Relative intensities in X-ray photoelectron spectra.: Part VIII

The photoionization cross-sections for the 3s and 3p shells of atomic Si, P, S, and Cl and the S²⁺ ion, and for the 2s and 2p shells of atomic F have been calculated using the random-phase approximation with exchange (RPAE) for the average-configuration term. Using the theoretical atomic cross-section values, the partial cross-sections for photoionization of the SF₆ molecule have been calculated for hv ? 54 eV and the photoelectron spectra have been interpreted. The calculation of relative intensities in the photoelectron spectra of H₂S is presented. The influence of the effective charge of an atom on the photoionization cross-section value for a molecular level is shown.     

Photoionization branching ratios and asymmetry parameters for inner- and outer-valence shells of H2S in the 18–70 eV energy range

Branching ratios, asymmetry parameters and relative partial cross sections have been obtained for photoionization of the outer and the inner valence shells of H₂S. These measurements were made in the photon energy range 18–70 eV using synchrotron radiation. Our results are compared to a set of calculations using a developed extension of the self-consistent field-Xα scattered-wave method to the continuum states. This comparison shows a qualitative agreement between the experimental and calculated β curves of the outer valence shells. The largely predominant sulfur 3p contribution to the outer valence orbitais 2b₁, 5a₁, 2b₂ is revealed in the corresponding β curves by a Cooper minimum also predicted in the same energy range for the β(3p) of the atomic sulfur. This comparison also shows discrepancies in the branching ratios curves and we suggest that this theoretical framework is better adapted to predict photoionization processes in the outer valence shells than in the inner ones.     

Photoelectron branching ratios and partial ionization cross-sections for CO and N2 in the energy range 18–50 eV

Branching ratios for photoionization to various ion states of CO and N₂ have been measured in the energy range 18–50 eV using an electron—electron coincidence technique at an ejected electron angle of 90°. The branching ratios, which show a marked variation with energy are shown to be in good agreement with the results of conventional photoelectron spectroscopy at those energies where quantitative PES measurements have been made. The results indicate that the effect of the assymmetry parameter, β, is relatively insignificant in the case of CO and N₂. Partial oscillator strengths have been determined from the branching ratios using recent total photoabsorption data. In addition to the expected one electron ion states, others are observed arising from two electron processes and the variation of these cross-sections with energy loss is also reported.     

Dipole oscillator strengths for the photoabsorption,photoionization and fragmentation of molecular oxygen

The photoabsorption, photoionization and fragmentation of O₂ have been studied using electron impact coincidence methods to obtain branching ratios and dipole oscillator strengths (cross-sections). The photoabsorption measurements cover the energy range 5–300 eV while the formation of electronic states of O₂⁺ (photoelectron spectroscopy) and the resulting ionic fragmentation (photoionization mass spectrometry) are both measured from close to threshold up to photon energies of 75 eV. The binding energy spectra of O₂ show peaks at 33, 47 and 57 eV in addition to those reported elsewhere in the literature. These peaks are assigned to multiple final ion states arising from photoionization of the inner valence orbitals. Structure in the O₂⁺ electronic state partial oscillator strength curves is in good agreement with recent theoretical work which predicts the existence of several shape resonances. A quantitative picture of the dipole-induced breakdown of O₂ is obtained for the energy range 12–75 eV. The photoionization efficiency is found to be constant above 20 eV.     

Relative intensities in x-ray photoelectron spectra. Part VI. The spectra of He(I), He(II), Y Mζ and Zr Mζ

The 2s- and 2p-electron photoionization cross-sections at photon energies up to 190 eV have been calculated, using the RPAE method for averaged configurations of the C, N, O and Ne atoms. The RPAE method ensures a more accurate relation between the cross-sections, 2s/2p, than that obtained using the Hartree—Fock method. Within the framework of the Gelius—Siegbahn model, but with the use of theoretical atomic cross-sections, we have calculated the photoionization cross-sections for He(I), He(II), Y Mζ, Zr Mζ for CH₄, C₂H₆, C₃H₈, C₂H₄, C₂H₂, NH₃, H₂O, CN^?, N₂, CO, CO₂, N₂O and NO₂^? molecules. For CO, N₂, CO₂, N₂O and H₂O molecules, a comparison is made between the theoretical and experimental cross-sections for hν < 60 eV. The calculated absolute and relative values of the molecular-orbital cross-sections are in reasonable agreement with experiment, especially at hν ? 40 eV. The calculations correctly reproduce the change in intensities under the transition He(I) → He(II). We have shown that our calculations have a significant advantage over those performed using the PW and OPW approximations. It is shown for NO, N₂, CO, H₂O, CH₄, NH₃ and N₂O molecules that the total photoionization cross-section calculated taking into account the real structure of the molecular orbitals is in better agreement with the experimental photoabsorption cross-section than is the sum of the cross-sections for the atoms in a molecule.     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

