J. Palaudoux

Single photon simultaneous K-shell ionization and K-shell excitation. I. Theoretical model applied to the interpretation of experimental results on H2O

We present in detail a theoretical model that provides absolute cross sections for simultaneous core-ionization core-excitation (K(-2)V) and compare its predictions with experimental results obtained on the water molecule after photoionization by synchrotron radiation. Two resonances of different symmetries are assigned in the main K(-2)V peak and comparable contributions from monopolar (direct shake-up) and dipolar (conjugate shake-up) core-valence excitations are identified. The main peak is observed with a much greater width than the total experimental resolution. This broadening is the signature of nuclear dynamics.

Single photon simultaneous K-shell ionization and K-shell excitation. II. Specificities of hollow nitrogen molecular ions

The formalism developed in the companion Paper I is used here for the interpretation of spectra obtained recently on the nitrogen molecule. Double core-hole ionization K(-2) and core ionization-core excitation K(-2)V processes have been observed by coincidence electron spectroscopy after ionization by synchrotron radiation at different photon energies. Theoretical and experimental cross sections reported on an absolute scale are in satisfactory agreement. The evolution with photon energy of the relative contribution of shake-up and conjugate shake-up processes is discussed. The first main resonance in the K(-2)V spectrum is assigned to a K(-2)π(∗) state mainly populated by the 1s→ lowest unoccupied molecular orbital dipolar excitation, as it is in the K(-1)V NEXAFS (Near-Edge X-ray Absorption Fine Structure) signals. Closer to the K(-2) threshold Rydberg resonances have been also identified, and among them a K(-2)σ(∗) resonance characterized by a large amount of 2s/2p hybridization, and double K(-2)(2σ(∗)/1π/3σ)(-1)1π(∗2) shake-up states. These resonances correspond in NEXAFS spectra to, respectively, the well-known σ(∗) shape resonance and double excitation K(-1)(2σ(∗)/1π/3σ)(-1)1π(∗2) resonances, all being positioned above the threshold.

Auger decay of molecular double core-hole and its satellite states: comparison of experiment and calculation.

Auger decay of the C(2)H(2) double core-hole (DCH) states, including the single-site DCH (C1s(-2)), two-site DCH (C1s(-1)C1s(-1)), and satellite (C1s(-2)π(-1)π∗(+1)) states, has been investigated experimentally using synchrotron radiation combined with multi-electron coincidence method, and theoretically with the assumption of the two-step sequential model for Auger decay of the DCH states. The theoretical calculations can reproduce the experimental two-dimensional Auger spectra of the C(2)H(2) single-site DCH and satellite decays, and allow to assign the peaks appearing in the spectra in terms of sequential two-electron vacancy creations in the occupied valence orbitals. In case of the one-dimensional Auger spectrum of the C(2)H(2) two-site DCH decay, the experimental and calculated results agree well, but assignment of peaks is difficult because the first and second Auger components overlap each other. The theoretical calculations on the Auger decay of the N(2) single-site DCH state, approximately considering the effect of nuclear motion, suggest that the nuclear motion, together with the highly repulsive potential energy curves of the final states, makes an important effect on the energy distribution of the Auger electrons emitted in the second Auger decay.

Triple ionization of CO2 by valence and inner shell photoionization

Spectra of triply ionized CO(2) have been obtained from photoionization of the molecule using soft x-ray synchrotron light and an efficient multi-electron coincidence technique. Although all states of the CO(2) (+++) trication are unstable, the ionization energy for formation of molecular ions at a geometry similar to that of the neutral molecule is determined as 74 ± 0.5 eV.

Spectra of the triply charged ion CS23+ and selectivity in molecular Auger effects

Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS(2)(3+) ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1+/-0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS(2)(2+) show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.

A local chemical environment effect in site-specific Auger spectra of ethyl trifluoroacetate

We have investigated a local chemical environment effect on Auger spectra of ethyl trifluoroacetate (C(4)H(5)F(3)O(2)), using multi-electron coincidence spectroscopy and high-resolution electron spectroscopy. Site-specific KVV Auger spectra for each carbon atom, and for the fluorine and oxygen atoms are presented. The extent of hole localization in the final dicationic states was investigated with the help of theoretical calculations based on a two-hole population analysis. The Auger spectra have been simulated using a statistical approach. It is found that all Auger decays populate mainly localized dicationic states, with the two holes located either on the same fluorine atom or on adjacent fluorine atoms. While the decay of the F 1s hole populates exclusively the former states, the latter class of states is also populated by the decay of the C and O 1s holes.

