A. Le Padellec

Dissociative recombination of D3O+ and H3O+: Absolute cross sections and branching ratios

Dissociative recombination of the polyatomic ions D3O+ and H3O+ with electrons have been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Absolute cross sections have been determined from 0.001 eV to 0.25 eV center-of-mass energy for D3O+ and from 0.001 eV to 28 eV for H3O+. The cross sections are large (7.3×10−13 cm2 for D3O+ and 3.3×10−12 cm2 for H3O+ at 0.001 eV). At low energies, the cross sections for D3O+ are E−1 energy dependent whereas it is slightly steeper for H3O+. A similar E−1 energy dependence was also observed by Mul et al. [J. Phys. B 16, 3099 (1983)] with a merged electron-ion beam technique for both H3O+ and D3O+ and by Vejby-Christensen et al. [Astrophys. J. 483, 531 (1997)] with the ASTRID storage ring in Denmark, who presented relative cross sections for H3O+. A resonance has been observed around 11 eV for H3O+. It reflects an electron capture to Rydberg states converging to an excited ionic core. A similar structure was reported by Vejb...

Dissociative recombination and excitation of N2+: Cross sections and product branching ratios

The absolute dissociative recombination and absolute dissociative excitation rate coefficients and cross sections have been determined for N2+ and electrons for collision energies between 10 meV and 30 eV. The ion storage ring CRYRING has been used in combination with an imaging technique with a position-and-time-sensitive detector. Information is retrieved on the ion beam vibrational state populations and on the product branching in the dissociative recombination process at 0 eV collisions. A hollow cathode ion source has been used to lower the vibrational excitation in the ion beam; a more traditional hot-cathode ion source was used as well. The most important findings are the following. The rate coefficient for an N2+ ion beam (46%, v=0, 27% v=1) versus electron temperature (K) is α(Te)=1.75(±0.09)×10−7(Te/300)−0.30 cm3 s−1. The dissociative recombination rate is found to be weakly dependent on the N2+ vibrational level. At 0 eV collision energy, the v=0 product branching is found to be 0.37(8):0.11(6)...

Dissociative Recombination and Excitation of CH5+: Absolute Cross Sections and Branching Fractions

The heavy-ion storage ring CRYRING was used to measure the absolute dissociative recombination and dissociative excitation cross sections for collision energies below 50 eV. Deduced thermal rates coefficients are consistent with previous beams data but are lower by a factor of 3 than the rates measured by means of the flowing afterglow Langmuir probe technique. A resonant structure in dissociative recombination cross section was found at 9 eV. We have determined the branching fractions in DR of CH+5 below 0.2 eV. The branching is dominated by three-body CH3 + H + H and CH2 + H2 + H dissociation channels, which occur with branching ratios of ≈ 0.7 and ≈ 0.2, respectively; thus methane is a minor species among dissociation products. Both the measured absolute cross sections and branching in dissociative recombination of CH+5 can have important implications for the models of dense interstellar clouds and abundance of CH2, CH3 and CH4 in these media.

Branching Fractions in Dissociative Recombination of CH2

The absolute cross section and branching ratios for dissociative recombination of CH -->+2 with electrons have been measured by means of the heavy-ion storage ring CRYRING. Contrary to what has been previously believed, recombination of CH -->+2 is dominated by the three-body channel C + H + H (63%), whereas breakup into the CH + H and C + H2 channels occurs with branching ratios of 25% and 12%, respectively. The thermal rate coefficient for dissociative recombination at 300 K is 6.4 × 10-7 cm3 s-1, which is higher by a factor of 2.5 than the value used in modeling dark molecular clouds. The low CH production and the high production of energetic carbon atoms could be favorable factors for the turbulence model to explain the large abundance of interstellar CH+. The cross section for dissociative excitation was also measured and found to be in good agreement with results from a crossed electron-ion beam experiment.

Branching Fractions in Dissociative Recombination of CH{sup {plus}} {sub 2}

The absolute cross section and branching ratios for dissociative recombination of CH -->+2 with electrons have been measured by means of the heavy-ion storage ring CRYRING. Contrary to what has been previously believed, recombination of CH -->+2 is dominated by the three-body channel C + H + H (63%), whereas breakup into the CH + H and C + H2 channels occurs with branching ratios of 25% and 12%, respectively. The thermal rate coefficient for dissociative recombination at 300 K is 6.4 × 10-7 cm3 s-1, which is higher by a factor of 2.5 than the value used in modeling dark molecular clouds. The low CH production and the high production of energetic carbon atoms could be favorable factors for the turbulence model to explain the large abundance of interstellar CH+. The cross section for dissociative excitation was also measured and found to be in good agreement with results from a crossed electron-ion beam experiment.

