4d Threshold Research Articles

A resonant photoemission study of the ZrO 2 valence band

Valence band photoemission spectra have been measured from in situ thermally grown ZrO 2 using synchrotron radiation. Resonant photoemission was observed at photon energies hv ≈ 39 eV, well above the Zr4p → 4d threshold. The phenomenon has been used to identify the Zr 4d derived valence states and good agreement was found with valence band structure calculations. Ar + bombardment of ZrO 2 causes the growth of an emission feature in the band gap at a binding energy of 1.5 eV associated with oxygen vacancies as well as a second band at 3.5 eV and other changes in the O 2p band, all of which are clearly associated with the formation of a highly oxygen deficient surface layer. Resonant photoemission of the bombarded oxide not only demonstrates the d-character of the states in the band gap but also shows a significant increase of the Zr contribution to the O2p band.

Resonant photoemission in CeNi single crystals.

The valence-band photoemission of the (010) surfaces of CeNi single crystals has been observed. The features occurring in angle-resolved normal-emission spectra do not show an appreciable amount of dispersion. The energy distribution curves are squeezed and narrower than those suggested by an existing energy-band calculation. Resonant photoemission has been observed both near the 3p core electron excitation threshold of Ni and the 4d threshold of Ce. An aspect of the super-Coster-Kronig final state with a post-collision interaction is found near the onset of the resonance in the two-hole bound-state satellite. In addition to the two-peak profile in the overall spectral aspect of the 4f spectrum, two additional features are resolved. The origin of these features is discussed. The ratio of Fano's q parameters between the 3p-3d resonance in Ni ions and the 4d-4f resonance in Ce ions agrees with a result of a simple theoretical analysis.

Spin-resolved-photoemission-spectroscopy study of the giant resonance in Gd overlayers on Fe(100).

It is shown that insight into the giant resonance of the lanthanides can be obtained by spin- and energy-resolved photoelectron spectroscopy. Gd has been evaporated onto the Fe(100) surface in the monolayer and submonolayer range of coverage. The spin-dependent electronic structure has been studied by spin-resolved photoemission with photon energies from 70 eV up to the Gd 4d threshold. Within the well-known Fano-like giant resonance, the intensity of the Gd 4f ${(}^{7}$F) signal shows an enhancement of about a factor of 20. The values of the total spin polarization of the Gd 4f ${(}^{7}$F) photoemission peaks on resonance at h\ensuremath{\nu}=152 eV and far away from the threshold at h\ensuremath{\nu}=70 eV are compared. Both values are found to be identical within the experimental error. The results indicate that the amount of non-spin-conserving emission processes in the resonant regime is not increased with respect to the direct photoemission.

Comparative Study of the Decay Mechanisms for 4d Resonances of lodine and 3p Resonances of Copper

This paper describes the approximations which allow to calculate the rates of the various decay processes for core resonances below inner-shell thresholds of open-shell atomic system (4d thresholds of iodine and 3p of copper). Moreover, it emphasizes the different mechanisms that predominantly produce either singly-charged ions when a valence state decay is considered, or doubly-charged ions in case of an excitation to a level of a Rydberg series. Despite different decay mechanisms, the main conclusions are similar to those obtained for closed-shell atoms. In particular, the rate of the double Auger with the Rydberg electron spectator or nearly spectator (shake-up process) in the first step is increasing as soon as the first resonances but this decay path is progressively replaced by shake-off (2 electrons ejected simultaneously) whose rate is increasing with the principal quantum number n of the Rydberg electron.

Photoion spectra in coincidence with threshold electrons near the Xe 4d ionization limits

The yield spectra of Xe2+ and Xe3+ ions near the 4d ionization limits have been measured by a threshold electron-photoion coincidence method. These spectra consist of some prominent peaks corresponding to the 4d-1np Rydberg states and two dominant peaks corresponding to 4d threshold ionization. It is proposed that threshold-electron emission from the inner-shell resonance states is due to not only shake-off (e.g. Xe 4d-16p to Xe2+ 5p-2) but also two-step autoionization (e.g. Xe 4d-16p to Xe+ 5s-15p-17p to Xe2+ 5s-15p-1). The two-step autoionization process plays an important role in decays from the lower n Rydberg states. Two dominant peaks somewhat above the 4d ionization limits show the effect of post-collision interaction, namely, shifts of peak positions and asymmetric profiles. The shifts are almost the same in the Xe2+ and Xe3+ spectra.

Observation and theoretical analysis of the odd J=3 autoionising spectrum of Sr up to the 4d threshold

The odd J=3 autoionising spectrum of Sr between the 5s and the 4d thresholds has been observed. In a Sr atomic beam, the ionised atoms produced by a two-step excitation scheme have been analysed via a time-of-flight spectrometer. A theoretical approach based on the R-matrix method combined with multichannel quantum defect theory calculations provided a good description of the position of the observed lines. Analysis of the observed profiles gave qualitatively good results although the intermediate 5p2 1D2 level is not perfectly described in the model.

