Fabian Österdahl

Dissociative Recombination of N2H+: Evidence for Fracture of the N-N Bond

Branching ratios and absolute cross sections have been measured for the dissociative recombination of N2H+ using the CRYRING ion storage ring. It has been found that the channel N2H+ + e- → N2 + H accounts for only 36% of the total reaction and that the branching into the other exoergic pathway, N2H+ + e- → NH + N, consequently amounts to 64%. The cross section of the reaction could be fitted by the expression σ = (2.4 ± 0.4) × 10-16E-1.04±0.02 cm2, which leads to a thermal reaction rate of k(T) = (1.0 ± 0.2) × 10-7(T/300)-0.51±0.02 cm3 s-1, in favorable agreement with previous flowing afterglow Langmuir probe measurements at room temperature, although our temperature dependence is very different. The implications of these measurements for the chemistry of interstellar clouds are discussed. A standard model calculation for a dark cloud predicts a slight increase of N2H+ in the dark clouds but a five- to sevenfold increase of the NH concentration as steady state is reached.

Dissociative recombination of NH4+ and ND4+ ions: storage ring experiments and ab initio molecular dynamics.

The dissociative recombination (DR) process of NH4+ and ND4+ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of NH4+ and ND4+ in the collision energy range 0.001-1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for NH4+ agrees well with earlier work and is about a factor of 2 larger than the cross section for ND4+. The dissociative recombination of NH4+ is dominated by the product channels NH3+H (0.85+/-0.04) and NH2+2H (0.13+/-0.01), while the DR of ND4+ mainly results in ND3+D (0.94+/-0.03). Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is NH3+H. The ejection of the H atom is direct and leaves the NH3 molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming NH2+H. It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel NH2+2H to isotopic exchange.

DISSOCIATIVE RECOMBINATION OF NITRILE IONS: DCCCN + AND DCCCND +

Branching ratios and absolute cross sections have been measured for the dissociative recombination of DCCCN+ and DCCCND+ using the CRYRING ion storage ring. In the case of DCCCN+ the dissociation y ...

Dissociative recombination of protonated methanol

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH3OH2+ and CD3OD2+ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH3OH2+ and CD3OD2+, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C-O bond, the major process being the three-body break-up forming CH3, OH and H (CD3, OD and D). The overall cross sections are best fitted by sigma = 1.2 +/- 0.1 x 10(-15) E(-1.15 +/- 0.02) cm2 and sigma = 9.6 +/- 0.9 x 10(-16) E(-1.20 +/- 0.02) cm2 for CH3OH2+ and CD3OD2+, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 +/- 0.9 x 10(-7) (T/300)(-0.59 +/- 0.02) cm3 s(-1) (CH3OH2+) and k(T) = 9.1 +/- 0.9 x 10(-7) (T/300)(-0.63 +/- 0.02) cm3 s(-1) (CD3OD2+) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH3+ and H2O and subsequent dissociative recombination of the resulting CH3OH2+ ion to yield methanol and hydrogen atoms is therefore very unlikely.

Rate constants and branching ratios for the dissociative recombination of CO2

Product branching ratios and thermal rate coefficients for the dissociative recombination of CO2+ have been measured in the cryogenic ion source ring ion storage ring. The rate constants were found to be 4.2×10−7(Te∕300)−0.75cm3s−1. The 300-K result is in agreement with previous flowing afterglow values and is somewhat smaller than a recent determination made at the Aarhas storage ring in Denmark (ASTRID) storage ring. The electron temperature dependence is, however, in good agreement with the ASTRID result of T−0.8. The present results show that only CO plus O are formed, other product branching ratios are zero within experimental error. This is in contradiction to the ASTRID results which show that 9% of the reactivity goes to C+O2. The new results show that the C+O2 channel does not need to be included in the models of the ionospheres of Venus and Mars.

Studies of electron cooling with a highly expanded electron beam

Abstract The electron cooler at CRYRING is now operating with a superconducting gun solenoid and an electron beam that is adiabatically expanded with a factor of up to 100. This paper describes the new gun solenoid and electron gun. It presents measurements made on longitudinal cooling forces with different expansion factors, electron densities, magnetic field strengths and beam alignments. It also presents studies of a transverse beam instability that appears when a misalignment is introduced between ion and electron beams. Finally, some measurements of dielectronic recombination that directly yield transverse and longitudinal electron temperatures are discussed.

