I. Nenner

Theoretical and experimental studies of the triatomic doubly charged ions CO2+2, OCS2+, and CS2+2

The term schemes of the doubly charged ions CO2+2, OCS2+, and CS2+2 have been calculated by the CIPSI method using a new and economical basis set of polarized atomic orbitals and difference orbitals. The calculated energies are compared with existing and new experimental data obtained from Auger spectra, double charge transfer, and photoionization including the PIPICO technique. A complete assignment of the manifolds of singlet states is achieved with good agreement between theory and experiment. Only the lowest doubly charged ion states are well described by simple two‐hole configurations, while three‐hole one‐particle configurations are important at higher energy. It is confirmed that the triplet ground states of these ions are well populated by photoionization, while the excited singlet states are revealed most clearly by the double charge transfer technique.

Photodissociation processes of water vapor below and above the ionization potential

Fluorescence excitation spectra of OH(A), OH(B), OH(C), Lyman α, Balmer α, Balmer β, and H2O+(A) are recorded in H2O in the 9–35 eV photon excitation range. The OH(A) fluorescence quantum yield is measured from 9.2 to 13.6 eV incident photon energy. The pressure dependence of the H2O+(A) fluorescence excitation spectrum is investigated and the quenching rate constant of H2O+(A) by H2O is found to be equal to (4±2)×10−10 cm3 s−1. New Rydberg states are observed: a continuum‐like state in the 10–12.6 eV range assigned to the (3a1)−1 (3pb2) 1B2 state and two broad bands lying at 27–29 and 30–32 eV, respectively, assigned to Rydberg states converging towards the (2a1)−1 innervalence state of H2O+. The valence character of the nsa1 Rydberg states is discussed. The decay processes of valence‐type vs pure Rydberg states are investigated. All energetically accessible dissociation channels producing fluorescent fragments are observed and the fragments appear at their thermodynamical threshold. The competition b...

Single photon double ionization studies of CS2 with synchrotron radiation

Single photon double ionization of CS2 has been investigated, using mass spectrometric coincidence techniques: both metastable and dissociative CS++2 states have been observed; three different dissociation pathways of CS++2 have been demonstrated, including one bond (S++CS+) and two bonds (S++C+S+ and S++C++S) breakings; simulation of the observed dissociation provided some insight into the dissociation mechanisms. Monochromatized synchrotron radiation enabled us to measure the excitation spectra of these relatively intense processes, in the 25–75 eV photon energy range. Our results provide an approach to the spectroscopy of the doubly charged CS++2 ion; comparison with a SCF‐LCAO‐MO calculation leads to a tentative assignment of the observed states.

Autoionization of OCS by threshold photoelectron spectroscopy

Autoionization of carbonyl sulfide between 12 and 16 eV has been investigated by photoionization using the pulsed synchrotron radiation from ACO Orsay’s storage ring. The threshold photoelectron spectrum and the total photoionization spectrum of carbonyl sulfide have been recorded at high resolution in the wavelength range between 112.0 and 65.0 nm (11–19 eV). Threshold energy electrons are observed in specific wavelength regions: (i) at excitation energies where the X, A, B, and C ionic states are formed by a direct process; (ii) in the A state region where resonant autoionization to A can be understood classically within the Franck–Condon approximation; (iii) in the A–X Franck–Condon gap between 90 and 110 nm, where resonant autoionization leads to very sharp electron energy distribution strongly peaked at zero energy. Here the mechanism must be more complex.

Experimental and theoretical study of configuration interaction states of CO+2

Configuration interaction states of CO+2 have been observed as satellites in a high resolution photoelectron spectrum of CO2 using an angle resolved electron spectrometer and far UV synchrotron radiation in the 30–55 eV photon energy range. Ten satellites have been identified in the 22–40 eV binding energy region and classified as the result of outer and inner valence ionization processes on the basis of their asymmetry parameter ( β) values. Theoretical calculations of configuration interaction states of CO+2 using an ab‐initio SCF‐CI method show that three‐hole‐two‐particle excited configurations are necessary to explain the line positions and intensities. The calculated satellite spectrum, corrected for transition moments at 45 and 1254 eV, shows an excellent agreement with experiment and allows assignments of both outer and inner valence satellite lines, with Πg, Πu, Σ+g, and Σ+u symmetries.

