4d Threshold Research Articles

Resonant Auger decay of dissociating CH3I near the I 4d threshold.

Resonant Auger processes provide a unique perspective on electronic interactions and excited vibrational and electronic states of molecular ions. Here, new data are presented on the resonant Auger decay of excited CH3I in the region just below the I 4d-1 ionization threshold. The resonances include the Rydberg series converging to the five spin-orbit and ligand-field split CH3I (I 4d-1) thresholds, as well as resonances corresponding to excitation from the I 4d5/2,3/2 orbitals into the σ* lowest unoccupied molecular orbital. This study focuses on participator decay that populates the lowest lying states of CH3I+, in particular, the X̃2E3/2 and 2E1/2 states, and on spectator decay that populates the lowest-lying (CH3I2+)σ* states of CH3I+. The CH3I (I 4d-1)σ* resonances are broad, and dissociation to CH3 + I competes with the autoionization of the core-excited states. Auger decay as the molecule dissociates produces a photoelectron spectrum with a long progression (up to v3+ ∼ 25) in the C-I stretching mode of the X̃2E3/2 and 2E1/2 states, providing insights into the shape of the dissociative core-excited surface. The observed spectator decay processes indicate that CH3I+ is formed on the repulsive wall of the lower-lying (CH3I2+)σ* potentials, and the photon-energy dependence of the processes provides insights into the relative slopes of the (4d-1)σ* and (CH3I2+)σ* potential surfaces. Data are also presented for the spectator decay of higher lying CH3I (I 4d-1)nl Rydberg resonances. Photoelectron angular distributions for the resonant Auger processes provide additional information that helps distinguish these processes from the direct ionization signal.

Photoionization from the Xe 4d orbitals of XeF2.

We present a comparison of the photoionization dynamics of the 4d shell of XeF2 from threshold to 250eV to those of the prototypical 4d shell of atomic Xe. The new experimental data include spin-orbit and ligand-field-resolved partial cross sections, photoelectron angular distributions, branching fractions, and lifetime widths for the 4d-hole states. The spin-orbit branching fractions and angular distributions are remarkably similar to the corresponding distributions from atomic Xe across a broad energy interval that includes both the intense shape resonance in the f continuum and a Cooper minimum in the same channel. The angular distributions and branching fractions are also in reasonably good agreement with our first-principles theoretical calculations on XeF2. Data are also presented on the lifetime widths of the substate-resolved 4d-hole states of XeF2. While the trends in the widths are similar to those in the earlier experimental and theoretical work, the linewidths are considerably smaller than in the previous measurements, which may require some reinterpretation of the decay mechanism. Finally, we present new data and an analysis of the Auger electron spectra for ionization above the 4d thresholds and resonant Auger spectra for several pre-edge features.

On the strong influence of inner shell resonances upon the outer shell photoionization of endohedral atoms

It is demonstrated by the example of the Xe atom stuffed inside the C60 fullerene, i.e., the endohedral Xe@C60, that the so-called confinement resonances in 4d subshell strongly affect the photoionization cross section of outer 5p and subvalent 5s electrons near the 4d ionization threshold. It is a surprise that these narrow inner 4d shell resonances are not smeared out in the outer shell photoionization cross section. On the contrary; the inner shell resonances affect the outer cross section by enhancing them enormously. Close to its own photoionization thresholds, 5p and 5s photoionization cross sections of Xe@C60 are dominated by their own confinement resonances greatly affected by the amplification of the incoming radiation intensity due to polarization by it of the C60 electron shell. Between 4d and 5p thresholds, the effect of 4d is becoming stronger while own resonances of 5p and 5s are becoming much less important.

Electronic structure of CeAg2Ge2 studied by resonant and angle-resolved photoemission spectroscopy

The electronic structure of CeAg2Ge2 single crystal has been investigated by using resonant valence band photoemission and angle-resolved photoemission spectroscopy at different photon energies. Resonant photoemission has been observed near the 4d threshold of Ce at 121 eV. The constant initial state spectra show two photoemission features having 4f character near the Fermi level that exhibits Fano-like sharp resonance profile. Experimental energy bands have been mapped from the normal-emission photoelectron spectra of CeAg2Ge2 (001) surface along the Γ–Z direction. Four photoemission features are observed to show the dispersion related to the Ce 4f states and the hybridized Ag 4d with Ge 4p states. The experimental spectra have been interpreted with the help of calculations based on full potential linear augmented plane wave method using density functional theory.

