2p Threshold Research Articles

Photoionization of the ground state of the Be-like C2+ ion leading to C3+ 2l and 3l states

Total and partial photoionization cross sections of the Be-like C2+ ion leading to the C3+ 2l and 3l(l = 0, 1) states from the 1S ground state have been calculated. The lower members of all autoionizing Rydberg series converging to the C3+ 2p, 3s, 3p, and 3d thresholds are identified and their energies and widths are determined. Except for the resonances converging to the C3+ 2p threshold, all of the resonance series are perturbed by overlapping series converging to other thresholds. In addition, the 4s4p 1P resonance is found to lie just below the C3+ 3d threshold. The calculations show excellent agreement between length and velocity gauges. Comparison with experiment for the photoionization of a mixture of the 1S ground state and 3P metastable states of C2+ in the photon energy range 40.8–56.9 eV shows good agreement with the present theoretical results; for the comparison, a calculation of photoionization of the metastable state was also performed. The present calculations are also compared with other previous theoretical and experimental results.

Resonant XPS study of the pyrite valence band with implications for molecular orbital contributions

High-quality valence band X-ray photoelectron spectroscopy (XPS) of vacuum-fractured pyrite (FeS2) was performed using photons from a high resolution undulator beamline. The spectra, collected at photon energies ranging from 50 to 65 eV, span the Fe 3p ionization threshold and reveal the nature of both final state and surface state contributions to the valence band. The lower valence band (1.5 to 10 eV binding energy) reflects primarily S 3p-like direct emission final state contributions. In contrast, the upper valence band (0 to 1.5 eV binding energy) incorporates both a ligand to metal charge transfer and a Fe 3d direct emission final state contribution, the latter perhaps derived in part from surface Fe atoms (a surface chemical state contribution). The results imply that calculations of the pyrite density of states, based on ground state configuration, and without consideration of final state contributions, should reflect closely the XPS valence band structure; this is confirmed by comparison of calculation and collected spectra. Although generally similar to low resolution valence band spectra previously collected, these high resolution spectra show little resonant enhancement of any valence bands above the Fe 3p threshold. The important implication concerns the state of Fe 3dt2g orbitals; the weakness of the direct emission final state (Fe3d5+) contribution and strength of the charge transfer final state contribution to the valence band is explained if Fe 3dt2g-derived orbitals are mixed with S valence orbitals, a possibility predicted by recent theoretical considerations (Eyert et al. 1998). Direct emission final state contributions are, however, observed in the upper valence band (0.40 to 1.3 eV binding energy) and some of the signal may result from surface Fe 3d non-bonding orbitals produced by fracture of the mineral

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.

Silane photoabsorption spectra near the Si 2p thresholds: the geometry of Si 2p excited SiH 4 **

Based on the multiple-scattering self-consistent-field method, we have studied the photoabsorption spectra near the Si 2p thresholds of silane. According to our calculations, the clear assignments of the inner-shell photoabsorption spectra are provided. In comparison with the high-resolution experimental spectra, the geometric structure of the Si 2p-excited SiH4** is recommended to be of a C2v symmetry. More specifically, the Si 2p-excited SiH4** have two bond lengths of 2.50 a.u. and another two bond lengths of 2.77 a.u. and the corresponding two bond angles are 104.0° and 112.5° respectively.

An energy resolved electron–ion coincidence study near the S 2p thresholds of the SF6molecule

The fragmentation dynamics of the SF6molecule following the excitations of S 2p electrons into unoccupied molecularorbitals has been studied using the energy-resolved electron–ion coincidencetechnique. Fragmentation patterns were found to depend on the particularexcitation and on the electronic state of the molecular ion. The spectatorresonant Auger decay at the 2p → 6a1gresonance induces changes in the ion distributions as compared to directphotoionization. Furthermore, coincidence spectra related to the sameAuger structure display different ion abundances at the 2t2g and4egshape resonances. Differences were also found in the Auger decay spectra. These findingsgive further support for the previously suggested many-electron character of the 4egshape resonance.

