Lutz Ackermann

Molecular photoionization cross sections by the Lobatto technique. I. Valence photoionization

A method for the calculation of electronic continuum wave functions is presented which is based on the logarithmic derivative version of the Kohn (LDK) variational principle. The variational principle is cast into algebraic form by introducing a finite basis set that consists of spherical harmonic Gaussian‐type functions (GTOs) and of Lobatto shape functions with the latter representing the translational part of the basis. A local effective potential which is obtained from density functional theory results in fairly accurate photoionization cross sections. Also studied are asymptotic corrections to the effective potential for the photoelectron which, in many cases, lead to improved results. The Lobatto procedure is applied to the diatomics N2 and CO and to benzene which may be regarded as a prototype for larger non spherical symmetric systems for which the method is targeted. For the two diatomics, results in excellent agreement with experiment have been found. For benzene the results are compared to thos...

Electronic structure and properties of nickel clusters: Ni6, Ni8, Ni19, and Ni44

Abstract All-electron, spin-polarized, LCGTO-LDF calculations have been performed on Ni clusters with face-centered-cubic (and simple cubic) geometry. The results are discussed in relation to theoretical and experimental data for gas-phase Ni clusters and for the extended metal. We analyze bonding, magnetic behaviour, cohesive energy, bond distances, ionization potentials, and DOS profiles, paying particular attention to the role of the Ni 4s and 3d electrons.

Paramagnetism of high nuclearity metal cluster compounds as derived from local density functional calculations

The electronic structure of high nuclearity carbonylated Ni clusters [Ni32C6(CO)32]n− and [Ni44(CO)48]n− (n=0–6) has been investigated by means of all‐electron calculations within the local density functional approach. The transition from the molecular to the metallic state was studied by determining the magnetic properties of bare and carbonylated clusters. The appearance of the metallic magnetic behavior is connected with the presence of metal atoms with bulklike coordination. The effect of interstitial impurity atoms in quenching the magnetic moment is also discussed.

Cyanogen on Ni(110): an experimental and theoretical study

Abstract Adsorption of cyanogen, C 2 N 2 , on a Ni(110) surface has been investigated by combining low energy electron diffraction, thermal desorption and angular resolved ultra-violet photoemission experiments as well as model cluster calculations using the linear combination of Gaussian-type orbitals local density functional method as well as a force field approach. Results of mirror plane photoemission experiments on the ordered c(2 × 2) monolayer could be rationalized by invoking adsorbates bonded to the surface via the π electrons, with their axis oriented along the [001] direction, across the grooves of the (110) surface, leading to an assignment of all six adsorbate-derived valence orbitals. However, unlike in a previous study on the analogous chemisorption system Pd(110)/C 2 N 2 , only one mirror plane was detected. A possible tilt of the adsorbates in the (110) plane was related to crowding on the closer spaced nickel surface by estimating the lateral interaction within the adsorption layer using force field models. Electronic structure calculations on various chemisorption model clusters confirmed the experimental orbital assignment and the orientation of the adsorbate axis essentially along the [001] direction. Best agreement with UPS data was found for the orbital splitting pattern of an adsorption geometry where the C-N groups bind sideways on-top of nickel atoms in the first crystal plane. On the other hand, bonding along in the troughs of the (110) surface leads to a distinctly different ordering of the valence orbitals, at variance with the experimental assignment.

On the coadsorption of CO and alkali atoms at transition metal surfaces: A LCGTO-LDF cluster study

Abstract The coadsorption system consisting of carbon monoxide and alkali atoms on transition metal surfaces has been studied theoretically by first principles electronic structure calculations. Special attention has been paid to the reduction of the CO stretching frequency in the presence of coadsorbed alkali atoms, an effect that indicates a weakening of the CO bond. To investigate CO coadsorbed with alkali atoms, a hierarchy of models has been constructed based on the clusters CO, Ni 2 CO and Ni n ( CO ) K 2 ( n = 8, 14). These models permit the separate and joint study of several interaction mechanisms and the evaluation of their relative contributions. In the smaller clusters, the electric field of the surface dipole layer is modeled by point charges. The electronic structure calculations have been carried out using the self-consistent linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. The calculated values for the reduction of the CO stretching frequency and the shifts of core and valence levels of CO and alkali atoms are in good agreement with experimental data. A comprehensive model for the CO/alkali coadsorption on transition metal surfaces emerges which allows the explanation of a variety of experimental findings. This model is corroborated by a detailed analysis of the electronic structure of the coadsorption system. In quantitative agreement with experimental data, about half of the reduction of the CO vibrational frequency has to be attributed to substrate-induced backbonding into the antibonding 2π ∗ orbital of CO as first suggested by Blyholder. The alkali-induced additional frequency shift is dominated by the electrostatic interaction between CO and the surface dipole layer which is modified by alkali coadsorbates. To a smaller extent, this frequency shift is also affected by enhanced backbonding due to a raised Fermi energy of the substrate. Direct orbital interactions between the coadsorbates were found to be negligible. Ionic models or rehybridization models are not supported by the present study.

Chemisorption of sulfur on nickel: A study of cluster convergence in the linear combination of Gaussian‐type orbitals local density functional approach

Chemisorption of sulfur at the (100), (110), and (111) surface of nickel has been studied, using the linear combination of Gaussian‐type orbitals local density functional (LCGTO‐LDF) method. Employing various cluster models consisting of 11 to 29 substrate atoms, adsorption at the experimentally known sites has been considered. Besides the equilibrium distance and the force constant of the vertical adsorbate motion, the dynamical dipole moment was evaluated and it turned out to provide a sensitive probe of cluster convergence. The influence of atoms from the third substrate layer on the various observables is found to be considerable in some cases. With increasing cluster size, bond lengths are stabilized to 0.02 A, frequencies to 20 cm−1, and dipole moments to 0.1 D. The converged results agree very well with experiment. Adsorption induced population changes are restricted to only four to six neighboring substrate atoms of the modifier atom. However, energy resolved charge density differences reveal a po...

The adsorption of sulfur on nickel: an electron energy loss spectroscopy investigation of Ni(110)/c(2×2)S and a LCGTO-LDF cluster model study

Abstract A c(2×2) sulfur overlayer on Ni(110), produced by adsorption and thermal dissociation of H 2 S has been investigated with vibrational electron energy loss spectroscopy (EELS) and a feature at 331 cm −1 is assigned as a nickel-sulfur stretch. Also, cluster models of sulfur adsorption at Ni(001), Ni(110) and Ni(111) have been studied using the linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. The equilibrium bond distances and the absorbate vibrational stretching frequencies are found to be in good agreement with experiment, confirming the experimental assignment of the Ni(110)/S vibrational data.

Cyanogen on Ni(110): a model cluster study using the LCGTO-LDF method

Abstract The linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method has been applied to cluster models of C2N2 absorbed on Ni(110). Various chemisorption sites and orientations of the molecule have been probed, optimizing the adsorbate geometry for each of them. Vibrational frequencies related to the corresponding internal modes have been determined. The electronic structure of the adsorbate is analyzed in terms of the 1πg and 1πu level splitting and compared to ARUPS measurements. From these results, preference is given to adsorption complexes with the cyanogen molecule adsorbed on top of nickel atoms of the first crystal plane with the molecular axis parallel to the surface, but orthogonal to the troughs of the surface.