Theoretical study of the 4d photoabsorption of the Cs+ ion

Calculations of the 4d absolute photoabsorption cross sections of the Xe-like Cs+ ion covering the energy region from 80 to 190 eV have been performed by using the multi-configuration Dirac–Fock method. The calculated cross sections are compared with the absolute experimental photoabsorption cross-section spectrum (Kjeldsen et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 2845) and other available theoretical results. In the 80–90 eV region, the discrete structure resulting from photoexcitation of a 4d electron into nf and np orbits are successfully identified. Above the 4d threshold, i.e. in the 90–190 eV energy region, a reasonable agreement between experiment and computations is found for the intense 4d → εf shape resonance.

Multi-coincidence in cascade Auger decay processes

Two-electron emission following inner shell ionization of rare-gas atoms by synchrotron radiation has been investigated experimentally with a magnetic bottle electron time of flight spectrometer. The detection in coincidence of a photoelectron with all Auger electrons allows a complete understanding of inner shell hole decay. This method has been applied to the study of single and double Auger decays following Xe 4d, Ar 2p and Kr 3d inner shell ionization. The double Auger decay scheme reveals direct double Auger process but is dominated by cascade processes involving inner-valence electrons in a first step followed by autoionization. Post collision interaction (PCI) associated to distorted Auger spectra and double photoionization paths in the vicinity of Xe 4d threshold, is also discussed.

Stability and electronic properties of silicates in the systemSiO2–Pr2O3–Si(001)

Pr2O3 is one of the most promising hetero-oxides that are the candidates of choice to replaceSiO2 as the gate dielectricmaterial for sub-0.1 µm CMOS technology. In order to enable process integration, however, hetero-oxides requiresubstantial characterization. In particular, the basic interaction mechanisms at theinterface to the silicon substrate are the key issues. A solid knowledge of thesemechanisms is required to address reliability arguments. The challenges in materialscience are to understand the chemical bonding of the hetero-oxides and Si on amicroscopic scale. We report on the specific variations in the electronic structurewhich are evident in the valence band features around resonant excitation atthe Pr 4d threshold. We also determine the valance band discontinuities at thePr2O3/Si(001) interface and follow the changes in the surface potentials to develop a band scheme, aprerequisite to understanding the properties of charge transport across that interface.

A study of the photoionisation dynamics of the cyanogen halides

The valence shell photoelectron spectra of BrCN and ICN have been studied using synchrotron radiation. In addition to the main bands associated with the single-hole states, complex satellite structure due to many-electron effects has been observed in the inner valence region. The Green’s function method has been employed to evaluate the ionisation energies and pole strengths of all valence states and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios for the valence shell, the Br 3d level in BrCN and the I 4d level in ICN have been measured in the photon energy range 14–120 eV. The continuum multiple scattering (CMS-Xα) approach has been used to calculate photoelectron asymmetry parameters and branching ratios for the valence orbitals of ClCN, BrCN and ICN. These have been used to assess the influence of Cooper minima and shape resonances on the photoionisation dynamics. In ICN, the experimental and theoretical results provide evidence of core-valence shell coupling at energies above the I 4d threshold. The experimentally determined asymmetry parameters and branching ratios for the spin–orbit split Br 3d5/2,3/2 and I 4d5/2,3/2 components have been compared to those for Kr and Xe, and a strong atomic-like behaviour is evident. The calculated spin–orbit averaged β-parameters for the Br 3d and I 4d levels confirm this interpretation. Hartree–Fock (HF) calculations have been carried out to study the evolution of the valence orbital characteristics through the series of halogen-substituted XCN (X=H, Cl, Br, I) molecules.

