P. M. Guyon

Double photoionization of SO2 and fragmentation spectroscopy of SO2++ studied by a photoion-photoion coincidence method

Abstract Doubly charged sulphur dioxide cations (SO 2 ++ ) are produced by photoionization with synchrotron radiation from ACO in the excitation-energy range 34–54 eV. A new photoion-photoion coincidence (PIPICO) experiment is described in which coincidences between photoion fragments originating from the dissociation of the doubly charged parent cation are counted. This PIPICO method enables us to study the fragmentation of individual electronically excited states of SO 2 ++ and to determine the corresponding absolute double-photoionization partial cross sections as a function of the excitation energy. A tentative assignment of the three observed α, β and γ SO 2 ++ states is given. The dissociation processes of the α and β states into the products SO + + O + are found to be non-statistical in nature; the γ state dissociates completely into three atomic fragments S + + O + + O. Three main observed features of the double-photoionization cross-section curves are discussed in the text: appearance potentials, linear threshold laws, and constant double-photoionization cross sections relative to the total ionization cross section at high energies.

Non‐Franck–Condon transitions in resonant autoionization of N2O

Autoionization of N2O between 12.89 and 16.4 eV was investigated by photoionization using the pulsed synchrotron radiation from ACO, Orsay’s storage ring. Measurements were performed of threshold photoelectron spectra, photoionization spectra, and of photoelectron energy spectra. The latter were obtained from photoelectron time of flight distributions at selected wavelengths. The results suggest that autoionization in the Franck–Condon gap between the ? 2Π and the ? 2Σ+ states of N2O+ proceeds via two distinct mechanisms. The major autoionization process (?90%) produces the ? 2Π state in its low vibrational levels, while a resonant autoionization path (?10%) produces vibrationally excited ? 2Π ions. The latter process is associated with the production of low energy electrons with a distribution peaking sharply at zero energy. This resonant autoionization process appears to be a general phenomena for polyatomic molecules.

A threshold photoelectron–photoion coincidence study of the N2O+ dissociation between 15 and 20.5 eV

Branching ratios and the kinetic energy released in the various fragmentation channels of energy selected N2O+ (15–20.5 eV) were investigated by the technique of threshold photoelectron–photoion coincidence. Pulsed synchrotron radiation from ACO, Orsay’s storage ring, dispersed by a monochromator, was used as a photon source. Threshold electrons were energy selected on the basis of angular and temporal discrimination against energetic electrons. The energy region below 16.388 eV and the ? state were investigated in detail. Below the ? state the most abundant fragment ion is O+, while above the ? state NO+ dominates. Results for the ? and ? states are also reported.

Observation of dissociative states of O2+ by threshold photoelectron-photoion coincidence

Using monochromatised synchrotron radiation as an excitation source, threshold photoelectron-photoion coincidence experiments have been performed on energy-selected states of O2+. Predissociation is observed for the b4 Sigma g- and c4 Sigma u- states. Two dissociation continua are observed between 23 and 24.5 eV.

O+2 ions dissociation studied by threshold photoelectron–photoion coincidence method

The threshold photoelectron–photoion coincidence method was used to study O+2 dissociation. Previous results were confirmed and a careful analysis of the time of flight peak shapes using a Monte Carlo simulation gave us new results. The lifetime of the B 2Σ−g state was measured to be 70∓25 ns independently of the vibrational quantum number. The III 2∏u state which dissociates towards the O+(2D0)+O(3P) and O+(2P0)+O(3P) limits shows anisotropic distribution of fragment ions. The c 4Σ−u(v=0) state which was previously observed to dissociate into O+(4S0)+O(1D), its adiabatic limit, is seen to predissociate also about 40% towards the ground state limit.

A fano-profile study of the predissociation of the 3pπ D1Πu+ state of H2

Abstract The predissociation of H 2 molecules in the 3pπ D 1 Π u + state has been studied by Fano-profile analysis of the R(0) and R(1) lines from υ = 3 to υ = 11. The predissociation probabilities have been measured. Strong indication of the opening of a new channel of dissociation is reported. Coupling with the B υ B 1 Σ u + state may be responsible for this.

Predissociation of the 1Π±u states of H2: measurement of the various dissociation yields

Abstract The predissociation behaviour of the n pπ 1 Π u levels ( n = 3 to 9) has been investigated by simultaneous observation of the absorption and Lyα excitation spectra. The predissociation yields were deduced from a peak-to-peak comparison on some 100 lines in the wavelength range from 840 to around 765 A. Most of the 1 Π + u levels are either partially or totally predissociated while the 1 Π − u are not. A theoretical interpretation is given.

A state-selected study of Ar+(2P32,12) O2 charge transfer at collision energies below 4 eV using synchrotron radiation and guided beam techniques

Abstract An experimental method is presented which combines state selection of reactant ions by threshold photoelectron-photoion coincidence following VUV photoionization with radiofrequency octopole ion guides. Absolute cross sections were measured for the charge transfer between state-selected Ar+(2Pj) ions and molecular oxygen as a function of collision energy in the 0.4–4 eV (CM) range. A critical analysis of the absolute values of both this study and studies reported in the literature is given, in which possible artifacts are identified mainly from coincidence time-of-flight spectra of O2+ product ions. Charge transfer mechanisms are discussed as a function of collision energy and fine structure state of Ar+ reactant ions.

Physics of molecular ion formation.

A review of the basic processes in molecular photoionization is made in the light of the classical as well as recent new theoretical models and illustrated on O(2) and N(2)O.

H(2s) and H(2p) branching ratio in the photodissociation of H2 near threshold

The excitation spectrum for producing Lyman‐α fluorescence by photodissociating H2 has been obtained with 0.12 A bandwidth in the dissociation threshold region. At 839 A, the peak in the fluorescence cross section, one observes Lyman‐α emission from several continuums and from the J=1 and J=2 predissociated rotational levels of the D 1Π+u(v′=3) state. At this wavelength the branching ratio σ (2s)/[σ (2s)+σ (2p)] was obtained using a quenching field method. It was determined that 57% of the excited atoms are produced in the metastable 2 2S1/2 state. From a study of the fluorescence signal versus pressure a cross section of 0.9×10−14 cm2 was derived from the chemi‐ionization reaction H2+H(2s) →H+3+e−.