Åsa Larson

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)...

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.

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.

Dissociative recombination of HCO

We report on ab initio calculations relevant for dissociative recombination of HCO+. From accurate quantum chemistry calculations, it is found that the electron collision is driven by capture into Rydberg states. The Renner-Teller effect is not important for higher Rydberg states. From calculated potentials, the effective quantum numbers are fitted in three dimensions. The model of the electron collision is simplified using an adiabatic approximation, where one of the Jacobi coordinates is treated adiabatically. Then the electron collision is described using two different theoretical methods. Preliminary results on autoionization widths for Rydberg states and dissociative recombination cross section are given.

Potential-splitting approach applied to the Temkin–Poet model for electron scattering off the hydrogen atom and the helium ion

The study of scattering processes in few body systems is a difficult problem especially if long range interactions are involved. In order to solve such problems, we develop here a potential-splitting approach for three-body systems. This approach is based on splitting the reaction potential into a finite range core part and a long range tail part. The solution to the Schrodinger equation for the long range tail Hamiltonian is found analytically, and used as an incoming wave in the three body scattering problem. This reformulation of the scattering problem makes it suitable for treatment by the exterior complex scaling technique in the sense that the problem after the complex dilation is reduced to a boundary value problem with zero boundary conditions. We illustrate the method with calculations on the electron scattering off the hydrogen atom and the positive helium ion in the frame of the Temkin–Poet model.

Imaging spectroscopy of recombination fragments of

Dissociative recombination of vibrationally cold has been studied at the ion storage ring CRYRING, with an accurate measurement of the kinetic energy of the recombination fragments. When the incident electron energy exceeds , fragments are observed with a kinetic energy of only . The energetics suggest that this low-energy channel derives from either recombination of the state or recombination of the metastable state. High-quality ab initio calculations have been used to obtain the state vibrational relaxation time (10 ms), excitation energy (2.25 eV) and radiative lifetime (0.030 s). The dissociative recombination cross section has been obtained in a model calculation. The results imply that the state has too small a recombination cross section and too short a radiative lifetime to explain the experimental results, which were obtained after 15 s of storage in CRYRING. We conclude that the 120 meV channel is due to recombination of into the asymptotic limit. Possible mechanisms are discussed.

A diabatic representation of the two lowest electronic states of Li3

Using the Multi-Reference Configuration Interaction method, the adiabatic potential energy surfaces of Li3 are computed. The two lowest electronic states are bound and exhibit a conical intersection. By fitting the calculated potential energy surfaces to the cubic E ⊗ ɛ Jahn-Teller model we extract the effective Jahn-Teller parameters corresponding to Li3. These are used to set up the transformation matrix which transforms from the adiabatic to a diabatic representation. This diabatization method gives a Hamiltonian for Li3 which is free from singular non-adiabatic couplings and should be accurate for large internuclear distances, and it thereby allows for bound dynamics in the vicinity of the conical intersection to be explored.

Quantum interference structures in trapped-ion dynamics beyond the Lamb-Dicke and rotating wave approximations

We apply wave-packet methods to study an ion-trap system imposing neither the rotating wave nor the Lamb-Dicke approximations. By this approach we show the existence of states with restricted phase-space evolution as a genuine consequence of quantum interference between wave-packet fractions. A particular instance of such a state oscillates between maximal entanglement and pure disentanglement between the constitute subsystems, where the characteristic crossover time is very rapid. Over longer time periods the dynamics of these states exhibits collapse-revival patterns with well-resolved fractional revivals in autocorrelation, inversion, and entanglement.

Studies of Dissociative Recombination in CRYRING

This article describes some of the most recent experiments at the ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm University. A comprehensive review of recent work at ion storage rings has recently been published,1 and there are several other reviews of recent date,2,3 as well as the proceedings from the previous dissociative recombination (DR) meeting at Nasslingen in Sweden.4 Finally, a brief review of three-body breakup dynamics has recently been published.5

Branching ratios in dissociative recombination of formyl and isoformyl cations

The energy dependence of the branching ratios in dissociative recombination of DCO+ with a known small admixture of DOC+ has been measured for collision energies from 0 to 25eV using an energy- and position-sensitive surface barrier detector which is designed for the analysis of multi-fragment events occurring in a molecular fragmentation study. The measurements are compared with theoretical calculations on the direct mechanism of dissociative recombination of HCO+ including the contribution from HOC+ at the experimental abundance fraction. At low collision energies, dissociative recombination of HCO+ is dominated by dissociation into H + CO. For collision energies above 2eV there is a transition into dissociation to HC+O, which can be explained by electron capture into resonant states. Signatures of DOC+ dissociative recombination are found experimentally and confirmed by the calculations for HOC+. Three-body breakup becomes important for collision energies above 6eV.