X-ray absorption and resonant photoemission spectroscopy of ZrB 2

We have performed X-ray absorption and valence band photoemission spectroscopy on ZrB 2, which has the same AlB 2 structure of MgB 2, to study the electronic structure. From X-ray absorption spectra near the B 1s–2p threshold, we found a small absorption edge at 188.23 eV with a negligible angular dependence upto 190 eV photon energy. This indicates that B 2p partial density of states near the Fermi level ( E F) is three dimensional, in sharp contrast to that of MgB 2. Resonant photoemission study across the B 1s–2p threshold was found to be difficult due to a large contribution of Auger electrons. On the other hand, resonant photoemission across the Zr 3p–4d threshold does show small change in intensity within 4 eV of E F. This indicates that the occupied states near E F have substantial Zr 4d character. These observations are in relatively good agreement with band structure calculations.

Evolution of angular anisotropy of Auger emission across krypton 3d and xenon 4d thresholds

Abstract The n dependence of the angular anisotropy parameters of the Kr 3d−15/2np→4p4mp (n,m⩾5) and Xe 4d−15/2np→5p4mp (n,m⩾6) transitions has been studied by experiment and theory. The spectator-induced mixing of core states strongly affects the anisotropy at first resonances. Hence the fine structure averaged angular distribution of a resonant Auger spectrum differs from that of the corresponding normal Auger spectrum. As the excitation energy is tuned to higher resonances and across the pertinent ionization thresholds, the differences gradually vanish. Because of the spin–orbit splitting, the Kr 3d−15/26p state is nearly degenerate with 3d−13/25p, and the Xe 4 d −1 5/2 n p (n=8–10) states are degenerate with 4d−13/26p. As these states are excited coherently the angular anisotropy coefficients cannot be written in a form describing the excitation and decay processes separately, but rather the one-step characteristics of the transition must be taken into account. The effect of coherence on the angular anisotropy is discussed.

Electronic-structure investigation ofCeB6by means of soft-x-ray scattering

The electronic structure of the heavy fermion compound CeB6 is probed by resonant inelastic soft X-ray scattering using photon energies across the Ce 3d and 4d absorption edges. The hybridization between the localized 4f orbitals and the delocalized valence-band states is studied by identifying the different spectral contributions from inelastic Raman scattering and normal fluorescence. Pronounced energy-loss structures are observed below the elastic peak at both the 3d and 4d thresholds. The origin and character of the inelastic scattering structures are discussed in terms of charge-transfer excitations in connection to the dipole allowed transitions with 4f character. Calculations within the single impurity Anderson model with full multiplet effects are found to yield consistent spectral functions to the experimental data.

Sudden interchannel interaction in the Tl 6p ionization above the 5d threshold

The linear magnetic dichroism in the angular distribution of Tl 5d and 6p photoelectrons and their dynamical spin polarization have been measured between hnu = 30 and 50 eV. In contrast to the Xe 5p photoionization at the 4d threshold, our results show that above the Tl 5d threshold strong interchannel coupling effects induce a sudden increase in the asymptotic phase difference of the s and d waves for the Tl 6p ionization. This shows that the valence excitation is different for resonant (Xe 4d) and nonresonant (Tl 5d) excitation from subvalence shells.

The photoabsorption of Eu, Eu + and Eu 2+ in the vicinity of 4d thresholds

The results of calculations of the total and partial photoionisation cross-sections of neutral Europium and its ions in the photon energy range from the threshold up to 270 eV are presented. The calculations were performed within the multi-channel Spin-Polarized Random Phase Approximation with Exchange. The main attention is given to the giant autoionizing resonance associated with the 4d → 4f discrete transition to vacant states in the 4f 7 subshell. The comparisons with experimental data obtained recently and the results of previous calculations are made.

Effects of electron correlation on the level widths, the photoelecton, Auger electron and ion-yield spectra above the 4d threshold in Ba

Electron correlation has been studied in the singly ionized 5p-1, 5s-1 and 4d-1 states and the double ionized (5s, 5p, valence)-2 states of Ba by carrying out extensive configuration interaction Hartree-Fock calculations including the mixing of configurations with outermost valence electrons (nl = 6s,5d,6p,4f, ... ). The results of calculations have been compared with existing experimental photoelectron, ion-yield, electron-ion coincidence, and Auger electron spectra.

