Ionizations Research Articles

A high-resolution N 1s photoionization study of the molecule in the near-threshold region

The angle-resolved N 1s photoelectron spectrum of has been measured with high resolution in the threshold region. An analysis of the vibrational fine structure yields a vibrational energy of and a lifetime broadening of for the core-ionized molecule. As in the case of C 1s ionization of CO, the vibrational fine structure changes considerably with photon energy in the region of both the shape resonance and the double excitations. Both the vibrationally resolved partial photoionization cross sections and asymmetry parameters have been determined in this photon energy range. The calculated cross sections in the literature are in reasonable agreement with experiment. In addition, we have measured the cross sections and asymmetry parameters of the most intense direct and conjugate shake-up satellites. The behaviour of the satellites is found to differ significantly from that of the corresponding C 1s satellites of CO.

Shape resonances and multielectron effects in the core-level photoionization of CO2.

The partial photoionization cross sections and angular distributions of the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ and O 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ single-hole states of ${\mathrm{CO}}_{2}$ have been measured with high precision near threshold. The 4${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}^{\mathrm{*}}$ shape resonance is well reproduced in the 1s single-hole cross sections, but strong site-specific effects are observed with respect to both its energy and intensity. A striking feature of the present data is the presence of fine structure in the 1s single-hole cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SH}}$ due to multielectron excitations: virtually all spectral details in the total 1s cross sections ${\mathrm{\ensuremath{\sigma}}}_{1\mathit{s}}$ are also present in ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SH}}$ implying that multielectron excitations couple strongly to the underlying 1s continuum. The decay dynamics of the double-excitation feature at 303.5 eV in the C 1s cross section of ${\mathrm{CO}}_{2}$ have been characterized by Auger spectroscopy. Partial cross sections and angular distributions have also been measured for the first group of strong shake-up satellites in the C 1s photoelectron spectrum. Similar to the C 1s ionization of CO, a strong enhancement of intensity near threshold is observed for some satellite channels as well as the occurrence of features that are not present in the sudden limit. The latter are attributed to conjugate shake-up.

Dissociation dynamics of core-excited BF 3 probed by the photoelectron—photoion—photoion coincidence

The present photoelectron—photoion—photoion coincidence measurement indicates that the dynamics of ion-pair production following the discrete resonance excitation B 1s → 2a″ 2 are completely different from those following the B 1s ionization and the direct double photoionization. The difference is attributed to deformation of the core-excited 2a″ 2 state with the help of ab initio SCF calculation.

Origin of the extended fine structure in the secondary-electron-emission spectrum of graphite.

An extended fine structure (EFS) has been observed in the electron-excited secondary-electron-emission (SEE) spectrum of graphite. Measurements were performed in the 0--150-eV electron kinetic range as a function of primary-electron energy above and below the threshold for carbon core-level 1s ionization. For primary electron energies of 200, 500, and 1000 eV the spectra display a well-defined fine structure that does not depend strongly on the primary electron energy. However, a dependence of the spectrum on orientation of the sample with respect to the primary electron beam and the electron analyzer was found. The effect of structural disorder on the EFS-SEE spectrum of graphite was investigated by performing in situ measurements as a function of the 2-keV Ar-ion irradiation dose. It was found that the EFS-SEE is very sensitive to imperfections created by the ion beam for very low dose levels of ion irradiation. The sensitivity of the spectrum to short-range crystalline order was assessed by comparing the spectrum obtained for graphite to that of glassy carbon. It is concluded that the EFS-SEE of graphite originates from interband electronic excitation processes involving excitations of valence electrons by primary electrons and secondary electrons of sufficient energy into unoccupied electronic states of high density in the graphite crystal field. This conclusion is supported by comparison with other electron spectroscopic measurements of graphite.

