Outer Valence Region Research Articles

Broken symmetry effects in the He(I) valence photoelectron spectrum of Se(CN)2

Results of quantum chemical calculations which take account of the full molecular symmetry are often at variance with the experimentally preferred assignments of the valence He(I) photoelectron spectra of molecules containing two or more symmetry-related cyano groups. It is demonstrated how on the basis of Hartree-Fock-Slater calculations the broken symmetry concept removes previous discrepancies in the assignment of the Se(CN)2 spectrum. Even in the outer valence region breaking the symmetry may have appreciable effects in the case of strongly localized orbitals.

Hole-mixing effects in the ionization of some unsaturated oxo-compounds

The He(II) photoelectron spectra of acrolein, propynal, propynol and propiolic acid are interpreted with the aid of ab initio many-body Green's function calculations in the outer and inner valence region. Koopmans' approximation fails to supply the correct ordering of electronic states in the radical cation if the lone pair orbital on the oxygen atom is of a′ symmetry. In the case of propynal the ordering of two ionic states o f equal symmetry is interchanged with respect to the Koopmans' orderin. A strong mixing of hole configurations occurs for outer valence states of propynol and propiolic acid. The implications of this mixing are briefly discussed.

The electronic structure and the photoelectron spectrum of (tetramethylallene) iron tetracarbonyl

The vertical ionization potentials of (tetramethylallene) iron tetracarbonyl (1) are investigated by means of many-body perturbation theory in the framework of a semiempirical INDO Hamiltonian. The ionization energies in the outer valence region are reproduced with high accuracy by the Green's function approach. The PE bands are traced back to molecular orbitals that are strongly localized at the 3d center or the result from bonding and antibonding linear combinations between fragment orbitals of Fe(CO) 4 and the allene ligand. The ground state properties of 1 are analyzed on the basis of the INDO results and are compared with MO properties of the Fe(CO) 4 ethylene complex 2.

∆SCF and Transition Operator (TO) Calculations in the Theoretical Determination of Vertical Ionization Energies in Transition Metal Compounds

The interrelation between theoretically determined vertical ionization potentials in transition metal compounds based on the ∆SCF approximation (Iv,j ∆SCF) and based on the “transition operator (TO)” method (Iv,j TO) has been investigated. Numerical results in the outer valence region have been obtained by means of semiempirical INDO calculations. The difference between the calculated ionization energies (Iv,j ∆SCF and Iv,j TO) is enlarged with increasing atomic numbers of the transition metal center due to noncompensating terms in third order of perturbation. The neces­sary conditions for the imbalance of both theoretical procedures are large two-electron interaction integrals and highly populated transition metal hole-states. Both requirements are enhanced by going from the left side of the 3d series to the extreme right. For d4-d6 complexes a simplified expression for Iv,j ∆SCF has been derived containing only one third order element.

A Green's function approach to the photoelectron spectrum of bis(?-allyl)nickel[1

The vertical ionization potentials of bis(π-allyl)nickel (see (1) in Fig. 1) are calculated by means of the Green's function approach within a semiempirical INDO extension to the first transition metal series. The computed ionization potentials are in good agreement with an experimentally deduced assignment. In contrast to earlier theoretical and experimental studies, the 7a u (π) level is predicted on top of the levels corresponding to the Ni 3d orbitals. Our approach leads to a complete assignment of the PE spectrum of (1) in the outer valence region.

Strong correlation effects in the ionisation of CS 2

The Hell photoelectron spectrum of CS 2 is presented. This spectrum is investigated by two different Green's function calculations. For the outer valence region the origin and assignment of satellite lines of significant intensity is clarified. Strong final state correlation effects are found for the inner valence region showing the so-called breakdown of single-particle picture of ionisation.

Correlation effects in the ionization of hydrocarbons

The spectral intensity for ionization as a function of binding energy for the valence electrons of ethylene, allene, butatriene, trans-butadiene, acetylene, benzene, methane, ethane, and cyclopropane is computed by a many-body Green’s function method. The results are used to interpret unidentified structures in experimental ionization spectra. For the ionization out of the inner valence orbitals of the unsaturated molecules the spectral intensity is found to be distributed over several lines, in sharp contrast to the ionization out of the inner valence orbitals of the saturated molecules where the greater part of the intensity appears in one main line. The reasons for this behavior are discussed. It is also found that there is a correspondence between the behavior of the spectral intensity in the inner valence region and the satellite structure in the outer valence region. For C6H6, C4H4, and C4H6 interesting satellite lines of considerable intensity are predicted to be situated in the outer valence region accessible with Hei radiation.