Relative partial photoionization cross-section of methylacetylene to the X2E ion state over the photon energy range 11–26 eV

Relative partial cross-sections for photoionization to the X²E ground state of the methylacetylene ion over the photon energy range 11–26 e V have been determined using synchrotron radiation. The observed cross-sections show a broad resonance feature similar to the results for photoionization to the X²II_u ground state of the acetylene ion. An explanation involving autoionization from a π^* (e) excited state is proposed.     

Relative intensities in x-ray photoelectron spectra. Part II

Experimental data on the relative intensities of X-ray photoelectron lines of some elements with 22 ? Z ? 56, and the calculations for the photoionization cross-sections for inner levels of some elements with 21 ? Z ? 63 are reported. The relationship between photoionization relative cross-sections and line relative intensities is examined. Theoretical values of the photoionization cross-sections were used in the calculations of relative intensities of molecular valence levels for AO₄^x? (A = Cl, S, P, Se, As), AF₆^x? (A = S, Si, Al), COS, CS₂ and H₂S.     

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 cross-section measurements from the 2p, 3d and 3s excited states of lithium

The photoionization cross-sections from the 2p²P_{1/2, 3/2}, 3d²D_{3/2, 5/2} and 3s²S_1/2 excited states of lithium have been measured at different ionizing laser wavelengths, above the first ionization threshold. The experiments are performed by using a thermionic diode working in the space charge limited mode and the cross-sections are measured by employing the saturation technique. By changing the ionization photon energy, a smooth frequency dependence of the cross-sections has been observed for the 2p and 3d states. The cross-section from the 3s excited state has been measured at a single photon energy. The measured values of the photoionization cross are compared with the available data.     

Photoionization cross-sections for atomic orbitals with random and fixed spatial orientation

Atomic photoionization subshell cross-sections and asymmetry parameters necessary for determining the differential cross-sections of randomly-oriented atoms have been calculated within the one-electron, central-potential model and the dipole approximation for all subshells of C, O, Al, Si, S, Ni, Cu, Ga, Ge, As, Se, In, Sb, Cs, Ba, Ce, Ta, W, Pt, Au, and Pb for a photon energy range from 20 to 1500 eV, and the relevant Cooper minima located to within 10 eV. These values are tabulated for general use, together with the associated radial matrix elements and phase shifts. Differential photoionization cross-sections for fixed-orientation s-, p- and d-orbitals have also been derived within the same model for a completely general experimental geometry, and closed-form expressions depending on radial matrix elements and phase shifts are given. For the special geometry of a polarized excitation source with polarization parallel to the electron emission direction, it is further shown that such oriented-atom cross-sections are exactly proportional to the probability distribution of the initial orbital, a result equivalent to that derived by using a plane-wave final-state approximation. However, detailed numerical calculations of cross-sections for oriented Cu 3d and O 2p orbitals in various general geometries and at various energies exhibit significant differences in comparison to plane-wave cross-sections. By contrast, certain prior angular-resolved X-ray photoemission studies of single-crystal valence bands are found to have been carried out in an experimental geometry that fortuitously gave cross-sections close to the plane-wave predictions.     

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.     

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.     

Photoionization of molecular oxygen in the X3Σg− and a1Δg states

Calculations are carried out, at similar levels of approximation, of the photoionization cross-sections for the ground X³Σ_g^? and metastable a¹Δ_g states of O₂ leading to the X²Π_g state of O₂⁺. Estimates, based upon measurements for the X³Σ_g^? state, are made of the photoionization cross-section of the a¹Δ_g state for transitions populating excited states of O₂⁺.     

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 partial photoionization cross-sections and photoelectron branching ratios of ethylene

Relative partial photoionization cross-sections and photoelectron branching ratios of the valence bands (~10–25 eV binding energy) of ethylene are reported over the photon energy ranges 18–100 eV and 21–100 eV, respectively. The four lowest ionization energy bands (1b⁻¹_1u, 1b⁻¹_1g, 3a⁻¹_g, and 1b⁻¹_2u) show monotonic cross-section decreases with photon energy from 33 eV, the 1b⁻¹_1u CC π band showing the least rapid decline. In contrast, the 2b⁻¹_3u and 2a⁻¹_g bands show almost constant cross-sections up to ~50 eV photon energy, followed by similar, although slower, monotonic decreases. This is attributed to the substantial carbon 2s character of the 2b_3u and 2a_g orbitais. The cross-section behaviour of all bands is interpreted with the aid of SCF-LCAO-MO calculations on the neutral molecule using the Gaussian-80 series of programs.     

40.81 eV photoelectron spectroscopy of the thallous halides

Photoelectron spectra (using 40.81 eV photons) of TlF, TlCl, TlBr and TlI are presented. Absolute binding energies of the outermost halogen np levels with respect to the vacuum level and the energy separations between the thallium 5d_5/2 level and the halogen np level are compared with the corresponding energies predicted using the Born model for ionic solids. The thallium ion ²D_5/2 and ²D_3/2 final states display a non-statistical branching ratio and this is explained in terms of the relative photoionization cross-sections of d_5/2 and d_3/2 electrons. Structure in the spectrum of TIF is interpreted in terms of molecular orbital-like states.     

