Core Hole States Research Articles

Symmetry-selective resonant inelastic x-ray scattering of C60.

The resonantly excited x-ray emission spectra of the ${\mathrm{C}}_{60}$ molecule are presented and analyzed in terms of symmetry- and polarization-selective resonant inelastic scattering processes (RIXS). The theoretical analysis implements recently derived formalisms for symmetry-selective x-ray scattering. Isolated properties of the RIXS spectra are simulated and their cooperative action for the buildup of various features in RIXS of ${\mathrm{C}}_{60}$ are analyzed. Apart from symmetry and polarization dependences, the role of Stokes shifts, tail excitation, vibrational excitation, and interference effects are simulated in detail. Other relevant aspects are discussed in a more brief manner, such as influence of vibronic coupling and nonresonant anomalous contributions. Several conclusions about the nature of RIXS from ${\mathrm{C}}_{60}$ have been derived. The symmetry- and parity-selective character of the RIXS spectra is clearly visualized by excitation in the band gap. The symmetry selectivity leads to a strong correlation between the shape of the RIXS spectrum and the shape of the spectral function describing the incoming excitation photons. It also implies that the RIXS spectra become sparse in the limits of long core hole state lifetime and narrow-band excitation. Spectra pertaining to higher resonant energies involving a higher density of core-excited states are less symmetry selective and turn progressively into their broadband excitation and nonresonant analogues.Tail excitation and Stokes shifts have strong influence on the appearance of the RIXS spectra, both depending crucially on frequency and form of the spectral functions of the incoming photons and on the vibrational progressions of the core-excited states. The band-gap generated spectra emerge as consequences of Stokes shifts when the absorption energies are detuned from the lowest unoccupied molecular orbital resonance. The polarization and angular dependences of RIXS in ${\mathrm{C}}_{60}$ are found to be comparatively weak, something which is rationalized by the highly degenerate electronic structure and the spherical shape of the molecule. The computer simulations in this work rest on transition moments and energies obtained by ab initio Hartree-Fock calculations in the full ${\mathit{I}}_{\mathit{h}}$ point-group symmetry.

Soft X-ray photoabsorption of the NO dimer

X-ray absorption spectra are reported for the first time for the (NO)2 dimer at both the nitrogen and oxygen K edges, the data being obtained from (NO)2 multilayers formed on Ag{111}. The spectra display distinctive features which suggest an electronic structure for excited states of this species. Density functional theory studies of the core-hole excited states provide the basis for spectral assignments at the nitrogen K edge, allowing the identification of the N 1s electron promotion into the 2b1, 2a2, 7b2, 8a1 and 8b2 orbitals. Correlations are found between the experimental integrated oscillator strengths of the two π* states, at the nitrogen K edge, and the N-localized 2p coefficients of the unoccupied orbitals of the (1A1) state.

The vibrationally resolved participator Auger spectra of selectively excited C 1s(2σ)−12π1 vibrational states in carbon monoxide

The fully vibrationally resolved participator Auger spectra originating from the decay of the C 1s(2σ)−12π1 resonance in CO are presented. The C 1s(2σ)−12π1 v′=0 resonance has been excited with a 75 meV monochromator bandpass, i.e., in Auger resonant Raman conditions, and the participator Auger spectrum observed. The C 1s(2σ)−12π1 v′=1 resonance is also excited and the corresponding participator Auger spectrum observed with a monochromator bandpass slightly larger than the inherent width. The results are compared to theoretical simulations using coherent lifetime-vibrational interference theory which accounts for the details of the spectrum. We have observed an interference shift on the transitions to different vibrational sublevels in the final state. A high resolution C 1s photoelectron spectrum of CO is also presented. The lifetime width of the C 1s core–hole state is determined to be 97(10) meV, whereas the C 1s(2σ)−12π1 resonance is measured to have a width of 86(10) meV.

Selectively excited core-to-core fluorescence of potassium halides.

Core-to-core fluorescence transitions from resonantly excited K 2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$3d states in KF and KI have been measured and specific interaction energies of the final 3${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$3d configuration were determined. Delocalization of the excited electron in the 2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$3d states was observed to compete with the core hole decay. Furthermore, evidence for an influence of the chemical surrounding on potassium properties such as the 3s core hole lifetime, the 3s3d exchange splitting, and the configuration interaction of the 3s core hole state was found.

