auger spectrum of N2.

Doubly ionized states of carbon tetrafluoride

Abstract High resolution Auger-electron and double-charge-transfer spectroscopies have been used to investigate the doubly ionized states in CF4. The double-charge-transfer spectra show that both the lowest singlet and triplet states in the CF4 dication have an energy of 38.0±0.4 eV. Auger-electron spectroscopy gives information concerning higher energy states in the CF2+4 ion. Peaks have been measured in the double ionization energy range 38–80 eV and have been assigned to the population of dication states using theoretically calculated spectra. Recent data from CF2+4 fragmentation studies (Coding et al., J. Phys. B 24 (1991) 951) are discussed and correlated with the present work. The double-charge-transfer and Auger-electron spectra have been analyzed in terms of results obtained by two-particle Greens function calculations.

On the auger spectra of CH3F

Abstract Auger spectra of methyl fluoride have been calculated by a Greens function method. Explicit assignment of the most important features in both spectra have been made. A partitioning of the spectra into three parts involving two outer-valence, one outer-valence and one inner-valence, or two inner-valence orbitals seems not to be possible.