Calibration of electron-energy spectrometers

Relative photoionization cross-sections of strong peaks in the He(I) photoelectron spectra of N₂, CO, CO₂, and O₂, are tabulated. These data have been measured with an electron energy analyzer whose relative luminosity has been calibrated to an accuracy of ± 5 ‰ Thus, the tables will be useful for calibrating the transmission of other analyzers for electron energies below 9 eV. Correction for angular distribution effects is discussed.     

Photoionization cross sections of Fe XXI

Total and partial photoionization cross sections for (Fe XXI+hν→Fe XXII+e) are presented for the ground and excited bound states with n?10 and l?9. Fe XXI is prevalent in high-temperature astrophysical plasmas as well as in photoionized plasmas excited by hard X-rays. Results are reported for the first time for the high-energy photoionization with core excitations to n=2,3 states. Details of photoionization, especially the high-energy features that often dominate considerably over the low energy ones, are illustrated. These prominent features will affect the photoionization and the recombination rates in high-temperature plasmas. Calculations are carried out in the close coupling (CC) approximation using the R-matrix method. A large CC wavefunction expansion for Fe XXII which includes the ground and 28 excited core states from n=2 and 3 complexes and spans over a wide energy range is used. A total of 835 discrete bound states of Fe XXI in the singlet, triplet, and quintet symmetries are obtained. Total photoionization cross sections, σ_PI(nLS), for ionization into all 29 states are presented for all 835 final bound states and partial photoionization cross sections, σ_PI(g,nLS), for ionization into the ground ²P⁰ state of the core are presented for 685 states. While the n=2 core excitations are at relatively lower energy range (within 15 Ry from the ionization threshold), the n=3 excitations lie at considerably higher energy, 73 Ry and above, yet introduce resonant features and enhancements more prominent than those of n=2 states. Larger numbers of resonances are formed due to Rydberg series of autoionizing states converging on to the 29 core states. However, most noticeable structures are formed in the excited state cross sections by the photoexcitation-of-core (PEC) resonances in the photon energy range of 73-82 Ry. All these high-energy features are absent in the currently available results. The present results should enable more accurate modeling of the emission spectrum of highly excited plasma from the optical to far-ultraviolet region.     

Absolute dipole oscillator strengths for photoabsorption and photoionization of carbon disulphide

The absolute dipole oscillator strengths (cross-sections) for photoabsorption and photoionization (total and partial) of CS₂ have been obtained in the 5–40 eV energy range by magic-angle dipole (e, 2e) spectroscopy. Very strong absorption is detected below 20 eV, much of which is attributable to the excitation of molecules decaying by autoionization processes. Analysis of binding energy spectra taken at energy losses above 20 eV reveals extensive satellite structure in the range up to 35 eV. This structure is attributed to many-electron effects consistent with theoretical calculations found in the literature. Photoelectron branching ratios for CS₂ are also reported.     

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.     

Photoionization cross-section and binding energy of shallow donor impurities in Ga1−xInxNyAs1−y/GaAs quantum wires

We have investigated the effects of the nitrogen and indium concentrations on the photoionization cross-section and binding energy of shallow donor impurities in Ga_1−xIn_xN_yAs_1−y/GaAs quantum wires. The numerical calculations are performed in the effective mass approximation, using a variational method. We observe that incorporation of small amounts of nitrogen and indium leads to significant changes of the photoionization cross-section and binding energy.     

Scattered-wave calculations of photoionization cross-sections and asymmetry parameters for CO,H2O and H2S

Partial photoionization cross-sections and asymmetry parameters are calculated for the valence orbitals of the molecules CO, H₂O, and H₂S and of the atoms O and S using a recently developed extension of the self-cosistent field— Xα—scattered-wave method to continuum states. The convergence of the partial-wave expansions for both initial and final states is studied for electron kinetic energies in the range 2–1000 eV. Since convergence is very slow at high kinetic energies, the interesting region between 2 and 50 eV is emphasized, and comparisons are made with experimental UV photoemission results where such data are available. Overall the method appears to be far more reliable than previous calculations for polyatomic molecules which have used plane-wave or orthogonalized plane-wave final states.     

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.     

Photoionization of the ground1 S e and twenty lowest excited states of the Ne-like Fe16+

TheR-matrix method is used to calculate cross-sections for the photoionization of Ne-like Fe¹⁶⁺ from ground 2s ²2p ⁶¹ S ^e and excited states belonging to 2s2p ⁶ 3l and 2s ²2p ⁵ 3l configurations. Configuration interaction wavefunctions are used to represent two target states of Fe¹⁷⁺ ion retained in theR-matrix expansion. The cross-sections are obtained as a function of kinetic energy (ε _k) of the ejected electron from 10 to 24 Ry. For low kinetic energy the cross-sections show series of Rydberg states which converge onto² S ^e threshold Fe¹⁷⁺. The calculations are carried out in the LS coupling.     

Theoretical study of EXAFS-like modulation observed in valence XPS spectra

Dynamic theory developed previously by the authors is applied to photoionization from delocalized molecular orbitals. EXAFS-like oscillation is observed in any case, but the origin, period and magnitude differ from those of EXAFS usually observed in core photoionization. Numerical calcu- lations are performed in the energy range k = 3–6 a.u. for three molecules: H₂O, CO and CO₂. Analysis of such oscillatory behavior aids in the characterization of photoionized orbitals.