Theoretical investigations of the N2H2+ cation and of its reactivity

Accurate ab initio calculations have been performed in order to investigate both the stable isomers and the reactivity of the N(2)H(2)(+) cation. In addition to the trans-HNNH(+) isomer already observed in the photoelectron studies, a formaldehyde type (isodiazene cation) and H(2)O(2)-like isomers are found. At the coupled cluster level of theory, the isodiazene cation is calculated to be as stable as trans-HNNH(+). We have also studied the reactivity of N(2)H(2)(+) and its implication on the reactive processes involving N(2)/N(2)(+) and H(2)(+)/H(2), H/H(+) and HN(2)(+)/HN(2), and HN and HN(+) by performing suitable one-dimensional cuts of the six-dimensional potential energy functions of the lowest electronic states of H(2)N(2)(+). We have pointed out the crucial role of this tetratomic intermediate cation and the importance of the short range internuclear distances during these processes. In the case of N(2)/N(2)(+) and H(2)(+)/H(2) reactions, we have shown that the initial orientation of the reactants may influence the N(2)H(2)(+) tetratomic intermediate: One can expect to form the trans isomer preferentially if the internuclear axes of the H(2)/H(2)(+) and the N(2)(+)/N(2) molecules are parallel to each other when these diatoms are colliding and after intramolecular isomerization process. However, if the internuclear axes of the diatomics are perpendicular to each other, the isodiazene cation is formed preferentially. Different branching ratios are expected for each collision scheme. These reactive processes are found to involve vibronic, Renner-Teller and spin-orbit couplings between the electronic states of N(2)H(2)(+). These interactions mix these electronic states, leading to the formation of atomic, diatomic, and triatomic species via the decomposition of the N(2)H(2)(+) intermediate complex.

Theoretical spectroscopy of acetylene dication and its deuterated species.

We mapped the six-dimensional potential energy surface of the electronic ground state of HCCH(++)(X (3)Sigma(g) (-)) dication using the coupled cluster approach. This potential energy surface is incorporated later into perturbative and full variational treatments to solve the nuclear motions. We derived a set of spectroscopic data for HCCH(++), HCCD(++), and DCCD(++). Our calculations reveal the presence of anharmonic resonances even at low energies, which complicates their assignment by vibrational quantum numbers. In light of our theoretical vibrational spectra, we propose an assignment of the experimental vibrationally resolved valence double ionization spectra of HCCH, HCCD, and DCCD. These spectra are viewed to be mostly composed by a pure vibrational progression involving the CC stretching mode together with a second progression involving both the CC stretching and the bendings.

Auger decay of core valence double photoionized states in atomic mercury

Multielectron coincidence spectroscopy has been used to study the Auger decay following the core (4f)—valence (5d, 6s) double photoionization of atomic mercury. Multiconfiguration Dirac–Fock calculations have been performed to predict the transition energies and populations of the triple ionized Auger final states. A good agreement between the experiment and simulated spectra is found.

Dynamics of electron emission in double photoionization processes near the Krypton 3d threshold

Two-electron emission following photoabsorption near the Kr 3d threshold is investigated both experimentally and theoretically. On the experimental side, electron/electron coincidences using a magnetic bottle time-of-flight spectrometer allow us to observe the complete double photo ionization (DPI) continua of selected Kr2+ final states, and to see how these continua are affected by resonant processes in the vicinity of the Kr 3d threshold. The analysis is based on a quantum mechanical approach that takes into account the contribution of three different processes: (A) Auger decay of the inner 3d vacancy with the associated post-collision interaction (PCI) effects, (B) capture of slow photoelectrons into discrete states followed by valence multiplet decay (VMD) of the excited ionic states and (C) valence shell DPI. The dominant process for each Kr2+(4p−2) final state is the photoionization of the inner shell followed by Auger decay of the 3d vacancies. Moreover, for the 4p−2(3P) and 4p−2(1D) final ionic states an important contribution comes from the processes of slow photoelectron capture followed by VMD as well as from double ionization of the outer shell involving also VMD.