Extended fine structure in the secondary electron emission spectra of graphite and glassy carbon

AbstractThe electron excited secondary electron emission (SEE) spectra of highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC) were measured in the 0–150 eV electron kinetic energy range.For HOPG, the SEE spectrum was measured as a function of primary electron beam energy above and below the threshold for carbon core level 1s ionization. The SEE spectrum in the 0–70 eV range reflects high energy conduction band states, in agreement with band structure calculations and other spectroscopic techniques sensitive to the empty density of states. For electron kinetic energies above ∼ 70 eV the spectrum of HOPG displays a well defined structure which has not been studied previously. We found that the SEE fine structure in the low energy range up to ∼ 40 eV shows some dependence on primary electron energy, whereas for larger energies a much weaker dependence was found. It has been established that the SEE spectrum is strongly affected by low‐energy Ar ion irradiation of the HOPG surface. This result suggests that the SEE spectrum is very sensitive to crystal defects induced by the irradiation process. The lower‐energy range of the spectrum, up to ∼ 40 eV was found to be more sensitive to damage induced by the ion irradiation process than the higher range of the spectrum.For GC, the SEE spectrum in the 0–50 eV range is smeared out compared to that of HOPG. However, in the higher electron kinetic range some of the peaks measured for HOPG were also observed for GC. These results suggest that the lower‐energy range of the SEE spectrum is more sensitive to long‐range order, whereas the higher‐energy range of the spectrum reflects a shorter‐range order. It is suggested that SEE spectroscopy can be used as a very sensitive tool for the characterization of different carbon structures.

The σ* absorption peak at the oxygen 1s edge of O2: Exchange splitting, ultrafast dissociation, and atomiclike Auger spectra

The x-ray absorption spectrum of solid O2 at the O 1s absorption edge is analyzed, using its polarization dependence and the Auger de-excitation spectra. Rydberg states are quenched in the solid, and the exchange splitting of the 1s ionization threshold (1.1 eV) can be observed. Below the ionization threshold, core electrons can be excited into unoccupied antibonding π* and σ* orbitals. We conclude from the relative intensities and from the polarization dependence, that the exchange splitting of the σ* final state is small (<1 eV). This is confirmed by LDA calculations of core-excited states. The calculated σ* potential surfaces are repulsive, which explains the large width of this absorption peak. Because of ultrafast dissociation (∼5 fs), core hole decay is likely to occur at large interatomic distances. Auger spectra at the σ* absorption of O2 do indeed show peaks that would be expected for free core-excited oxygen atoms.

Generalized oscillator strength studies for electron-impact 2s ionization of some low- and intermediate-Z atoms

Generalized oscillator strengths (GOS) for ionization of the 2s subshell of several atoms are reported. The atomic wavefunctions used in the work have been obtained from a relativistic local density potential method known as the RC Xi E method. The GOS has been obtained using the Born approximation for 2s subshell ionization of Be, C, O, Ne, Al, Ar, Fe and Zn. The GOS behaviour has been studied both as a function of momentum transfer and energy transfer to understand the Bethe surface properties. Besides the RC Xi potential, the non-relativistic Hartree-Slater (HS) potential has also been used in a separate calculation for comparison. There is in general reasonably close agreement between the results of the RC Xi and the Ha calculations. The main feature of the GOS for 2s ionization is that in the case of small-Z atoms the GOS at threshold has its maximum at a finite value of K, away from the optical limit (K to O). At the position of the peak, significant contribution from optically forbidden partial wave channels has been observed. The 'Bethe ridge' is found to occur when delta approximately=(Ka0)2 at energies above the ionization threshold. At the ionization threshold itself, a ridge on the Bethe surface has been found to occur due to contributions from non-optical channels.

Determination of the lifetime width of the argon L1-hole state.

