Double Ionization Energies Research Articles

An experimental and theoretical investigation of the valence double photoionisation of the iodine molecule

The double photoionisation spectrum of molecular iodine has been measured at three wavelengths by the TOF-PEPECO technique and is interpreted using relativistic molecular structure COSCI calculations of potential curves for a large number of electronic states connected to the three lowest groups of dissociation limits. The lowest double ionisation energy is 24.85 ± 0.02 eV (adiabatic) or 24.95 ± 0.02 eV (vertical). It is associated with the 3 Σ g , 0 - electronic state of I 2 2 + . The double ionisation process is mainly direct, and positions and widths of the bands are well reproduced by the calculations.

Microsolvation of F-in Water

Microsolvation of F- in water is studied by ionization and double ionization spectra of (H2O)1-3F- calculated by ab initio methods. It is shown that the presence of the fluorine electrons introduces many-body properties in the spectra which cannot be reproduced by the presence of a negative point charge. The increase of the solvation shell increases the complexity in particular of the double ionization spectra. Ionization and double ionization energies slowly increase with continued solvation, and many-body effects in the inner valence spectra become more prominent.

Single and double photoionizations of methanal (formaldehyde)

Single and double photoionization spectra of formaldehyde have been measured at 40.81 and 48.37 eV photon energy and the spectrum of the doubly charged cation has been interpreted using high-level electronic structure calculations. The adiabatic double-ionization energy is determined as 31.7+/-0.25 eV and the vertical ionization energy is 33 eV. The five lowest excited electronic states are identified and located. The potential-energy surfaces of the accessible states explain the lack of stable H2CO2+ dications and the lack of vibrational structure. The experimental double-ionization spectrum can be decomposed into two distinct contributions, one from direct photoionization and the second from indirect double photoionization by an inner-valence shell Auger effect.

First-principles T-matrix calculations of double-ionization energy spectra of atoms and molecules

Strong electron correlation plays an important role in the determination of double ionization energy, which is required for removing or adding two electrons, particularly in small-sized systems. Starting from the state-of-the-art GW approximation, we evaluate the particle-particle ladder diagrams up to the infinite order by solving the Bethe-Salpeter equation of the T-matrix theory to calculate the double-ionization energy spectra of atoms and molecules (Be, Mg, Ca, Ne, Ar, Kr, CO, C(2)H(2), Li(2), Na(2), and K(2)) from first principles. The ladder diagrams up to the infinite order are significant to calculations of double-ionization energy spectra. The present results are in good agreement with available experimental data as well as the previous calculations using, e.g., the configuration-interaction method.

C 2H 22+: An experimental and theoretical study of the spectrum of the ethyne dication

Double photoionisation spectra of ethyne and ethyne-d 2 have been measured by the TOF–PEPECO technique and by the TPEsCO method with vibrational resolution. With the help of new calculations of the dication structure and Franck–Condon factors we deduce adiabatic double ionisation energies for the first three states of the normal hydrogen isotopomer as 31.74 eV ( X 3 Σ g −), 32.42 ( a 1 Δ g) and 33.07 ( b 1 Σ g +). The two techniques give spectra with very different vibrational structure and reasons for the difference are discussed.

Photo double ionization spectra of CO: comparison of theory with experiment

High level ab initio calculations have been undertaken of potential energy curves of CO2+ (and for the CO neutral ground state). The accuracy of the potentials was tested by a synthesis of the available vibrationally resolved threshold photoelectrons in coincidence (TPEsCO) and time of flight, photo electron photo electron coincidence (TOF-PEPECO) spectra of CO2+. Good agreement was found between experimental and theoretical spectra once relative energies of the calculated double ionization energies were slightly adjusted (by approximately 1%) to match experiment. Vibrational separations within individual electronic states are very well reproduced (the worst error is 0.07%).

