Paul Morin

Shape resonance and electronic autoionization in vibrationally resolved partial photoionization cross sections of O2

Vibrationally resolved partial photoionization cross sections of O2 leading to the b4Σ−g and B2Σ−1g(3σ−1g) tonic states have been measured at high resolution in a wide photon energy range from 0.5 eV above threshold to 24.5 eV. The σu shape resonance is observed around 21.5 eV, in fairly good agreement with “one-electron” theoretical predictions. The series of resonances from 19 to 20 5 eV result from an autoionization process with a strong vibrational selectivity, explained by the similar geometries of the resonance states and the b ionic state. The large resonance widths originate from a strong Rydberg-valence orbital mixing Calculations using multichannel quantum defect theory reproduce the most important features.

Fragmentation of doubly charged argon clusters

Fragmentation of doubly charged argon clusters is reported. Neutral argon clusters are excited with monochromatized synchrotron radiation in the energy regime of the argonL3/L2 absorption edges (240–260 eV) leading predominantly to cluster dication formation. All charged particles are detected in a photoelectron-photoion-photoion-concidence (PEPIPICO) experiment. Symmetric and asymmetric charge separation reactions (Coulomb explosion) are identified for clusters below the critical size of stable dication formation. The peak shapes of the coincidence signals are investigated as a function of neutral cluster size. Characteristic changes in peak shape are observed which are used to derive fragmentation mechanisms involving sequential evaporation of neutrals before and after charge separation. The spectra indicate in accordance with low kinetic energy releases occurring in charge separation of large dissociative cluster dications (Arn2+, withn>50) that due to large charge separation distances the momenta of both singly charged fragments are not any more directed into opposite direction, as it is typical for Coulomb explosion. The results are compared to collision induced fragmentation of mass selected argon cluster dications as well as photon stimulated desorption spectra of condensed argon.

DISSOCIATION DYNAMICS OF CORE-EXCITED BF3 PROBED BY THE PHOTOELECTRON-PHOTOION-PHOTOION COINCIDENCE

Abstract 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.

Photoionization branching ratios and asymmetry parameters for inner- and outer-valence shells of H2S in the 18–70 eV energy range

Abstract Branching ratios, asymmetry parameters and relative partial cross sections have been obtained for photoionization of the outer and the inner valence shells of H 2 S. These measurements were made in the photon energy range 18–70 eV using synchrotron radiation. Our results are compared to a set of calculations using a developed extension of the self-consistent field- X α scattered-wave method to the continuum states. This comparison shows a qualitative agreement between the experimental and calculated β curves of the outer valence shells. The largely predominant sulfur 3 p contribution to the outer valence orbitais 2 b 1 , 5 a 1 , 2 b 2 is revealed in the corresponding β curves by a Cooper minimum also predicted in the same energy range for the β( 3p ) of the atomic sulfur. This comparison also shows discrepancies in the branching ratios curves and we suggest that this theoretical framework is better adapted to predict photoionization processes in the outer valence shells than in the inner ones.

Photodissociation of core excited molecules

Abstract We review several aspects of the fragmentation of photoexcited molecules near shallow core edges especially in polyatomic systems, because of possible site selective processes. We analyse the role of repulsive states of the core excited species itself and of the residual doubly charged molecular ion, using recent experiments on CH 3 Br photoexcited near the bromine 3d edge, as an example. We also show how potential wells in the potential surface of such doubly charged ions can lead to unexpected bond breaking on the basis of recent theoretical calculations on N 2 O 2+ . Finally, we discuss possible localisation or Coulomb explosion mechanisms and some of the limitations of site selective fragmentation processes using some recent experimental results on the BrF 2 C-CF 2 I molecule.

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.

Multipathway dissociation dynamics of core-excited methyl chloride probed by high resolution electron spectroscopy and Auger-electron-ion coincidences

Core excitation triggers nuclear dynamics on the femtosecond time scale. A multiparametric electron/ion coincidence approach has been used to disentangle complex decay processes occurring at short (molecular) or long (atomic) time scales. Methyl chloride has been excited by scanning along the dissociative Cl2p-->sigma* resonance. The detailed chronology of the competing decay processes, leading to either the rearrangement product HCl(+), or an ultrafast dissociation leading to Cl(+), has been investigated. The observed Auger-Doppler shift has been analyzed for various orientations of the electron and fragment ion.

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.

New results on core excited N2O obtained by electron–ion coincidence spectroscopya)

In this paper we show and discuss new results on the dissociation of N2O after core (N 1s) excitation and ionization. We have developed an experimental setup combining an electrostatic electron analyzer with a time‐of‐flight mass spectrometer without angular discrimination, operated with a strong pulsed extraction field. Taking advantage of the pulsed character of the synchrotron radiation light, we have been able to efficiently remove fortuitous coincidences, without perturbing the electron energy analysis. We relate in detail the experimental setup and show new results obtained on 14N15NO. A selective fragmentation is shown depending on the excited site (central or terminal nitrogen) and the influence of the electronic excited state of the ion on the fragmentation pattern is pointed out.