Recombination of simple molecular ions studied in storage ring: dissociative recombination of H2O+

Dissociative recombination of vibrationally relaxed H2O+ ions with electrons has been studied in the heavy-ion storage ring CRYRING. Absolute cross-sections have been measured for collision energies between 0 eV and 30 eV. The energy dependence of the cross-section below 0.1 eV is found to be much steeper than the E-1 behaviour associated with the dominance of the direct recombination mechanism. Resonant structures found at 4 eV and 11 eV have been attributed to the electron capture to Rydberg states converging to electronically excited ionic states. Complete branching fractions for all dissociation channels have been measured at a collision energy of 0 eV. The dissociation process is dominated by three-body H + H + O breakup that occurs with a branching ratio of 0.71.

A Storage Ring Study of Dissociative Excitation and Recombination of D3

Dissociative recombination and excitation of D3+ have been studied in CRYRING, a heavy-ion storage ring at the Manne Siegbahn Laboratory at Stockholm University. The measured cross section for dissociative recombination was used to deduce a 300 K rate constant of 2.7 × 10-8cm3s-1. This is a factor of four smaller than the corresponding value for H3+ measured earlier in CRYRING. Dissociative excitation into both the D and 2D channels (D + D or D2) were studied. The 2D channel occurs at energies below threshold for the ions dissociative states, which indicates that resonant enhanced dissociative excitation via autoionizing resonances takes place.

Statistical universal branching ratios for cosmic ray dissociation, photodissociation, and dissociative recombination of the C, CH and C3H2 neutral and cationic species

Context. Fragmentation-branching ratios of electronically excited molecular species are of first importance for the modeling of gas phase interstellar chemistry. Despite experimental and theoretical efforts that have been done during the last two decades there is still a strong lack of detailed information on those quantities for many molecules such as Cn ,C n Ho r C 3H2. Aims. Our aim is to provide astrochemical databases with more realistic branching ratios for Cn (n = 2t o 10), Cn H( n = 2t o 4), and C3H2 molecules that are electronically excited either by dissociative recombination, photodissociation, or cosmic ray processes, when no detailed calculations or measurements exist in literature. Methods. High velocity collision in an inverse kinematics scheme was used to measure the complete fragmentation pattern of electronically excited Cn (n = 2 to 10), Cn H( n = 2t o 4), and C 3H2 molecules. Branching ratios of dissociation where deduced from those experiments. The full set of branching ratios was used as a new input in chemical models and branching ratio modification effects observed in astrochemical networks that describe the dense cold Taurus Molecular Cloud-1 and the photon dominated Horse Head region. Results. The comparison between the branching ratios obtained in this work and other types of experiments showed a good agreement. It was interpreted as the signature of a statistical behavior of the fragmentation. The branching ratios we obtained lead to an increase of the C3 production together with a larger dispersion of the daughter fragments. The introduction of these new values in the photon dominated region model of the Horse Head nebula increases the abundance of C3 and C3H, but reduces the abundances of the larger Cn and hydrocarbons at a visual extinction AV smaller than 4. Conclusions. We recommend astrochemists to use these new branching ratios. The data published here have been added to the online database KIDA (KInetic Database for Astrochemistry, http://kida.obs.u-bordeaux1.fr).

Electric dipole moments and polarizabilities of single excess electron sodium fluoride clusters: Experiment and theory

In this article we present the first measurement of the electric dipole susceptibility of one excess electron NanFn–1 clusters. The static electronic polarizability and the permanent electric dipole of these clusters have been calculated with a one-electron model. Experimental values for the susceptibility are clearly related to the calculated values of the permanent dipole. The size evolution of the dipole moments is interpreted in terms of the interplay between the lattice and electron properties. This study outlines that the response of the cluster to the electric field cannot be fully understood with only equilibrium structure calculations and that the coupling between the permanent dipole and the vibrational motion of the cluster has to be taken into account

Dissociative recombination of 3HeH+: comparison of spectra obtained with 100, 10 and 1 meV temperature electron beams

The heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory at Stockholm University has been used for the study of dissociative recombination of 3HeH+. The new adiabatically expanded electron beam at CRYRING, which is achieved by means of a superconducting magnet, was used. The electron-beam expansion factor of 100 gave a transverse electron temperature of about 1 meV. This allowed the observation of several new resonances in the recombination cross-section.