Threshold excitation in Fe Kα3,4 satellites

Fe Kα3,4 satellite spectra are investigated using a Johann-type spectrometer at the BL15XU undulator beam line, SPring-8. The intensity of the Kα3,4 satellite relative to the Kα1 emission line is found to increase asymptotically with excitation energy up to 9000 eV, almost saturating at 10 000 eV. The satellite threshold energy is found to be 7882 ± 14 eV. This corresponds to the ionization energy of Fe 1s + 2p*, where * indicates that the value is from the Z + 1 approximation. The appearance of a small peak around the [1s2s] double ionization energy suggests that the satellite intensity with the excitation energy can be increased not only by the formation of the 2p spectator holes, but also by a combination of the [1s2s] resonance and [2s2p] Coster–Kronig transition.

Double ionization energy spectra of small alkali-metal clusters

In this article, we calculate the spectra of the double ionization energy (the energy required for adding two electrons to the neutral system) of small alkali-metal clusters (K2 and Li2) including the effect of electron–electron short-range correlation by adding two one-particle energies within the GW approximation and taking into account the electron–electron repulsive interaction effectively.

The effect of particle–hole interaction on the XPS core-hole spectrum

How the effective particle–hole interaction energy, U, or the polarization effect on a secondary electron in a final two-hole one-particle (2h1p) state created by the Coster–Kronig (CK) transition can solely affect the density of the CK particle states and consequently the core-hole spectral function, is discussed. The X-ray photoelectron spectroscopy (XPS) core-hole spectrum is predominantly governed by the unperturbed initial core-hole energy relative to the zero-point energy. At the latter energy, the real part of the initial core-hole self-energy becomes zero (no relaxation energy shift) and the imaginary part (the lifetime broadening) approximately maximizes. The zero-point energy relative to the double-ionization threshold energy is governed by the ratio of U relative to the bandwidth of the CK continuum. As an example, we study the 5p XPS spectra of atomic Ra ( Z=88), Th ( Z=90) and U ( Z=92). The spectra are interpreted in terms of the change in the unperturbed initial core-hole energy relative to the zero-point energy. We explain why in general an ab initio atomic many-body calculation can provide an overall good description of solid-state spectra predominantly governed by the atomic-like localized core-hole dynamics. We explain this in terms of the change from free atom to metal in both U and the zero-point energy (self-energy).

Double-ionization energies of some n-alkyl isocyanate molecules; studies with double-charge-transfer spectrometry and ab initio propagator theory

Double-charge-transfer (DCT) collisions of H +, OH + and F + 3 keV beam ions with a series of alkyl isocyanate molecules were studied using mass spectrometric techniques. Measurement of the kinetic energies of H − ions so produced enabled the determination of double-ionization energies (DIE) for transitions to singlet doubly ionized states of the target molecules; those for triplet doubly ionized states were obtained similarly from measurements of the kinetic energies of OH − and F − ions. Values up to approximately 40 eV were obtained in most cases and were found to be in close agreement with the predictions of ab initio calculations using propagator theory, also presented here. For n-butyl isocyanate (and by implication heavier molecules in the series) the density of doubly ionized states above 30 eV was both observed and predicted to be too large and featureless for meaningful analysis, so establishing an effective upper limit on molecular size for the current application of these techniques. Significant configuration interaction was predicted for the final doubly ionized states, which justified theoretical analysis with a relatively complex method that accounts well for correlation effects.

Structure and Binding Energies of Monohydrated Cd and Cd2+

High-level ab initio calculations have been performed on the neutral Cd·H2O complex, and the dication, Cd2+·H2O. Effective core potentials (ECPs) are employed for cadmium, augmented with a large, flexible polarization space, which also includes diffuse functions. The calculated double ionization energy for Cd is within 0.03 eV of the experimental value, suggesting that the basis set is able to describe both Cd and Cd2+ well. For both complexes, three main structures were considered: C2v with the cadmium atom interacting with the oxygen atom of H2O; C2v with the cadmium atom interacting with both hydrogens of H2O; and planar Cs where the cadmium interacts with only one of the hydrogen atoms of H2O. The global minimum for Cd·H2O is found to be the trans Cs structure, whereas for Cd2+·H2O, the charge−dipole interaction leads to the C2v geometry, with the cadmium interacting with the oxygen atom of H2O being the lowest in energy. Our best values for the binding energies, De, are: Cd·H2O, 134 cm-1 (0.4 kcal ...