Relativistic R-matrix close-coupling calculations for photoionization of Ne-like Al IV

We present relativistic close-coupling photoionization calculations of Al IV using the Breit-Pauli R-matrix method to obtain photoionization cross-section of Al IV from the ground state and the lowest two J=0 (even) excited states. A multi-configuration eigenfunctions expansion of the core Al V is employed with spectroscopic configurations 2s²2p⁵, 2s2p⁶, 2s²2p⁴3s, 2s²2p⁴3p, 2s²2p⁴3d and 2s²2p⁴4s. We have included, for the first time, the lowest 68 level target states of Al V in the photoionization calculations of Al IV. Extensive configuration interaction wavefunctions are used to describe both the initial Al IV states and the final Al V states. Cross-sections are compared from three level calculations including only 2s² 2p^{5 2}P^o _3/2, _1/2 and 2s 2p^{6 2}S_1/2 levels of Al V. The present calculation using the lowest 68 target levels of Al V are presented for the first time and should provide reasonably complete database for practical application for photoionization cross-section for Al IV, where high-energy cross-sections along with near-threshold photoionization cross-section is required.     

Photoelectronic investigations of semi-insulating p-type GaAs:Cr containing neutral chromium acceptors

A photoelectronic analysis of p-type GaAs:Cr, i.e. measurements of thermally stimulated currents and the dependences of photoconductivity and photo-Hall effect on photon energy, temperature and light intensity, have enabled trap locations and densities as well as properties of neutral chromium acceptors to be determined. Hole traps proved to be located at 0.15 and 0.23 eV above the valence band, and their densities have been estimated to be 10¹⁵ cm^?3 and 5 × 10¹⁶ cm^?3 respectively. Their occurrence is related to the presence of copper in the samples investigated. Neutral chromium acceptors are located at 0.77 eV above the valence band and are at a constant distance from the conduction band. Their photoionization cross-section is 3 × 10^?17 cm² while the photoexcited electron escape cross-section is about 10^?20 cm². The potential of a neutral Cr acceptor is of the delta function type with weak coulombic tails. The maximum radius of the Bohr orbit of an electron in the ground state is 4 atomic units.     

Subshell photoionization cross-sections,electron mean free paths and quantitative X-ray photoelectron spectroscopy

Subshell photoionization cross-sections (SPC) for elements of atomic number in the range 3?, z ? 82 have been determined from measurement of relative photoelectron intensities at a photon energy of 1486.6 eV (Al Kα). A correction procedure has been developed which permits such determinations even when sample surfaces are not atomically clean. The results obtained suggest that Scofield's SPC calculations [2] are reasonably reliable for 1s, 2p and 3d subshells but that the present, experimentally derived data are to be preferred for quantitative use. As a result of this work, a rapid method of quantitative surface-constituent analysis, accurate to ± 20%, has also been developed, which is expected to be of value particularly in industrial situations. A semi-empirical method of extracting electron mean free paths from measurements of relative photoelectron intensities is also illustrated.     

Photoionization cross section of atomic nitrogen calculated by the many-channel quantum defect method

The many-channel quantum defect method has been used for the calculation of the photoionization cross section of ground-state nitrogen atoms in the dipole length approximation. The calculations couple three channels corresponding to the 2s ² 2p ² and 2s2p ³ configurations of the residual ion. The results obtained are in agreement with experiment for the energy region between the ionization limit and 1 rydberg unit beyond.     

Some lifetimes and transition probabilities in Cu(I)

Radiative lifetimes of 7 levels in the Cu(I) 3d⁹ 4s4p configuration have been measured using a delayed coincidence technique; copper vapor was excited in an argon buffer gas and cross sections for collisional destruction of copper levels by argon were also obtained. Transition probabilities of the lines originating from these levels have been determined using the measured lifetimes and branching ratios.     

Many-electron character of Rydberg series converging to the ionization thresholds of the 4<Emphasis Type="Italic">p</Emphasis><Superscript>4</Superscript>5<Emphasis Type="Italic">s</Emphasis> states of KrII

The photoionization cross sections of the 4p shell and the 4s main level and 4p ⁴(³ P) 5s ⁴ P _{5/2, 3/2} satellite subvalence levels of KrII have been calculated in the 4s-near-threshold range of excitation energies from 28.48 to 28.70 eV. The calculation takes into account the core relaxation by the methods of the theory of non-orthogonal orbitals, the interaction between resonant states through autoionization channels by solving the complex secular equation, and the interaction between the channels of the continuous spectrum in all orders of the perturbation theory by the K-matrix method. Good quantitative agreement between the energy-integrated theoretical and experimental photoionization cross sections for the satellite levels has been obtained for the first time. It is shown that only simultaneous consideration of the above-mentioned effects leads to such agreement. The resonant structure of the photoionization cross sections in this excitation energy range is related to the autoionization decay of the 4p ⁴5s(⁴ P _1/2)np and 4p ⁴5s(² P _3/2)np Rydberg series. The specificity of this process is that both series manifest themselves not independently but owing to their strong electrostatic interaction with the prominent 4p ⁴(¹ D)5s ² D _5/2 6p _3/2 resonance, which lies in this excitation energy range.     