Site-specific dissociation of (CH 3) 3SiI involving I(4d), Si(2p), C(1s) and I(3d) inner-shell excitations in the range of 88–1000 eV

Dissociative multiple photoionization of iodotrimethylsilane [(CH 3) 3SiI] has been investigated in the I(4d), Si(2p), I(4p), I(4s), C(1s), I(3p), and I(3d) core level excitation/ionization regions in the range from 88 to 1000 eV by time-of-flight (TOF) mass spectrometry and the photoion–photoion coincidence (PIPICO) method coupled to synchrotron radiation. The inner-shell spectra below 160 eV are dominated by a powerful giant resonance which has been identified as a resonance owing to the diffusion of the outgoing photoelectrons by the centrifugal barrier in the I(4d 9ϵf) outgoing channel. In addition to the huge resonance, discrete photoabsorption bands around the Si(2p) (106.77 eV) and the C(1s) (289.95 eV) edges are observed in both the total photoion and PIPICO yield spectra. Site-selective dissociation patterns were elucidated by the coincident measurements of the fragment ions formed via a single photon absorption. At low energies below the Si(2p) threshold region (88–107 eV), dissociation pathways leading to the formation of H +–I 2+ and H +–I + are dominant. Above the energy, however, near total destruction giving rise to CH 3 +–SiH +, H +–CH p + ( p=0–3), H +–SiH p + ( p=0–3), SiCH n +–I +, and H +–SiCH p + ( p=0–5) are dominant.

PROBING THE Mn3+ SUBLATTICE IN La0.5Ca0.5MnO3 BY RESONANT INELASTIC SOFT X-RAY SCATTERING AT THE MnL2,3 EDGE

Resonant soft X-ray scattering spectra of a La 0.5 Ca 0.5 MnO 3 thin film were measured at varying energies of the incident photon beam across Mn 2p thresholds. The data are analyzed within the framework of the Anderson impurity model and full multiplet theory. The scattering profiles of the film are reasonably well reproduced by model calculations for the Mn 3+ system, in contrast to the absorption spectrum at the Mn L2,3 edge for which the contribution of the Mn 4+ sublattice is required to be taken into account. The obtained agreement between calculations and experiment indicates that in this case resonant inelastic scattering mainly probes the local electronic structure at the Mn 3+ sites in the La 0.5 Ca 0.5 MnO 3 film. In turn, Mn 4+ sites mainly contribute to ordinary Lα,β X-ray emission due to relaxations in the intermediate state of the spectroscopic process. Such a situation can occur when the system favors a phase separation rather than checkerboard charge ordering of Mn 3+ and Mn 4+ sites.

FEASIBILITY STUDIES OF THE THREE-DIMENSIONAL DETECTOR FOR SOFT X-RAY EMISSION SPECTROSCOPY

A time-resolving two-dimensional position-sensitive detector was applied to soft X-ray emission spectroscopy (SXES) and its feasibility was successfully studied by using synchrotron radiation. A soft X-ray emission spectrum synchronously acquired with bunch signal of the storage ring has elucidated that a threefold higher signal-to-noise ratio could at least be achieved. By adopting a pump–probe technique to SXES, we have preliminarily studied the effect of the electron–hole plasma (EHP) on the Si 2p soft X-ray emitting processes of Si crystal. A meaningful change in the excitation curve for the elastic scattering yields near the 2p thresholds was found to be probably due to the many-body interactions caused by the EHP.

Resonant structure due to the 3 p→3 d transition in the photoionization of Ca +

Many-body perturbation theory has been used to calculate the resonance structure (3 p→3 d) into the photoionization (4 s→ kp) for ion Ca +. This resonance structure results from the interference between a photoionization process or a photoionization with excitation process and a resonant Auger process. The coupled equation method has been improved to calculate this interference by the summation of specific classes of diagrams of perturbation theory to infinite order. The resonance structure in the region of the 3p threshold is enhanced via a super-Coster–Kronig transition. The theoretical results are in good agreement with experiment.