Measurement of the partial photoionization cross section and the asymmetry parameter of the Ag valence band near the 4d threshold.

Measurements of the partial photoionization cross section (\ensuremath{\sigma}) and of the photoelectron asymmetry parameter (\ensuremath{\beta}) have been performed on polycrystalline Ag around the 4d threshold. A description of the technique used to obtain \ensuremath{\sigma} and \ensuremath{\beta} from the photoemission spectra is provided. It is shown that in the condensed phase, the photoelectron asymmetry parameter can be experimentally determined much more accurately than the partial cross section, because many experimental uncertainties, such as the dependence of the intensity of the light source with the photon energy, the variation of the electron mean free path, and the transmission and counting efficiency of the electron analyzer as a function of the kinetic energy of the photoelectron, which introduce a considerable amount of distortion in the measurements of \ensuremath{\sigma}, do not play a role in determining \ensuremath{\beta}. Near the threshold, the \ensuremath{\sigma} from bulk Ag is very close to the atomic results, whereas the \ensuremath{\beta} is severely distorted with respect to the free atom, being essentially equal to zero. We propose that the isotropy of the photoelectron angular distribution in polycrystalline Ag is primarily a consequence of a double randomization by a diffraction process of the final states due to the crystal potential and by random orientation of the crystallites. We also point out that there is an interesting analogy between the results of our photoemission experiment and the ``Debye-Scherrer pattern'' of an x-ray-diffraction experiment from a powder of crystals oriented at random.

Single and double photoionisation in Xe and Ba above the 4d threshold

The authors advance and examine a new mechanism which strongly influences the photoionisation processes above the 4d threshold in Xe and Ba. It is shown that the elastic and inelastic scattering of the inner-shell photoelectron by the outer-shell electrons determines the rate of single and double ionisation, decomposing the cross section of the inner-shell photoabsorption into these two channels. The results of these calculations are in good agreement with experimental data.

Threshold effects in ultrasoft X-ray emission

The N4,5(4d) spectra of the atoms of Rare-Earth elements in metals and oxides have been examined under monoenergetic electron beam-excitation with energies varying in the region containing 4d thresholds. The full pattern of the radiative decay of the states of Rare-Earth atoms with a vacancy in the 4d shell have been obtained. Besides, the process of radiative electron capture into free 4f states (inverse photoemission, IPE) has been studied. Attention has been paid to the results of the competition between the capture and 4d shell excitation. The results obtained have been used for the analysis of the structure of 4d appearance potential spectra.

Electron-impact excitation of the resonance transition in Ca+

Detailed calculations of the electron-impact excitation of ${\mathrm{Ca}}^{+}$ are performed using both perturbation theory and the close-coupling approach. Particular attention is focused on the resonance (4s-4p) excitation since experimental emission-cross-section data are available for this transition. The results of the most sophisticated model, a six-state (4s, 3d, 4p, 5s, 4d, 5p) close-coupling calculation with semiempirical Hartree-Fock target wave functions and including one- and two-body core-polarization potentials are in better agreement with the experimental cross section and resonance-fluorescence polarization data than any other calculation. At incident electron energies below the 5s, 4d, and 5p thresholds, the six-state calculations are essentially in agreement with the experimental data, although rich resonance structures predicted by theory are not seen experimentally due to the finite energy resolution. At energies above the 5s, 4d, and 5p thresholds the six-state emission cross sections exceed the experimental cross sections by about 20%, once allowance is made for cascades from the 5s, 4d, and 5p levels.

R-matrix calculation of eigenchannel multichannel quantum defect parameters for strontium

Within the framework of the eigenchannel R-matrix method, theoretical calculations of the multichannel quantum defect (MQDT) parameters have been performed for 1.3Po, 1.3De and 1.3Se spectra of Sr. For the singlet spectra, both discrete and autoionisation regions below the Sr+ 4d threshold are investigated, while the analysis of the triplets is restricted to the bound spectra below the Sr+ 5s threshold. Near the Sr+5s threshold, the calculated parameters are in good agreement with those previously fitted to spectroscopic data. The main improvements with respect to the empirical MQDT studies concern evidence of the large energy dependence of the parameters and identification of isolated perturbers of the 1D2 and 1S0 spectra. In addition, the energy levels and the spectral intensities are accurately reproduced and channel mixing and correlation effects in doubly excited states are analysed in detail. A close similarity is found between the 5snp-4d np 1Po mixing in Sr and the msnp-mpns 1Po mixing in the light alkaline earths Be and Mg. Such near invariance does not hold for the 1De and 1Se spectra.