The width of the argon 2s level was experimentally obtained from x-ray emission-spectroscopy (2.25\ifmmode\pm\else\textpm\fi{}0.15 eV) and x-ray photoelectron-spectroscopy (XPS) (2.25\ifmmode\pm\else\textpm\fi{}0.05 eV) measurements. A theoretical value of 1.85 eV was obtained by a Green's-function calculation. A high-energy satellite located 2.7\ifmmode\pm\else\textpm\fi{}0.2 eV above the ${\mathit{L}}_{1\mathrm{\ensuremath{-}}}$${\mathit{M}}_{2,3}$ x-ray emission transition was confirmed and identified as the 2${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$3${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\rightarrow}3${\mathit{p}}^{\mathrm{\ensuremath{-}}2}$ double-hole transition. The XPS measurement of the argon 2s ionization energy gave a value of 326.25(5) eV.

K-shell ionization of O4+ and C2+ ions in fast collisions with H2 and He gas targets.

Cross sections for 1s ionization of the 1s 2 2s 2 and 1s 2 2s2p 3 P (metastable) states of C 2+ and O 4+ ions in 0.5-1.8 MeV/u collisions with H 2 and He targets were measured using projectile Auger electron spectroscopy at 0 o

Multiple photoionization of Ne in the K-shell ionization region

Multiple photoionization of Ne in the K-shell ionization region (810-1340eV) has been studied using a time-of-flight mass spectrometer. The singly charged ion Ne+ appears only at the 1s → 3p excitation, except for production through direct L-shell ionization. The doubly charged ion Ne2+ shows the most intense yield above the 1s → 3p excitation, and K-shell excited Rydberg states mainly turn into Ne2+ through Auger shake processes. The partial photoionization cross sections of Ne3+ and Ne4+ have maxima at about 130eV and 230eV above the 1s ionization threshold, respectively. These maxima are attributed to shake-off processes at the initial photoabsorption step. Double Auger processes following the 1s photoionization are estimated to happen at about 6% relative to normal Auger processes.

Core level photoionization mass spectroscopy of the fluoromethylsilanes, Si(CH 3) xF 4- x ( x = 0–4), around the Si 2p ionization edges

Total ion yield spectra, total electron yield spectra, photoionized mass spectra and mass resolved partial photoion yield spectra of the five fluoromethylsilane compounds, Si(CH 3) x F 4- x ( x = 0–4), measured around the Si 2p core ionization edges using monoc synchrotron radiation are reported. Photoionized mass spectra of the five compounds were also measured around the Si 2s and F 1s ionization edges. For each of the fluoromethylsilane compounds, total ion yield and total electron yield spectra are very similar to each other and to the corresponding photoabsorption spectra with the exception of the decreased intensities of the discrete resonances. The photoion mass spectra show that almost all possible molecular fragments are generated by excitation and ionization of the Si 2p, Si 2s and F 1s core electrons. Branching ratios of the photoion mass spectra peak areas indicate that the methyl groups are much more labile than the fluorine atoms at all photon energies, both below and above the core ionization edges, in the mixed fluoromethylsilane compounds. Relative areas of the methyl peaks in the photoion mass spectra are enhanced at the discrete resonances below the ionization threshold with more pronounced enhancements above the ionization threshold. An explanation for the greater lability of the methyl ligands is proposed based on the effect of the electron withdrawing and donating properties of the fluorine atoms and methyl groups which also affect the relative Auger transition rates and core hole lifetimes. Partial ion yields of the SiF + and SiMe + fragment ions also exhibit some specific enhancements at Si 2p→σ* resonances below the ionization edge.

Effect of electron-electron interaction on projectile K-shell ionization in energetic ion-atom collisions studied by 0° Auger electron spectroscopy

Cross sections for 1s ionization of the ground state of O 4(1s 22s 2) were obtained in 10–30 MeV collisions of O 4+ with H 2 and He targets. High resolution Auger electron spectroscopy was used to detect at 0° with respect to the beam direction, the electrons emitted from the Auger decay of O 5+ (1s2s 2) → O 6+ (1s 2) + e − following 1s ionization of the O 4+ ions. The electron-nucleus ( σ enI) and electron-electron ( σ eeI) ionization cross sections, due to ionization of the projectile electron either by the target nucleus or the target electrons, respectively, were independently computed. σ enI was calculated within a PWBA treatment, while σ eeI was computed within an impulse approximation (IA) treatment of electron impact ionization of O 4+. The incoherent sum σ enI + σ eei was found to be in good agreement with the data. K-shell ionization of the metastable O 4+(1s 22s2p 3P) component of the beam was also observed.