Angle resolved autoionization spectra of adsorbates on metal oxide surfaces

The autoionization of excited core hole states of simple adsorbates has attracted considerable attention. It provides information on the local electronic density of the valence levels. However, mainly simple adsorbates on different metal surfaces have been investigated. Here it was of considerable interest to study autoionization on oxide surfaces because effects due to the substrate are expected to be different from metals. Angle resolved autoionization spectra after resonant C ls→2 π excitation of CO on different epitaxially grown metal oxide surfaces are presented for the first time. The spectra resemble the autoionization spectrum of CO in the gas phase. Differences between spectra on oxide surfaces and on metal surfaces are discussed.

Core-valence (KLV, KVV, LVV) auger and high resolution valence band XPS spectra of aluminium: a comparison with the results of cluster MO calculations

Core-valence (KLV, KVV, LVV) Auger and valence band XPS spectra of aluminium have been measured by high resolution and high sensitivity electron spectrometers and interpreted by different theoretical models. For core-core-valence transitions both the Green function formalism, applied to account for the distortion of the local density of states due to the presence of the final state core hole, and the cluster MO (DVXα) calculation, using a cluster potential with a core hole, describe well the shape of the measured spectra. In the case of the Al KVV spectrum, a spectral width (FWHM) of 14.5 e.V has been observed which is considerably smaller, than predicted by previous experiments. The shape of the core-valence-valence Auger spectra can be interpreted successfully using the ground state density of states obtained from the cluster MO model.

Initial and final state rules in X-ray spectroscopies of adsorbates

The so-called “initial and final state rules” for X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are applied to CO on Ni(1 0 0). In XAS the final state core hole strongly modifies the distribution of unoccupied valence states from that of the ground, whereas in XES the final state reflects the ground state (without core hole) occupied density of states. The intensities of the X-ray transitions are related to the electronic state occupancy in the initial state. This is used to derive electronic state populations from XAS since the initial state corresponds to the ground state.

High-resolution study of the Xe 4d5/2:4d3/2 branching ratio.

The Xe 4${\mathit{d}}_{5/2}$:4${\mathit{d}}_{3/2}$ branching ratio has been determined in the photon energy range 80--250 eV using high-resolution electron spectroscopy and undulator radiation at the new Finnish beamline at the MAX I storage ring. The results were found to be in a satisfactory agreement with the calculated literature values obtained with the relativistic random-phase approximation. A few discrepancies between the theoretical predictions and our experimental results indicate additional correlation effects not accounted for in theory. The branching ratio is now shown to have a maximum of about 1.6 at around 230 eV. Lifetime widths of 114(4) and 121(4) meV are obtained for the 4${\mathit{d}}_{3/2}$ and 4${\mathit{d}}_{5/2}$ core hole states, respectively.

Strategy for an evaluation of core excitation energies in complex molecules: application to SF 6

Modelling of single core-hole states is proposed, which consists of adding an external charge exerting on the remaining electrons to an outer repulsion globally equivalent to the inner attraction resulting from the partial unscreening of the nucleus. For a given complex molecule and an ionized core level, this outer equivalent charge (oech), which may be fractional, can be computed by comparing results from exact core-hole calculations and from applying Koopmans' theorem to valence-ion results for a related simpler molecule. For double core-hole states, the total relaxation energy is computed by adding to the single relaxation energies an induced relaxation energy approximated by the geometric average of the former. This model helps to assign two satellites in the photoelectron spectrum of SF 6 to singlet and triplet double core-hole states.

Ionic Fragmentation of NO Following Excitation of the NK-Shell and the OK-Shell Electron

Ionic fragmentation of NO stimulated by soft X-ray absorption has been studied using a monochromatized synchrotron radiation and a time-of-flight mass spectrometer. In photoexcitation of a 1s electron, the singly charged molecular ion NO+ was formed only at 410 and 533 eV (transitions to the 2π orbital), and a fragment ion N+ had the highest intensity in all the energies. The doubly charged molecular ion was produced appreciably, and fragment ions (N2+ and O2+) were formed considerably even below the 1s ionization thresholds. The measured time-of-flight spectra were converted into kinetic energy distributions of N+, O+, N2+ and O2+ at photon energies for characteristic excitation by a simulation calculation. The dissociation pathways from the core-hole states of NO were discussed using the kinetic energy distribution and ion intensity ratios as well as Auger electron spectra in the literature.

Charge-transfer effect on the linewidth of Fe K alpha x-ray fluorescence spectra.