Photoelectron study of the valence level cross sections of XeF2above the Xe 4d threshold: many-body effects

Using monochromatised synchrotron radiation, the valence band and F 2s photoelectron spectra of XeF2 have been recorded at photon energies of between 50 and 110 eV, below and above the Xe 4d ionisation edge. Branching ratios and partial cross sections have been obtained from the experimental spectra and compared with theoretical results from Ms X alpha calculations. There is generally reasonable agreement between theory and experiment. The 5 pi u cross section is enhanced at a pre-edge resonance, while only the 9 sigma g orbital (about 80% Xe 5s) character shows strong interchannel correlation. The results show that this type of interchannel correlation may only be important for non-bonding valence levels.

Decay of the giant 4d photoabsorption resonance in atomic Cs and Sm

The photoelectron spectra and the relative partial photoionisation cross sections of atomic Cs and Sm have been determined in the photon energy range of 4d-4f, epsilon f excitations (Cs50-130 eV; Sm 120-160 eV). For Cs direct 4d ionisation dominates whereas the Sm '4d to 4f, epsilon f resonance' mainly autoionises via the emission of a 4f electron. The Sm 4d partial cross section goes through a maximum about 12 eV above threshold, where it becomes comparable with the 4f partial cross section. The 4f type character of the Sm to excited wavefunction at threshold turns into 4f, epsilon f type character about 7 eV above. Close to the 4d threshold the 5s, 5p and two-electron satellite photoemission lines are strongly enhanced for both atoms.

Rydberg series in the photoionization spectrum of Pd I

We present the first experimental investigation of the photoionization in Pd I using the synchroton radiation facility of the 800 MeV electron storage ring in Berlin (BESSY) and atomic beam technique. The experiments have been concentrated on the study of the Rydberg series of autoionizing levels 4 d 9( 2 D 3 2 )n p and nf which lie between the two 4d thresholds.

Investigation of the connection between 'giant' resonances and 'atomic' bremsstrahlung

On excitation of gaseous xenon by 0.6 keV electrons a wide emission band has been found in the photon energy range 70-150 eV. On the basis of the analysis of experimental and theoretical investigations the band is attributed to the 'atomic' component of the bremsstrahlung spectrum of 0.6 keV electrons on xenon formed by atomic excitations in an ionisation continuum above the 4d threshold.

Theoretical study of photoabsorption for the palladium 4d subshell

Photoabsorption by electrons in the 4d10 subshell of palladium atoms is studied theoretically below, above, and between the two 4d thresholds using the relativistic random-phase approximation. In the energy regions above and between the 4d threshold cross sections, angular distributions, and spin polarisation parameters are determined. Between the thresholds resonances in the photoionisation parameters caused by autoionisation are analysed using the multichannel quantum defect theory (MQDT). Below the lower 4d threshold the dipole allowed excitation spectrum is studied using MQDT, and oscillator strengths of the high Rydberg states converging to the 4d thresholds are determined.

The controlled raising of discrete levels into the far continuum

A controlled raising of discrete levels into the far continuum is achieved within the configuration average Hartree-Fock scheme by acting on the valence electron of Cs I. The LS-dependent scheme is also considered, in which 4d94f 1P-based levels do not rise above the associated 4d thresholds. It is inferred that controlled transfer of oscillator strength to a 'shape resonance' in the 4d9 epsilon f 1P continuum will occur.

The absolute photoabsorption cross section of atomic cadmium from the 4d threshold to 250 eV

The photoabsorption cross section of cadmium has been determined in the range 40-250 eV using a heat-pipe absorption cell and a synchrotron light source. The results are compared with previous experiments at low energies and with a single-electron calculation. A more recent calculation using many-body perturbation theory is in considerably better agreement with experiment.

Experimental Verification of Many-Electron Effects in the 4d, 5s, and 5p Partial Photoionization Cross Sections of Cs

The partial 4d10, 5s2, and 5p6 photoionization cross-sections of metallic Cs have been measured in the energy range 50–210 eV at the Stanford Synchrotron Radiation Laboratory. The shapes of the absorption resonances are in good agreement with RPAE theory, particularly when relaxation is considered. There is, however, some discrepancy between theory and experiment concerning the relative contribution of each component. In addition, comparison with the total absorption cross-section indicates a considerable contribution to absorption near the 4d threshold which is presumably due to two-electron excitation.

The 4d, 5s and 5p partial photoionization cross sections of xenon above the 4d threshold

The partial cross sections for the 4d, 5s and 5p electrons in xenon have been measured between 60 and 135 eV. The results indicate the importance of electron correlation effects between the 4d inner shell and the 5s and 5p outer subshells.