Photoionization and double excitation spectrum of Be2+

The photoionization cross-section of Be²⁺ has been measured in the range from threshold at 154 eV up to 420 eV. The value at threshold of [1.4 ± 0.2] × 10^?18 cm² is in good agreement with theoretical predictions. The method used was that of absorption using two laser produced plasmas. The doubly excited resonance 2s2p ¹p⁰ at 281.25 ± 0.07 eV has been observed and its Fano profile measured.     

Photoelectron asymmetries and two-electron satellites near the 3p → 3d giant-resonance region in atomic Mn

The partial cross-sections and photoelectron angular distributions for several lines in atomic Mn have been measured at photon energies between 50 and 72eV. The intensities of the 3d correlation satellites at 24–26 eV binding energy behave similarly to the mainline intensity near the 3p → 3d giant resonance, but show an enhancement near the 3p threshold which is not present for the main line. A configuration-interaction analysis is applied to help identify the origins of the satellites. The 3p/3d branching ratio from 55–72eV and the shape of the 3d cross section in the resonance region are in good agreement with many-body perturbation-theory calculations.     

Photoabsorption of Atomic Sodium in the VUV

The absorption spectrum of atomic sodium in the photon energy region from 30 to 150 eV has been investigated. A great number of sharp absorption lines which can be attributed to the excitation of a 2p- or a 2s-electron has been detected. Simultaneous excitation of one 2p- and one 3s-electron gives rise to considerably strong broad and asymmetric absorption structures above the highest series limit (¹ P ₁) for the excitations of a single 2p-electron. Some of the assignments have been confirmed by Hartree-Fock calculations. The relative spectral dependence of the absorption cross-section in this energy range has been determined for the first time. The spectrum of free Na atoms has been compared with theL _{II, III} spectrum of solid sodium.     

Branching ratios and partial oscillator strengths for the photoionization of NH3 in the 15–50 eV region

The branching ratios for dipole ionization from the three valence orbitals of NH₃ have been determined up to 50 eV using an improved electron impac     

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

Many-body perturbation theory has been used to calculate the resonance structure (3p→3d) into the photoionization (4s→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.     

Partial photoionization cross-sections for CO2 and N2O from 20 to 6OeV

Measurements of the partial photoionization cross-sections of the molecules CO₂ and N₂O are reported for the energy range 20–6OeV. Large contributions to the photoionization cross-section from multiple electron transitions are observed, particularly above 40eV, in accord with the results of recent many-body Green's function-calculations. The cross-section for total photabsorption as well as the ionization efficiency have also been measured.     

Synchrotron-radiation photoemission study of the ultrathin Ba/3C–SiC(111) interface

Electronic structure of the Ba/3C–SiC(111) interface has been detailed studied in situ in an ultrahigh vacuum using synchrotron radiation photoemission spectroscopy with photon energies in the range of 100–450 eV. The 3C–SiC(111) samples were grown by a new method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. Valence band photoemission and both the Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage. Under Ba adsorption two induced surface bands are found at binding energies of 2 eV and 6 eV. It is obtained that Ba/3C–SiC(111) interface can be characterized as metallic-like. Modification of both the Si 2p and C 1s surface-related components were ascertained and shown to be provided by redistribution effect of electron density between Ba adatoms and both the Si surface and C interface atoms.     

Ionization of ytterbium atoms from an excited state

The cross sections of the photoionization and the electron impact-induced ionization of Yb atoms from the excited 6s6p(³ P ₁) state are numerically calculated. Matrix elements are computed in multielectron relativistic and nonrelativistic approximations with allowance for the superposition of configurations and a relaxation effect. The radial part of the electron wavefunction in a continuous spectrum is calculated using the solutions to one-configuration Hartree-Fock and Dirac-Fock equations. The cross sections calculated by a relativistic method are compared to those for a nonrelativistic approximation. The ratios of the radiation reduced matrix elements and the phase shifts of the wavefunctions of a continuous spectrum calculated for the 6p ɛs and 6p → ɛd transitions are compared to the values obtained by approximating the experimental dependences of the angular distribution of photoelectrons for the photoionization by ultraviolet radiation from an oriented excited state.