Valence and core photoelectron spectroscopy of C 60, buckminsterfullerene

The MgKα X-ray (XPS) and He(I)/He(II) ultraviolet (UPS) photoelectron spectra of several thin films of C 60 (one to three monolayers) prepared by vapor deposition on gold are reported. The core XPS spectrum shows a single narrow carbon 1s ionization at 285.1 eV. The valence UPS spectra show very sharp valence ionization bands, comparable to gas phase spectra, with wide separations between the lowest ionization energy features. The ionizations are consistent with theoretical calculations based on the highly symmetric truncated icosahedral structure. The first vertical ionization energy relative to the vacuum level is estimated to be 7.6 ± 0.2 eV from these solid state measurements.

Atomic data for opacity calculations. XIV. The beryllium sequence

The authors have calculated 1814 energy levels in LS coupling, 33030 oscillator strengths and 859570 photoionization cross section values for the following fifteen members of the beryllium isoelectronic sequence: Be I, B II, C III, N IV, O V, F VI, Ne VII, Na VIII, Mg IX, Al X, Si XI, S XIII, Ar XV, Ca XVII and Fe XXIII. Photoionization is from 1554 bound states lying below the 2s ionization thresholds. The continua are perturbed by resonances converging onto the 2p, 3s, 3p and 3d states of the residual Li-like ions. They delineate the resulting autoionization features by using sufficiently small energy step lengths in regions where the cross sections vary rapidly. Photoexcitation of the core produces large PEC resonances which dominate photoionization of Be I and the less highly ionized ions. They discuss some selected examples drawn from the entire data set and make comparisons with the work of other investigators.

Kinetic energy released in positive ion pair formation from inner-shell excited oxygen molecule

Total kinetic energy released in dissociation from inner-shell excited states of O2 induced by soft x-ray irradiation has been measured using a time-of-flight mass spectrometer operated in a photoion–photoion coincidence mode. The distribution of the total kinetic energy released in the dissociation into O++O+ at a photon energy above the 1s ionization threshold shows three peaks at about 6, 11, and 15 eV and that at an energy of the 1s→2pπg transition has a broad maximum at 11 eV. The total kinetic energy distribution of O++O2+ shows a broad maximum at about 20 eV in all the photon energies. The dissociation pathways are discussed in detail using the kinetic energy distributions obtained.

Polarized copperK-edge x-ray-absorption spectra inYBa2Cu3O7−y

The polarized copper K-edge x-ray-absorption near-edge spectra (XANES) in the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and related semiconductor oxide ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ were calculated. The initial states and the final bound states were obtained from the self-consistent-field discrete-variational X\ensuremath{\alpha} method (SCF DV-X\ensuremath{\alpha}). Continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV-X\ensuremath{\alpha} potentials. XANES for each of the two Cu sites in these compounds were identified by using the calculated difference in Cu 1s ionization potentials. Features in XANES were interpreted with the help of the projected density of states and photoelectron trapping times. A quantitative comparison with the measured spectra leads us to assign shakedown features with relaxation energy of 8.3 eV and intensities of 20% and 12% of the total transition in the a-axis- and b-axis-polarized Cu K-edge XANES of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, respectively. This shakedown feature apparently involves ligand\ensuremath{\rightarrow}3d transitions in the core-hole excited state. It is found that this relaxation process is present in the metallic ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and absent in the semiconducting phase.

Electron-impact ionization of highly charged hydrogenic atomic ions.

