M. Lavollée

State-selected ion-molecule reactions: N2+(X, v″), N2+(A, v′) + Ar → N2 + Ar+

Abstract The total, absolute cross sections for charge transfer between N 2 + (X, A, v ) and Ar have been measured at 8, 14 and 20 eV center of mass translational energy. The internal energy of the N 2 + ions was selected by threshold photoelectron-photoion coincidence using pulsed synchrotron radiation from the ACO storage ring. The vibrational levels investigated were v ″ = 0−4 for the X state, and v ′ = 0−6 for the A state. The data for the A state were corrected for the fraction of ions which fluoresced to the X state prior to reaction with Ar. The N 2 + (X, v ″ = 0) state was found to be much less reactive (by a factor ≥ 10) than the other X-state levels, at all three translational energies. The levels N 2 + (A, v ′ ⩾ 3) were found to react with cross-sections which depend strongly on the relative translational energy. The data are interpreted in terms of the interaction between vibronic curves as discussed by Bauer, Fisher and Gilmore. This model accounts well for the low reactivity of the N 2 + (X, 0) level. It is proposed that the variation in the cross section of the N 2 + (A, v ′ ⩾ 3) + Ar reactions is a result of competition with a radiationless transition which converts N 2 + (A, v ′) ions into N 2 + (X, v ″) ions.

Threshold photoelectrons coincidence spectroscopy of N22+ and CO2+ ions

Abstract Threshold photoelectrons coincidence (TPEsCO) spectra of the nitrogen and carbon monoxide dications have been recorded with improved resolution (≈ 40 meV) allowing us to clearly resolve the lowert three vibrational progressions in each species. For the nitrogen dication we have identified, for the first time, the 3 Σ g − state predicted by theory and determined its constants. In the case of the CO dication, we have determined that the ν = 0 level of the 1 1 Σ + state is almost degenerate with X 3 Π, ν = 1 and is thus one vibrational quantum lower than previously believed.

Dissociative double photoionization of CO2 molecules in the 36–49 eV energy range: angular and energy distribution of ion products

Dissociative double photoionization of CO(2), producing CO(+) and O(+) ions, has been studied in the 36-49 eV energy range using synchrotron radiation and ion-ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO(+) and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO(2)(2+) dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO(+) and O(+) product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.

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.

Double photoionization of CO2 molecules in the 34-50 eV energy range.

The double photoionization of CO(2) molecules has been studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. Three processes have been observed: (i) the formation of the CO(2)(2+) molecular dication, (ii) the production of a metastable (CO(2)(2+))* that dissociates, with an apparent lifetime of 3.1 micros, giving rise to CO(+) and O(+) ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 micros. The relative dependence on the photon energy of the cross section for such processes has been measured. While for the production of the molecular dication a threshold is observed, in agreement with the vertical threshold for double ionization of CO(2), for the dissociative processes the threshold appears to be lower than that value, indicating the presence of an indirect dissociation, probably leading to the formation of CO(+) together with a neutral autoionizing oxygen atom.

Mechanism of collision-induced intersystem crossing in CO

Abstract We have studied inert-gas pressure effects on the fluorescence decay in CO selectively excited to the υ = 0 to 7 vibronic levels of the A 1 Π electronic state. It is shown that the dependence of the quenching cross section σ isc on the average value of the ST mixing coefficient (β 2 ) has a quasi-logarithmic form. A simple two-level model describing semiquantitatively this behavior is proposed.

Vibrational relaxation by reversible intersystem crossing

Abstract The time- and energy-resolved fluorescence spectra of CO obtained under selective excitation of the A 1 Π (υ′ = 1) level have been studied. The apparent vibrational relaxation in the singlet manifold is shown to be mainly due to the three-step process: the singlet → triplet crossing from υ′ = 1 level, vibrational relaxation in the triplet manifold and reverse crossing to the A 1 Π (υ′ = 0) level. The decay form may be fitted by assuming the relaxation rate constants in triplet (and singlet) manifolds of the order of 10 5 s −1 Torr −1 i.e. smaller by one order of magnitude than previously proposed.

Double photoionization of below the double ionization potential

Three separate experiments demonstrate that an indirect process of double ionization can populate part of the potential energy surface of the ion at energies below the lowest bound vibrational level. The process seems to involve autoionization at a curve crossing of a singly ionized (or possibly neutral) state with a repulsive part of the adiabatic ground state doubly charged ion surface.

Normal incidence monochromator for synchrotron radiation

Abstract The design of a normal incidence monochromator is described, taking into account the small divergence and the polarization of the synchrotron radiation in order to raise at a maximum the photon flux and the polarization of the light on the exit slit. It is quite common to see a succession of very different experiments at the extremity of a given beam line of a synchrotron radiation laboratory. The required wavelength range and band pass of the exciting light may vary a lot from one experiment to another. We will describe a method we have used to modify completely the characteristics of the monochromator by using the properties of holographic concave gratings, without any mechanical modification.

Post-collision interaction effects in near-threshold Ar -shell photoionization

Near-threshold photoionization of argon in the () inner-shell region has been studied by means of threshold photoelectron - photoion coincidence spectroscopy. The observed line profiles are clearly influenced by the post-collision interaction effect and this influence is observed to be stronger for double than for single-Auger decay. The spectra have been interpreted using a classical theory which takes into account the possibility that a single photoelectron can interact with several Auger electrons.