4f spectral function of cerium in Ce/Fe(100) and Ce/Pd(100) interfaces

Resonant photoemission experiments at the 4d threshold have been carried out in ultra-thin films of cerium deposited on monocrystalline Fe and Pd. In the Ce/Fe system, we have unambiguously demonstrated the reduced Ce configuration admixture in the surface layer by evaporating one monolayer of an appropriate element to suppress the surface contribution. In the Ce/Pd system, the spectra clearly show an exceptional configuration mixture suggesting an intermediate valence regime but they cannot be described in the single impurity Anderson model.

Resonant inelastic soft-X-ray scattering at the 4d edge of Ce-based heavy-fermion materials

Resonant X-ray scattering measurements were performed on CeB6, CeAl, γ-Ce, and α-Ce at various incident-photon energies near the Ce 4d threshold. A pronounced inelastic scattering structure which h ...

Probing electron correlation, charge transfer, and Coster-Kronig transitions at the3dand4dthresholds of Nd by resonant inelastic scattering

Soft x-ray emission near the 3d-4f and 4d-4f thresholds of Nd{sub 2}O{sub 3} is studied with monochromatic synchrotron radiation excitation. At excitation energies above the M{sub IV} threshold, we observe Coster-Kronig enhanced fluorescence from the refill of the 3d hole via the 5p and 4f channel. Fluorescence occurs mainly due to transitions in which charge-transfer provides additional 4f electrons. Resonant inelastic scattering dominates the emission process and we observe two energy loss features at 2.3 and 21 eV, which are due to net transitions within the 4f shell (4f{sup 3}{r_arrow}4f{sup 3}) and 5p{sup 6}4f{sup 3}{r_arrow}5p{sup 5}4f{sup 4}, respectively. The emission spectra resulting from excitations at various energies through the 3d and 4d thresholds were compared and a number of differences were found. Our atomic calculations are in excellent agreement with the experimental results indicating strong localization and a low degree of correlation of the 4f electrons. {copyright} {ital 1999} {ital The American Physical Society}

Theoretical treatment of the bremsstrahlung process in the vicinity of giant atomic resonances.

The frequency and the angular distributions of photons emitted in the bremsstrahlung process of a 0.2--10 keV electron in collision with Ba are calculated for the photon frequencies above the 4d threshold: \ensuremath{\omega}=100--200 eV. The polarizational bremsstrahlung is taken into account. A simple method is described for the approximate calculation of the polarizational bremsstrahlung cross section. This method is particularly efficient in the case, when the many-electron correlations are important. \textcopyright{}1996 The American Physical Society.

Surface electronic structure of CeCo 2, CeRh 2 and CeRh 3 probed by valence band resonant photoemission spectroscopy

The 4f surface versus bulk electronic structure of three strongly hybridized Ce intermetallic compounds (CeCo 2, CeRh 2 and CeRh 3) is investigated by exploiting the 4f photoemission resonant enhancement at the Ce 3d–4f and Ce 4d–4f thresholds. The much higher surface sensitivity at the shallow 4d threshold allows to extract the surface and bulk 4f electronic structure. Our results show a decreasing of the hybridization strength at the surface as compared to the bulk. The analysis of the whole set of results highlights the effect on the 4f spectral function obtained by varying the Ce metallic partner in the same crystal structure (CeCo 2 versus CeRh 2) and by changing the stoichiometry in the CeRh phase diagram (CeRh 2 versus CeRh 3).

Photoionization of Ba+ ions due to creation of 4d hole states

Photoion spectrometry has been applied to determine the relative photoionization yields of Ba+ ions around the 4d ionization threshold. A ground state Ba+ ion beam was merged with monochromatized synchrotron radiation over a length of 15 cm. Counting the number of doubly and triply charged ions produced in the interaction region, we have obtained partial photoion yields as a function of photon energy. Prominent peaks due to resonance Auger processes have been observed below the threshold. A giant resonance structure above 4d threshold was seen only for the Ba3+ spectrum.