Dissociative recombination of D+(D2O)2 water cluster ions with free electrons

Dissociative recombination (DR) of the water cluster ion D+(D2O)2 has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Cluster ions were injected into the ring and accelerated to an energy of 2.28 MeV. The stored ion beam was merged with an almost monoenergetic electron beam, and neutral fragments produced by DR were detected by an energy-sensitive surface barrier detector. The first experimental determinations of the absolute DR cross section and branching ratios for a cluster ion are reported. The cross section for the process D+(D2O)2+e− is large and reaches 6⋅10−12 cm2 at a low center-of-mass collision energy of 0.001 eV. The cross section has an E−1.19±0.02 dependence in the energy range 0.001–0.0052 eV, and a steeper slope with an E−1.70±0.12 dependence for E=0.052–0.324 eV. The general trends are similar to the results for previously studied molecular ions, but the cross section is higher in absolute numbers for the cluster ion. Thermal rate coefficients for electron temperatures of 50–2000 K are deduced from the cross section data and the rate coefficients are consequently also large. Branching ratios for the product channels are determined with a grid technique. Break-up into 2D2O+D is the dominating dissociation channel with a probability of 0.94±0.04. The channel resulting in the fragments D2O+OD+D2 has a probability of 0.04±0.02, and the probability for formation of D3O+D2O is 0.02±0.03. The results are compared with data for molecular ions, and the cluster dissociation dynamics are discussed.Dissociative recombination (DR) of the water cluster ion D+(D2O)2 has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Cluster ions were injected into the ring and accelerated to an energy of 2.28 MeV. The stored ion beam was merged with an almost monoenergetic electron beam, and neutral fragments produced by DR were detected by an energy-sensitive surface barrier detector. The first experimental determinations of the absolute DR cross section and branching ratios for a cluster ion are reported. The cross section for the process D+(D2O)2+e− is large and reaches 6⋅10−12 cm2 at a low center-of-mass collision energy of 0.001 eV. The cross section has an E−1.19±0.02 dependence in the energy range 0.001–0.0052 eV, and a steeper slope with an E−1.70±0.12 dependence for E=0.052–0.324 eV. The general trends are similar to the results for previously studied molecular ions, but the cross section is higher in absolute numbers for the cluster ion. Thermal rate coef...

Dissociative recombination branching ratios and their influence on interstellar clouds

Cross sections and branching ratios for the dissociative recombination (DR) reactions of the astrophysically important ions HN2+, HCO+, DOCO+, and SO2+ at reactant kinetic energies from 1 to 1000 meV have been measured using the CRYRING ion storage ring facility at the Manne Siegbahn Laboratory, Stockholm University. Whereas the break-up of the N-N bond leading to NH + N is the major pathway in the DR of HN2+, the analogous reaction in HCO+ almost exclusively leads to H and CO. In the DR of both DOCO+ and SO2+ three-body break-up was observed. Inclusion of the newly measured branching ratios into a standard model on dark interstellar clouds leads to an improvement of the predictions of such models, especially concerning the abundances of nitrogen compounds. The impact of these newly found branching ratios and reaction rates on the chemistry of different astronomical environments like dark clouds, circumstellar envelopes and planetary ionospheres, is discussed.

Dissociative Recombination of the Thioformyl (HCS+) and Carbonyl Sulfide (OCS+) Cations

Branching ratios and absolute cross sections have been measured for the dissociative recombination of HCS+ and OCS+ at the CRYRING ion storage ring. In the case of OCS+, the channel leading to CO + S (83%) dominates, whereas the other exoergic pathways leading to CS + O (14%) and C + SO (3%) are of lesser importance. In the case of HCS+, fracture of the C–S bond is predominant (81%), with the production of H + CS accounting for the remainder (19%). The cross section of the reaction could be fitted by the expressions σ = 1.41 × 10-15E(eV)-1.11 and 4.47 × 10-16E(eV)-1.14 cm2 for HCS+ and OCS+, respectively. The derived energy dependences of the thermal reaction rate coefficients can be fitted by k(T) = 9.7 × 10-7(T/300)-0.57 and 3.5 × 10-7(T/300)-0.62 cm3 s-1 for HCS+ and OCS+, respectively. We use these data to perform model calculations on the HCS+/CS abundance ratio in dark clouds and find that the models using the UMIST and Ohio State University databases have even more difficulty in accounting for the large observed ratio.

Dissociative recombination study of Na+(D2O) in a storage ring

The dissociative recombination of Na(+)(D(2)O) ion has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The cross section has been measured as a function of center-of-mass energy ranging from 1 meV to 0.1 eV and found to have an E(-1.37) dependence. The rate coefficient has been deduced to be (2.3+/-0.32)x10(-7)(T(e)/300)(-0.95+/-0.01) cm(3) s(-1) for T(e)=50-1000 K. The branching ratios have been measured at 0 eV. Of the four energetically accessible dissociation channels, three channels are found to occur although the channel that breaks the weak Na(+)-D(2)O bond is by far dominant.