Satellites and double ionization continua in the CS2 photoelectron spectrum

The CS+2 valence satellites have been studied with photoelectron spectroscopy using synchrotron radiation in the 35–75 eV photon energy range. Partial cross sections (σ) and asymmetry parameters (β) for most bands have been measured as a function of photon energy. Below 28 eV binding energy, the symmetry of satellite bands has been identified on the basis of their β value and a comparison between experimental photoelectron spectra and new ab initio SCF‐CI calculations of CS+2 corrected for transition moments. For the bands with binding energies above 28 eV no configuration interactions states of CS+2 are predicted with significant intensity. We suggest that they originate from the opening of direct double ionization continua. This is supported by the similarity of their cross sections with the total double ionization one.

Relaxation processes following excitation and ionization of SiF4 in the vicinity of the silicon 2p threshold. II: Dissociation of the molecular ions

Partial ion yields from photoionization of SiF4 have been measured with time‐of‐flight mass spectrometry, photoion–photoion coincidence and triple photoion coincidence techniques using synchrotron radiation from 100 to 136 eV, in the region of the silicon 2p edge where strong resonances are found. From the photon energy dependence of positive ion pairs and of doubly charged fragment intensities, with a suitable normalization procedure, we have estimated the total double photoionization cross section. Below the Si2p edge, the double to single dissociative ionization branching ratio follows the resonance behavior, and is consistent with the photoelectron results of de Souza et al. (Paper I) and discussed in terms of multibond breaking dissociation pathways of residual excited ions (singly and doubly charged) produced by resonant Auger decay processes. Above the Si2p edge, the fragmentation is found to originate from dissociative double ionization occurring after normal Auger processes. The small additional ...

Towards high‐resolution spectroscopy and dynamics studies in complex multi‐ionized molecules

New experimental developments for high‐resolution spectroscopy and dynamics studies of complex multi‐ionized molecules are presented. Using photoion‐photoion coincidence measurements in the doubly ionized C6F6 (hexafluorobenzene) molecule obtained with a conventional time‐of‐flight spectrometer and the ACO storage ring as an example, we demonstrate the importance of improving (i) the light source by using the new storage ring Super‐ACO, (ii) the detection mode by using the photoelectron‐photoion‐photoion coincidence method, and (iii) the mass resolution of the spectrometer by constructing a reflectron time‐of‐flight mass spectrometer.

Autoionization observed in the CO+ (A 2Π–X 2Σ+) and (B 2Σ+–X 2Σ+) fluorescence excitation spectra

The CO+ (A 2Π–X 2Σ+) and (B 2Σ+–X 2Σ+) fluorescence excitation spectra (FES) using VUV synchrotron radiation, have been measured from 16 to 38 eV excitation energy. Total photoionization cross section has also been measured from 17 to 20 eV for comparison. Sharp ionization onsets typical of direct ionization process are observed for both A 2Π and B 2Σ+ thresholds. New autoionization structures have been observed above 20 eV besides the broad shape resonance located at 32 eV in the (B–X) FES. Sharp structures observed above the A 2Π (v=0) threshold are interpreted in terms of electronically induced vibrational autoionization. Branching ratios for electronic autoionization of four Ryberg series converging to B into the CO+ A and X states have been measured. Those states decay preferentially into the CO+ A 2Π state as opposed to X 2Σ+.

N+2(A→X) and N+2(B→X) fluorescence excitation spectra by photon impact

N+2(A 2Πu→X 2Σ+g) and N+2(B 2Σ+u→X 2Σ+g) fluorescence excitation spectra were measured using synchrotron radiation from threshold to 38 eV. Rydberg series converging to N+2 B 2Σ+u is seen to autoionize, with a 85% branching ratio into the A 2Πu continuum. For series converging to B 2Σ+u(v = 1), vibrational autoionization observed into the B(v = 0) continuum, is shown to compete efficiently with electronic autoionization. Above 20 eV, Rydberg series converging to N+2, C 2Σ+u as well as a number of unidentified resonances are shown to be coupled to each or both the A and B ionization continua.