High-resolution K\ensuremath{\alpha} x-ray fluorescence spectra of FeO, ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, ${\mathrm{K}}_{3}$[Fe(CN${)}_{6}$]\ensuremath{\cdot}${3\mathrm{H}}_{2}$O, ${\mathrm{K}}_{4}$[Fe(CN${)}_{6}$], and Fe (metal) are measured. It is found that the linewidth of Fe K${\mathrm{\ensuremath{\alpha}}}_{1}$ (K-${\mathit{L}}_{3}$) does not follow Van Vleck's theorem for oxides, but it follows Van Vleck's theorem for cyanides; the line shapes of FeO, ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ are nearly identical. These results are rationalized by the spin-unrestricted DV-X\ensuremath{\alpha} molecular-orbital calculations of model clusters. The model clusters we tried are [${\mathrm{FeO}}_{6}$${]}^{10\mathrm{\ensuremath{-}}}$, [${\mathrm{FeO}}_{6}$${]}^{9\mathrm{\ensuremath{-}}}$, [Fe(CN${)}_{6}$${]}^{3\mathrm{\ensuremath{-}}}$, and [Fe(CN${)}_{6}$${]}^{4\mathrm{\ensuremath{-}}}$ as model clusters of FeO, ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{K}}_{3}$[Fe(CN${)}_{6}$]\ensuremath{\cdot}${3\mathrm{H}}_{2}$O, and ${\mathrm{K}}_{4}$[Fe(CN${)}_{6}$], respectively. The electronic structures of these clusters are calculated for their ground states and 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ hole states. It is concluded from these calculations that the charge-transfer effect induced by the creation of the 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ core hole, which is the initial state of the K\ensuremath{\alpha} x-ray emission, reduces the effective number of 3d unpaired electrons of iron oxides, resulting in the K${\mathrm{\ensuremath{\alpha}}}_{1}$ width being reduced from that expected from Van Vleck's theorem. On the other hand, the effective number of 3d unpaired electrons of cyanides in the ground state is conserved in the 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ core-hole state. Thus Van Vleck's theorem holds for cyanides.

Deexcitation processes in adsorbates.

It is shown theoretically that there is competition between core-hole decay and delocalization of a resonantly excited electron from the adsorbate to the substrate metal with simultaneous substrate-to-ligand charge-transfer core-hole screening [relaxation from the resonantly core excited neutral state to the x-ray-photoemission-spectroscopy (XPS) core ionized state] or between the core-hole decay and the charge-transfer core-hole screening from the substrate metal to the adsorbate (relaxation from the XPS core ionized state to the resonantly core excited neutral state). In the former case a spectral feature due to the Auger decay from the ionic core-hole state appears in the autoionization spectrum, while in the latter case a spectral feature arising from the spectator and participant decay from the neutral screened core-hole state appears in the Auger spectrum measured high above the core ionization limit. The relaxation time can be determined from the relative Auger and autoionization spectral intensity. Applications of the Auger-photoelectron coincidence spectroscopy and x-ray-emission spectroscopy to adsorbates are also discussed.

3p fine structure of ferromagnetic Fe and Co from photoemission with linearly polarized light

Linear Magnetic Dichroism in the Angular Distribution of the photoelectrons is observed in angular resolved photoemission experiments with linearly polarized light on 3p core levels of the ferromagnetic transition metals. The measured magnetic asymmetry is strictly related to the one observed in photoemission with circularly polarized radiation. Atomic theory shows that the effect is proportional to the state multipoles that characterize the polarization of the hole levels under the exchange interaction. By peak-fitting the experimental dichroic core level spectra and by calculating the relevant multipoles one can resolve the fine structure of the 3p core hole states of ferromagnetic Fe and Co and obtain the m j ordering of the sublevels. We show that the energy order for the 3p hole-levels of Fe and Co is reversed for the J= 3 2 and J= 1 2 multiplets, and that spin-selected lineshapes can be derived from our analysis and compared with spin-resolved photoemission data.

Inner-shell electron excitation in the methylmethoxysilanes

The effect of electron-donating substituents on the low-lying unoccupied valence orbitals in the series of silanes Si(CH 3) x (OCH 3) 4− x is investigated in an examination of electron-impact energy-loss spectra near the Si 2p ionization edge, and by means of ab initio calculations of the core-hole states populated by the inner-shell excitation. The substituent effect on the LUMO energies, as well as upon the 29Si NMR shieldings, is considered along with the consequences of valence—Rydberg mixing.

Shake-off processes in photoionization and Auger transition for rare gases irradiated by soft X-rays

Branching ratios of multiply charged Ne, Ar, Kr and Xe ions produced following soft X-ray absorption by the rare gases have been measured using a time-of-flight mass spectrometer and monochromatized synchrotron radiation. Ratios of double, triple and quadruple valence photoionization to single valence photoionization, of multiple Auger transitions to the normal Auger transition from a core hole or core-valence two-hole state, and of the multiple photoionization with a core hole to single core photoionization have been determined using the measured branching ratios of the multiply charged ions.

