N. V. Dobrodey

Charge transfer effects in molecule–negative ion complexes induced by core ionization

A variety of charge transfer effects can take place as a result of core ionization of anion–molecule complexes. As specific examples we study the core ionization of Na−H2O and Cl−H2O clusters using an ab initio Green’s function method. The site-localized character of the electron distribution in the ground state of these weakly bound clusters favors a transfer of an electron from the negative atomic ion to the H2O molecular unit upon ionization of the O1s core level. The charge transferred screens the created core hole giving rise to the appearance of low-lying satellites which are completely absent in the spectrum of the isolated H2O molecule. Energies and intensities of the charge-transfer satellites are found to depend strongly on the chemical type of the atomic anion. While the bandshape of the O1s−1 spectrum of Cl−H2O is very similar to that of isolated H2O, the spectrum of Na−H2O has virtually nothing to do with the core-ionization spectrum of the water molecule. Interestingly, the charge-transfer s...

Charge transfer in the Cl−CO cluster induced by core ionization

Ab initio calculations of core-ionization spectra of the anion-molecule Cl-CO cluster are performed. Particular attention is paid to the investigation of charge-transfer screening processes accompanying core ionization of the CO molecule in the cluster. The charge-transfer processes are very efficient and favored by the presence of a low-lying unoccupied pi* orbital in CO capable of accepting an electron from Cl-. The O1s(-1) and C1s(-1) core-ionization spectra are calculated and compared. Both reveal a breakdown of the quasiparticle picture of core ionization caused by the charge-transfer processes. Remarkable differences between these two spectra are found which manifest themselves in distinct intensity distributions in the prominent low-energy spectral bands. The underlying reason for these differences is elucidated and linked with the preference of the pi* orbital to localize mainly on carbon. Core-ionization spectra of anion-molecule clusters are very sensitive to the type of the molecule involved as the comparative analysis of the O1s(-1) core-ionization spectra of the Cl-CO and Cl-H(2)O clusters show.

Foreign and native coordination effects in core-level spectra of mixed Be-Mg clusters

The core-ionization spectra of mixed Be-Mg clusters are computed by accurate ab initio methods. They are found to exhibit a variety of strong and unexpected effects originating from the presence of neighboring atoms of the same (native) or the other (foreign) element. The spectra reveal unusually low-lying intense satellites and the band shapes are sensitive to the local environment of the core-ionized atom. Foreign and native screening processes compete with each other resulting in specific spectral features which uniquely characterize the clusters.

Interatomic response to core ionization of atomic clusters

Abstract Fourth-order Green function calculations on very many core-ionized states of Mg n clusters ( n =2,3,4) are reported. Among several interesting features the results reveal unusually strong low-lying satellites in the core ionization spectra of the clusters. These are absent in the respective atomic spectra and not observed in the spectra of typical molecules. A thorough analysis attributes these satellites to distinct interatomic processes. These processes comprise charge and energy transfer from one atom to neighboring atoms. Other cluster systems are briefly addressed.

Strong charge-transfer effects in the Mg 2 p − 1 core-level spectrum of MgB 2

The two available Mg 2p - 1 core-level spectra of the recently discovered high-temperature superconducting MgB 2 crystal exhibit interesting structures, but contradict each other. This motivated us to perform ah initio calculations on the core-level spectra of cluster models. The computed spectra reveal unusual rich and intense structures triggered by a B 2p z →Mg 3s,3p charge transfer.

Equivalent core model: Extended theory and applications

The ability of the recently developed corrected equivalent core model (cECM) to predict properties of core hole states is examined for the CO molecule. It is shown that systematic corrections derived in this approach significantly improve the results of the conventional equivalent core model (ECM). This opens new possibilities to apply the equivalent core formalism to calculate the energy of core hole states which cannot be usefully done by the ECM itself. On the self-consistent-field level the predictions of geometry changes upon core ionization made by the cECM and those of direct core hole calculations are found to be very similar to each other. There exists, however, an appreciable difference between the total energies of the core hole states obtained in both approaches. A new procedure enabling us to improve the results of the cECM, in particular, to reduce this energy difference is proposed. In contrast to the cECM, where the corrections to the ECM energy are found by deriving the Hamiltonian HZ of ...

Core-hole Hamiltonians and corrected equivalent core model for systems with equivalent atoms

Core ionization in systems with several equivalent atoms gives rise to a set of near-degenerate core-hole states each associated with the removal of an electron from one of the delocalized orbitals. The energy splitting between the core-hole states is the intrinsic feature of core ionized systems and should not be neglected. The conventional equivalent core model (ECM) predicts strictly degenerate core-hole states because the core-hole is thought of as completely localized on one of the equivalent centers. This failure as well as several others inherent to the ECM are successfully removed in the framework of the corrected ECM presented in this paper. Two approaches are available in accordance with the two representations of the core-hole, either delocalized or localized. The Z+1 approximation is an excellent starting point in the localized representation. It must be modified, however, when the delocalized picture is used. To this end we introduce a (Z+Q) system where Q is the magnitude of point charges ad...

Strong dynamical screening in weak chemisorption systems

Greens function calculations have been carried out on the N Is XPS of a linear NiN 2 cluster used as a model of the N 2 /Ni(100) system. A good agreement with the experimental spectra is obtained. A new method. based on exact effective Hamiltonians ( EEH ) is used for a detailed analysis of adsorbate core hole screening. Two lowest energy states in the spectrum. split by 1.47 eV are classified as π→π * shake-down satellites, whereas two states dominating the region of the so-called giant satellite are attributed to the main states. The EEH technique has disclosed a dominant role of dynamical screening in the formation of the strong shake-down satellites in the spectrum.

On core-hole screening in chemisorption systems

Core-hole screening in adsorbates was studied by the example of the CO/Ni(100) chemisorption system. The C 1s and O 1s core-hole spectra of a linear NiCO cluster used as a model of the system were obtained by large-scale ab-initio configuration interaction (CI) calculations. The computed spectra are consistent with the experimental ones of the CO/Ni(100)-c(2 x 2) system. Contrary to the previous restricted CI calculations (Surf. Sci. 258 (1991) 91; Chem. Phys. 156 (1991) 309), our results show that the screening mechanism of the core hole on the ligand atoms is dominantly metal-ligand π-π* charge transfer.