Photoionization of the excited 3s3p 1,3Po states of atomicmagnesium

Calculational results for the photoionization cross sections leading to theionic Mg+(\\rm3s,3p,4s and 3d) states from theexcited3s3p 1,3Po states of atomic magnesium are reported in the energyregion from the first ionization threshold up to the Mg+(4p)threshold limit. For these calculations, we have used an enhancednon-iterative variational R-matrix approach combined withmultichannel quantum-defect theory at the R-matrix surface.Autoionizing doubly excited 1,3S, 1,3P and 1,3D resonancestructures converging to the Mg+(3p, 4s, 3d and 4p)thresholds are discussed with resonant energies Er, effective quantumnumbers n* and widths Γ. The present photoionizationcross sections are in good agreement with previous available calculations.There is excellent agreement between length andvelocity gauges in our calculations.

Autoionizing resonances in the photoionization of groundstate atomic magnesium

Photoionization of the ground state 1s22s22p63s2 1S ofatomic magnesium is studied theoretically in the energy regionbetween the Mg+(3s) and Mg+(4p) thresholds usingthe non-iterative variational R-matrix method combined withmultichannel quantum defect theory at the R-matrix surface. Theinitial and final states are represented byconfiguration-interaction wavefunctions. We calculated totaland partial photoionization cross sections in both length andvelocity formulations. Photoionization cross sections aredominated by Rydberg series of autoionization resonancesconverging to Mg+(3p, 4s, 3d and 4p) thresholds. We alsocalculated the photoelectron angular distribution asymmetryparameter β for the process leaving the ion in theMg+(3p) state. The present photoionization cross sections arein good agreement with available experiments and previouscalculations. There is excellent agreement between length andvelocity gauges in our calculation.

Absolute photoionization cross section of K+ ions from the 3p to the 3s threshold

The absolute photoionization cross section of ground state K+ ions has been measured from the 3p threshold to above the 3s threshold (31-49 eV) by VUV radiation using a merged ion-photon beam set-up. The cross section, which reaches a maximum value of 32 Mb ±10% at 31.8 eV and then decreases with energy, is in overall good agreement with absolute measurements by Peart and Lyon, but the present investigation also reveals the 3s3p6np 2P (n = 4-6) resonances and eliminates a previously unassigned structure at 35.5 eV. The energies, widths and shape parameters of the autoionizing 2P resonances are determined and compared with recent data from dual-laser plasma measurements and with theoretical predictions. The present experimental set-up will allow absolute photoionization cross sections 0.1 Mb to be measured, making it possible to measure the absolute photoionization cross sections of ions of astrophysical importance.

High resolution spectroscopy of 2p6→2p53d resonantly excited atomic Ca

Abstract Free calcium atoms were excited in the energy region about 10 eV below the 2p thresholds. For the excitation, synchrotron radiation from the high resolution undulator beamline BW3 (HASYLAB, Hamburg) equipped with a SX-700 monochromator was used. The excited atoms were studied by means of ion as well as electron spectroscopy. Two lines could be resolved within one of the two dominating features of the 2 p →3 d resonance. Resonance enhanced 3 p −1 photoelectron spectra resolving the 2 P 1 2 and 2 P 3 2 lines are presented.

Resonant photoemission from vanadium compounds with excitation near V 2p threshold

Abstract Valence-band photoemission measurements were performed on VC 0.80 , VN 0.89 , VF 3 , and VF 2+ x at various photon energies across V 2p absorption edges. While on-resonance spectra of VC 0.80 and VN 0.89 are dominated by V LVV Auger lines, strongly resonating structures in spectra of vanadium fluorides remain at constant binding energies, thus indicating a Fano-type resonance. Analysis of the data indicates the importance of strong metal 3d–anion 2p hybridization for the description of the electronic structure of early transition metal compounds.

The contribution of the ligands around Cr to the resonant inelastic L X-ray emission spectra

Abstract L X-ray emission spectra in the energy region of the Cr 2p threshold on Cr compounds were measured and analysed using tunable synchrotron radiation. The X-ray absorption and emission spectra (XAES) of Cr (VI) are very different from those of Cr (III). It is considered that the difference in the soft X-ray emission is mainly attributed to the effect of the covalency in bonding, because the transition intensity of the Kβ 2 spectral line is sensitive for the valence electronic configuration. Therefore, it follows that XAES is very sensitive to the ligand environment in Cr compounds and relatively insensitive to the metal oxidation state, as Grush et al. suggested for Mn complexes [J. Electron. Spectros. Relat. Phenom. 92 (1998) 225].