The problem with the copper Auger data for the high temperature superconductors

A summary of the L 3 VV Auger data reported for the high temperature superconductors is given. The variations and trends in the data for different materials and processing conditions are contrasted with those reported for the L 3 VV line shape of the copper dihalides and for the L 3 M 23 V Auger line shape of the superconductors. At least four different mechanisms involving shakeup or shakeoff in the intermediate core-hole state or in the Auger final state have been given in the literature to explain the existence of the satellite in the L 3 VV line shape. Contrasts are made with how this satellite varies compared with the XPS core level satellite. Advantages and drawbacks of each proposed mechanism are given, with the final state mixing model favored; but no firm conclusions are reached on which mechanism is acting.

The bonding between NO and the NiO(100) surface

The interaction between NO and different possible adsorption sites of the NiO(100) surface is studied. The Ni2+ cation gives a bonding to NO in reasonable agreement with experiment, but only if a crystal potential corresponding to less than completely ionic charges is assumed. The computed angle of 43° is also in good agreement with experiment. O1− sites in both weak and strong crystal potentials also give a strong interaction with NO, 1.3 and 0.5 eV, respectively. In this case the angle is larger or around 70°. The O2− anion and Ni1+ sites do not give any significant bonding irrespective of assumed crystal potential and can be excluded as adsorption sites. The computed vibrational frequency for the adsorbed NO show shifts of +50, −85 and −200 cm−1 for adsorption on Ni2+ in the weak potential, and O1− in strong and weak potential, respectively. Only one, downwards shifted, frequency has been observed in the experiment but the most likely candidate for the experimentally observed adsorption site with a binding of 0.5 eV, is still the Ni2+ in a weak potential. Nitrogen core level shifts are also computed and discussed and the fully screened core-hole state is obtained for a cluster model, NiF4O+NO, of Ni2+ in NiO with an ionicity lower than the standard ± 2.

Binding of radical species to surfaces: Cluster models for OH on Cu(111)

The chemisorptive interaction of the hydroxy radical (OH) with the Cu(111) surface and its consequences for the surface electronic structure are studied by CunOH (n ranging from 1 to 25) cluster models using ab initio Hartree–Fock and correlated wave functions. A comparison of the adsorbate binding at the Cu(111) on top, bridge, and the two threefold hollow [face-centered-cubic and hexagonal-close-packed (fcc and hcp)] sites based on extended geometry optimizations yields the fcc site energetically favored with a binding energy EB=3.1 eV at the configuration interaction (CI) level. Correlation contributions account for about 0.9 eV of the binding and are almost independent of the binding site and cluster size. The OH–metal interaction is characterized by considerable Cu to OH charge transfer filling the partially occupied O 2p lone pair orbitals (OH 1π), while OH to Cu charge transfer involving the OH 3σ orbital is small. As a result, the adsorbate is negatively charged in the presence of the surface and ionic binding contributions become important. This makes the OH–metal binding very similar to that of other radicals such as, e.g., OCH3. The adsorbate binding character is also reflected in the results of cluster core and valence hole states corresponding to OH adsorbate ionization. These results may be compared with experimental photoionization data as they become available.

Relaxation and correlation contributions to molecular double core ionization energies

We describe self-consistent field optimization of double core hole states for single- (SCF) and multi-configuration (MCSCF) wavefunctions. With the proposed schemes it is possible to separate contributions to the double core hole ionization energies due to relaxation from those due to correlation, and to obtain, optionally, both symmetry broken and symmetry adapted solutions of such states. Sample calculations are performed to elucidate the role of single versus double core hole relaxation and static versus residual relaxation when the two core holes are in the same main shell but on different sites and when they are on the same site but in different main shells. Perturbation theory results for the double equivalent core replacement species are presented and discussed. The “singlet-triplet separability problem” previously identified for certain double core hole states is verified and remediated by second order SCF/MCSCF.

Core level shifts in amorphous transition metal-tin alloys

X-ray and UV photoemission data are presented for core level binding energy shifts in amorphous TmSn (TMFe, Co, Ni, Pd) films prepared in situ at low temperature by means of a vapour quenching technique. The experimental results are compared to predictions based on the thermodynamical model of Martensson and Johansson using the Born-Haber cycle and the equivalent core approximation. The overall good agreement found between theory and experiment emphasizes the strong influence of the conduction electrons in the photoionization process. It is observed that relaxation effects and screening of the final core hole state can result in binding energy shifts of the same order of magnitude as those of the ground state due to alloying. As a consequence, photoemission core level binding energies in amorphous alloys generally cannot be interpreted within the ground state picture, but have to be corrected for final state contributions.