Electron-impact-ionization cross sections are calculated for hydrogenic ions with nuclear charge between 26 and 92, for incident electron energies up to 8 times the ionization energy, by a relativistic distorted-wave method. The results demonstrate the increasing importance of relativistic effects with Z and with incident electron energy, particularly for 1s ionization. The effects of including magnetic and retardation interactions as Z increases are also studied. Results are compared with scaled K-shell ionization measurements for $^{47}\mathrm{Ag}$ and $^{79}\mathrm{Au}$.

C 1s ionization in C2H2 studied by asymmetric (e,2e) experiments.

The dynamics of core ionization by electron impact is investigated through the measurement of the triply differential cross section of the C \ensuremath{\sigma}1s orbital in the molecule ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$. The (e,2e) experiments have been performed under asymmetric conditions and at small scattering angles, with a scattered electron energy of 1500 eV and low energies of the ejected electrons (9.6 and 41.0 eV). The measured angular distributions are characterized by large-size recoil lobes, breaking of the symmetry around the momentum-transfer direction, and unusual deviations of the maxima of the recoil peaks towards smaller deflection angles. In the (e,2e) energy spectrum a shift is observed in the position of the C \ensuremath{\sigma}1s peak with respect to the expected value as measured by x-ray photoelectron spectroscopy. The amplitude of the shift amounts to 0.46\ifmmode\pm\else\textpm\fi{}0.23 eV at 9.6 eV excess energy, and it is too large to be explained only in terms of postcollision interactions.

Theoretical calculations of x-ray-absorption spectra of copper in La2CuO4 and related oxide compounds.

We report the results of theoretical calculations of copper K-edge x-ray-absorption near-edge spectra (XANES) in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ and related oxides ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$. The final bound states were obtained from the self-consistent-field discrete-variational X\ensuremath{\alpha} method (SCF DV X\ensuremath{\alpha}), and continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV X\ensuremath{\alpha} potentials. Composition of the final-state wave functions was analyzed. The 1s ionization potentials of the three reference compounds obtained from the SCF DV X\ensuremath{\alpha} transition-state calculations were used to set up the relative energy scale for the calculated cross section. The principal features of measured Cu K-edge XANES for ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$ were reproduced satisfactorily by our calculations along with their relative energy positions. Our calculated polarized XANES of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ were compared with measured spectra, and two shakeup features were identified with radiation polarized along the crystal c axis. By quantitatively comparing the measured spectra with a model based on our calculated cross sections, their intensities were found to be \ensuremath{\sim}24% and 10% of the main transition with shakeup energies of 5.7 and 9.5 eV, respectively. We suggest that these multielectron excitation features involve Cu 3d\ensuremath{\rightarrow}4p transitions.

Chemical effects in the X-ray emission (F K ? ) and X-ray photoelectron spectra of alkali-metal and alkaline-earth-metal fluorides

X-Ray emission (XE) and both core and valence-band X-ray photoelectron (XP) spectra have been measured for LiF, NaF, KF, BeF2, MgF2, CaF2, SrF2 and BaF2. Both the F 1s ionisation energies and the F Kα X-ray energies decrease as the cation atomic number increases within a group of the Periodic Table. Knowledge of the F 1s binding energy enabled the valence-band XP spectra to be aligned on a common energy scale for direct comparison with the F Kα peak profile. Both the XE and XP spectra showed broader bands for the lighter cations, which for Li, Be, Mg and Ca could be resolved into two distinct components. Symmetric broadening of the valence-band peaks was rationalized as due to anion–anion interaction whilst additional structure on the low (kinetic) energy side and low (X-ray) energy side of the main XP and XE peaks was related to covalent character in the cation–fluoride bond. Differences in the relative intensities of these components for XE (reflecting F 2p character only) and the XP spectra were ascribed to the presence of F 2s and possibly cation s character in the more tightly bound orbitals of the valence band. Measurement of the BeKα and Mg Kβ XE spectra gave a direct indication of the covalent nature of the bonding in BeF2 and MgF2.