Transverse magneto-optical Kerr effect at the 2p threshold of 3d magnets

The transverse magneto-optical Kerr effect (T-MOKE) was investigated for thin epitaxial films of Fe grown on Ag(100) in the region of the 2p excitation threshold. The experimental reflectivity and asymmetry spectra are compared to those derived from the complex dielectric function tensor. The T-MOKE asymmetry is a mixture of both the real and imaginary parts of the off-diagonal elements of the dielectric function tensor, depending sensitively on the relative magnitudes between the real and imaginary parts of the diagonal elements. We demonstrate the possibility to determine the exchange coupling between a ferromagnetic thin film and an adsorbed monolayer by T-MOKE.

Kinematics of the three-body dissociation of SO23+, after sulphur 2p photoexcitation of the SO2 molecule

An original three-dimensional detector, which allows the precise measurement of the kinetic energy release and angular distributions of charged fragments, has been used to investigate the kinematics of the charge separation reaction SO23+→S++O++O+ after core excitation in the S 2p threshold region. Kinetic energy release distributions and angular distributions are reported at various photon energies. The role of bending and stretching of the molecule in the repartition of the kinetic energies in the dissociative kinematics is demonstrated by means of a simple, parameter free, Coulomb repulsion model. At certain photon energies different geometries of SO23+ have been observed experimentally. An ab initio study, at the multireference single- and double-excitation configuration interaction level, of the potential curves corresponding to the neutral core-excited SO2 molecule is presented. This theoretical approach demonstrates that it is possible to explain this observation from the nuclear dynamics of the neutral core-excited molecule before the Auger decay occurs.

Theoretical study of the excited and continuum states in the NEXAFS regions of Cl2

The NEXAFS regions of Cl2 near the 2p and 1s core thresholds have been theoretically considered. The electron excitations below the ionization limit have been treated with a highly correlated abinitio method (QDPT-CI). The position of doubly excited states above 1s ionization has also been considered at the CI level. The region above the threshold is described with an explicit continuum method, based on a LCAO formalism within the framework of density functional theory. The results are in very good accordance with the experiment and support detailed assignment of the spectral feature. Comparison of the present results with respect to previous work on HCl shows a more atomic behaviour of Cl2 with respect to HCl below edge and the opposite in the continuum. The former is traced to the stronger chemical bond in HCl, the second to the larger relaxation of Cl2 in the ionic state.

Excitation energy dependence of X-ray emission spectra and electronic structure of Eu1−xCaxMnO3

Abstract The results of measurements of X-ray photoelectron spectra (valence band and core levels), the O 1s X-ray absorption spectra and fluorescence X-ray emission valence spectra (O K α and Mn L 2,3 ) near the O 1s and the Mn 2p threshold of EuMnO 3 and Eu 0.7 Ca 0.3 MnO 3 are presented. They are compared with LSDA and LDA+U band structure calculation of orthorhombic and cubic LaMnO 3 . Comparison of XPS valence band spectra and O K α and Mn L 3 X-ray emission spectra with band structure calculations shows that the LSDA is more accurate than the LDA+U approach as a description of the spectroscopic properties of orthorhombic EuMnO 3 .

Inner shell excitations of the correlated insulator MnO(100)

Abstract One- and two-electron excitations of core electrons into unoccupied states of the correlated 3d transition metal monoxide MnO(100) were investigated by core electron energy loss spectroscopy (CEELS) and appearance potential spectroscopy (APS: SXAPS and TCAPS) at the metal 2p and the ligand 1s thresholds. Unlike in the spectra of NiO, the number of the structures in the APS spectra at the oxygen 1s threshold exhibits no direct influence of the electron correlation, because all dominant peaks can be explained by one-electron transitions of an oxygen core electron into unoccupied manganese 3d, 4s and 4p states. Integrated CEELS spectra and APS spectra at the oxygen 1s threshold show good agreement. But in the APS spectra at the manganese 2p threshold we find two-electron excitations which are absent in one-electron excitation spectra. In this case a value for the electron correlation energy Udd can be estimated.