Electronic structure studies of six-atom gold clusters

Combined theoretical and experimental studies of the hexagold phosphine-stabilized complex [Au6(PPh3)6][BF4]2 (1) and of related systems are reported. The goal of these studies is to gain a better understanding of how 1 interacts with the TiO2(110) substrate to yield finely dispersed supported Au particles that are effective for practical catalytic reactions. The experimental efforts involved the measurement of the visible-ultraviolet (UV) absorption spectra of 1 and Au(PPh3)Cl in solution. The theoretical efforts involved the determination of the electronic structure of molecular models of 1 based on density functional theory (DFT), Hartree–Fock (HF), and configuration interaction (CI) methods. The CI wave functions and energies were obtained for a range of excited states and were used to simulate the absorption spectra of Au6 and Au62+ clusters. The theoretical CI absorption spectra for Au6 can be correlated with the visible-UV absorption spectra while the theoretical spectra for Au62+ cannot be correlated with the experiments. This suggests, even though the [Au6(PPh3)6] unit of 1 carries a +2 charge, that the Au6 portion is essentially neutral. More direct evidence for this distribution of the ionized charge has been obtained from HF and DFT calculations of the double ionization energies of models of 1. It is found that the energy required to remove two electrons from a bare Au6 cluster is much larger than that from an Au6 cluster with phosphine ligands present; this is again consistent with the +2 charge in 1 being delocalized onto the triphenylphosphine ligands. It is possible that this delocalization of positive charge is responsible for facilitating the adhesion of the gold cluster as finely dispersed particles onto the metal oxide support.

New coincidence techniques to study valence double photoionisation

Abstract Three novel coincidence techniques for the study of single photon double ionisation are described and illustrated by applications in the valence double ionisation energy region. Target gases studied are CS 2 , OCS, furan, SF 6 , CO, O 2 and atomic Kr.

An experimental and computational study of the double ionization of CH 3Cl, CH 2Cl 2, and CHCl 3 molecules to singlet and triplet electronic states of their dications

Experimental and computational methods were applied to study the double ionization of the molecules CH 3Cl, CH 2Cl 2, and CHCl 3. Double-charge-transfer spectroscopy was applied to measure the double-ionization energies of the molecules. The translational energies of the negative ions generated when fast-moving singly charged positive projectile ions acquire two electrons in collisions with the molecules provides information on the states of the dications populated. The use of H +, OH +, and F + projectile ions allowed transitions to singlet and triplet electronic states of the dications to be studied. Double-ionization energies to those states were calculated using the second order algebraic diagrammatic construction Green’s function method. The calculated and measured values for CH 3Cl were found to be in good agreement. The computed data predicted that the density of states for CH 2Cl 2 2+ and CHCl 3 2+ were much higher than for CH 3Cl 2+. On grouping the calculated double-ionization energies to close-lying states together, however, good agreement with the measured values was evident. The combined experimental and computational study reported here thus provides a detailed understanding of the double ionization of the three molecules to singlet and triplet electronic states of their dications.

Experimental and theoretical study of the double ionization of the sulphur hexafluoride molecule to singlet and triplet electronic states of its dication

The results of a combined experimental and theoretical study of the double ionization of sulphur hexafluoride are reported. Double-charge-transfer (DCT) spectroscopy was used in the experimental investigation. Double-electron-capture (DEC) reactions, on which DCT spectroscopy is based, are known to be subject to spin conservation. Consequently, double-ionization energies to singlet and triplet electronic states of SF 6 2+ were measured by using H + and F + projectile ions, respectively. In the theoretical investigation, ADC(2) Green’s function calculations indicated a high density of singlet and triplet electronic states for SF 6 2+. It was found, however, that groups of states could be identified that were well separated from adjacent groups. The average double-ionization energies (DIEs) for these groups are in good agreement with those measured. The combined experimental and theoretical approach has thus provided a considerably improved understanding of the double ionization of the SF 6 molecule.

