M. Richard‐Viard

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.

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.

EXPERIMENTAL STUDY OF THE DISSOCIATION OF SELECTED INTERNAL ENERGY IONS PRODUCED IN LOW QUANTITIES : APPLICATION TO N2O+ IONS IN THE FRANCK-CONDON GAP AND TO SMALL IONIC WATER CLUSTERS

Abstract A new experimental apparatus to study gas phase photoionisation of cold species and clusters is available at SuperAco, the Orsay synchrotron radiation facility. This equipment brings together the advantages of synchrotron radiation, supersonic expansions, and photoelectron-photoion coincidence techniques. We present here two types of preliminary results to illustrate some important technical features. The use of multicoincidences to study the photoionisation of N 2 O in the Franck-Condon gap between the X ground state and the A first excited state clearly reveals that the parent ion should be fully predissociated above 15.1 eV. A specific design of a Wiley-McLaren type time-of-flight (TOF) spectrometer for ion detection and analysis allow us to distinguish between the ionic clusters, resulting from direct ionisation of neutrals and those resulting from dissociation of larger ionic clusters, as will be shown for small water clusters, especially for the dimer.

A Born–Oppenheimer‐type separation for the treatment of the dynamics of heavy + light‐heavy systems

The dynamics of heavy + light‐heavy systems are investigated using a Born–Oppenheimer‐type separation between the light and the heavy nuclear motions. The method is illustrated using the IH+I system with the extended LEPS hypersurface A of Manz and Romelt. Hydrogenic wave functions are obtained for clamped iodine nuclei and provide potential energy curves and couplings that govern the I–I motion. Two objectives are pursued, namely, the study of the H‐atom exchange process in the IH (v=0)+I collision at thermal energy and the study of the dissociation of the I2H quasimolecule that follows the photodetachment of the IHI− anion. The proposed approach allows for an understanding of the origin of bound states and resonances involved in these processes. Simple calculations of total cross sections for the H exchange reaction show that they exhibit a strong dependence upon the rotational state of the reactant IH molecule. The widths of quasibound states of I2H are calculated and the rotational levels of the IH mo...

Fluorescence excitation spectrum of the Si–Ar van der Waals complex

We report here the fluorescence excitations spectrum of the Si–Ar van der Waals complex in the region of the (3p4s)3 P–(3p 2)3 P atomic transition. Long progressions are observed, which have been assigned to a Π–Σ transition. The potential curves derived from the analysis of these progressions are discussed in terms of effects of spin–orbit coupling on van der Waals interactions.

Experimental evidence of vibrational mode selectivity in the indirect predissociation of N2O+A 2Σ+. Energy distribution of the diatomic fragment and comparison with a model prediction

The predissociation of the A 2Σ+ state of N2O+ has been reinvestigated with a threshold photoelectron–photoion coincidence experiment using synchrotron radiation as the excitation source and a newly designed spectrometer. The NO+ fragment vibrational distribution resulting from the predissociation of (1,0,0), (0,0,1), (2,0,0), (1,0,1), and (3,0,0) levels varies with vibrational excitation of the parent ion and in all cases is inverted and narrow. Both NO+(X 1Σ+)+N(4S0) and NO+(X 1Σ+)+N(2D0) limits are observed. Predissociation of the A state to the 2 4Σ− dissociative continuum is shown to proceed via an indirect mechanism involving the 1 4Π bound state. Close coupled quantum calculations were carried out with a simplified model for indirect predissociation in which the bending motion is neglected. They show the occurrence of vibrational population inversions in the fragment distributions, as observed experimentally.

Ionic substitution reaction in small dihalogenated benzene-(NH3)n clusters studied through threshold photoelectron-photoion coincidences

Abstract Substitution reactions in small ionic fluorochlorobenzene-(NH 3 ) n clusters have been studied through threshold photoelectron-photoion coincidence techniques using synchroton radiation. In this experiment the internal energy of the cluster ion is well defined and therefore the change of the reactive pathways with the internal energy content of the parent ion can be evidenced.

Isotope effect in the predissociation of the c4Σu- state of O+2

The dissociation of the v=0 and v=1 vibrational levels of the c4 Sigma u- state of O2+ has been reinvestigated using a photoion-photoelectron coincidence method. A different branching ratio for the fragmentation into O+(4S0)+O(3P) and O+(4S0)+O(1D) has been observed for 16O+2 and 18O+2 in the v=0 level. This isotope effect shows that the predissociation to the O+(4S0)+O(1D) adiabatic limit is mainly due to the tunnelling through the c-state potential barrier; no spin-orbit or rotational interaction seems to be involved.

Intracluster ion-molecule reactions induced by the synchrotron radiation in allyl bromide-ammonia clusters

Abstract The ionization of allyl bromide-ammonia clusters produced in a supersonic expansion leads to an intracluster nucleophilic substitution reaction: C3H5Br+ − NH3 → C3H5NH3+ + Br. Larger clusters also react leading to ammonia solvated C3H5NH3+ clusters. The geometries of the complexes and the energetics of the different reactive channels have been computed through ab initio calculations. A barrier to the reaction is evidenced.