Single and double electron-impact ionization of dichlorine monoxide (Cl 2O)

Electron-impact single and double ionization of dichlorine monoxide (Cl 2O) have been investigated using time-of-flight mass spectrometry and ion–ion coincidence techniques. Relative partial single ionization cross sections of Cl 2O have been determined for incident electron energies from 30 to 450 eV using time-of-flight mass spectrometry. The experiments detect stable parent ions, Cl 2O +, and the fragmentation products Cl +, ClO +, and Cl 2 +. The formation and fragmentation of the Cl 2O dication has been investigated using ion–ion coincidence techniques coupled with time-of-flight mass spectrometry. The coincidence spectra show that Cl 2O 2+ dissociates to form the following pairs of ions: Cl + + Cl +, O + + Cl + and Cl + + ClO +. Interpretation of the coincidence spectra suggests that there is a multiple kinetic energy release upon dicationic dissociation to form O + + Cl +, indicating that two distinct electronic states of Cl 2O 2+ dissociate to form this ion pair. The experiments also yield a determination of the excitation energy required to form the lowest energy dication electronic state (30.6 ± 1 eV) which dissociates to form Cl + + ClO +. Comparison of this excitation energy with estimated values of the double ionization energy of Cl 2O indicates that this state is probably the ground electronic state of Cl 2O 2+ and, hence, these investigations provide a first estimate for the double ionization energy of Cl 2O.

Search for evidence of double K shell ionization in Mg by single X-ray photon inelastically scattered in the near threshold regime

Electron correlation is studied using multiple ionization techniques. The present work investigates the possibility of extending the use of conventional experimental arrangements in the study of K-shell double ionization phenomena in the X-ray inelastic scattering regime. A Mg sample was studied using a conventional Compton spectrometer. The inelastic scattering spectrum of 59.54 keV incident radiation does not present any discontinuity in the expected double ionization energy while it presents a clear discontinuity in the single ionization energy (X-ray Raman). The discontinuity height and the statistical fluctuation lead to the conclusion that the ratio of double to single ionization cross section is less than 1%.

Vertical double ionization of the sulphur dioxide molecule

Vertical double-ionization energies, for transitions from the ground state of the neutral sulphur dioxide molecule to both singlet and triplet energetically low-lying electronic states of the SO 2 2+ dication, have been studied both experimentally with double-charge-transfer spectroscopy and theoretically with ab initio propagator calculations; consistently good agreement is found between the theoretical and experimental results. The experimental data for transitions to singlet states of SO 2 2+ are also found to be consistent with the results of earlier studies of SO 2 employing Auger spectroscopy, which also match closely the theoretical predictions in the higher-energy range not currently accessible with double-charge-transfer spectroscopy. The results of this paper, therefore, provide a comprehensive survey of the vertical double ionizations of SO 2 up to 50 eV in energy, allowing an analysis of some earlier PIPICO data for SO 2.

Formation and dissociation of the nitric acid dication

Electron-impact double ionization of nitric acid, HNO 3 has been investigated by using ion–ion coincidence techniques coupled with time-of-flight mass spectrometry. The coincidence spectra show that HNO 3 2+ dissociates via a variety of two- and three-body reactions to form the following pairs of ions: OH + + NO +, O + + NO +, OH + + NO 2 +, O + + HNO 2 +, H + + N +, H + + O +, H + + NO +, and H + + NO 2 +. The experimental data are consistent with the HNO 3 2+ ion decaying to form the observed ion pairs via an initial charge-separating bond cleavage followed by the dissociation of the resulting singly charged ions. Interpretation of the coincidence spectra yields a determination of the excitation energy required to form the lowest energy dicationic state that dissociates to form OH + + NO 2 +. Comparison of this excitation energy with estimated values of the double ionization energy of HNO 3 indicate that this state is probably the ground electronic state of HNO 3 2+ and, hence, these investigations provide a first estimate for the double ionization energy of HNO 3.

Calculated and measured double-ionization energies of CH3Br, CH2Br2 and CHBr3 to singlet and triplet electronic states of their dications

Calculated and measured double-ionization energies of CH3Br, CH2Br2 and CHBr3 to singlet and triplet electronic states of their dications