A. Källberg

Destruction rate of h3+ by low-energy electrons measured in a storage-ring experiment.

Knowledge of the abundance of H3+ is needed in interstellar and planetary atmospheric chemistry. An important destruction mechanism of H3+ is low-energy electron impact followed by dissociation, but estimates of the reaction rate span several orders of magnitude. As an attempt to resolve this uncertainty, the cross section for dissociative recombination of vibrationally cold H3+ has been measured with an ion storage ring down to collision energies below 1 millielectron volt. A rate coefficient of 1.15 x 10-7 cubic centimeters per second at 300 kelvin was deduced. The cross section scaled with collision energy according to E–1.15, giving thee rate a temperature dependence of T–0.65.

CRYRING — a synchrotron, cooler and storage ring

Abstract CRYRING is a small synchrotron and storage ring equipped with electron cooling. Highly charged ions from the electron beam ion source CRYSIS or singly charged ions from the plasmatron source MINIS are injected via an RFQ into the ring. The facility is in the commissioning phase. Full design energy has been achieved and electron cooling demonstrated both for atomic and molecular ions. The experimental program started in August with two projects, dissociative recombination of H+3 ions and radiative recombination to deuterons. The status as of September 20, 1992, is reported.

First storage of ion beams in the Double Electrostatic Ion-Ring Experiment: DESIREE

We report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (C(n)(-), n = 1, 2, 3, 4) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C2 (-) molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s ± 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10(-14) mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity.

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.

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.

Experimental determination of dissociative recombination of CH2OH+, CD2OD+, and CD2+

Measurements of the cross-sections and branching ratios of the dissociative recombination of the ions CH2OH þ, CD2OD þ and CD2OD þ 2 have been performed at the CRYRING storage ring located in Stockholm, Sweden. Evaluation of the data yielded reaction rate coefficients of: 7.0 10 (T/300) 0.78 cmmol 1 s 1 for CH2OH; 7.5 10 (T/300) 0.70 cmmol 1 s 1 for CD2OD þ and 1.51 10 (T/300) 0.66 cmmol 1 s 1 for CD2OD2 . Calculation of the branching ratios for CH2OH þ and its deuterated isotopologue gave the following results for the DR reaction channels involving C–O bond fissure: H2OþCH (2.2%) and CH2þOH (5.5%) in the reaction of CH2OH þ as well as D2OþCD (5%) and CD2þOD (18%) for the dissociative recombination of CD2OD þ. The remainder of the reaction flux kept the C–O bond intact: 92% for CH2OH þ and 77% for CD2OD þ, respectively. Other recent measurements on the CH3OH þ 2 ion indicate dominating bond breaking between the heavy atoms, which is in contrast to this experiment. For CD2OD þ 2 CO-bond breaking was observed for 43% of the reaction flux.Measurements of the cross-sections and branching ratios of the dissociative recombination of the ions CH2OH+, CD2OD+ and CD2OD2+ have been performed at the CRYRING storage ring located in Stockholm, Sweden. Evaluation of the data yielded reaction rate coefficients of: 7.0 x 10-7( T/300) -0.78 cm3mol-1s -1 for CH2OH+; 7.5 x 10-7(T/300) -0.70 cm3 mol-1s-1 for CD2OD+ and 1.51 x 10-6(T/300)-0.66 cm3 mol-1s-1 for CD2OD2+. Calculation of the branching ratios for CH2OH+ and its deuterated isotopologue gave the following results for the DR reaction channels involving C-O bond fissure: H2O+CH (2.2%) and CH2+OH (5.5%) in the reaction of CH2OH+ as well as D2O+CD (5%) and CD2+OD (18%) for the dissociative recombination of CD2OD+. The remainder of the reaction flux kept the C-O bond intact: 92% for CH2OH+ and 77% for CD2OD+, respectively. Other recent measurements on the CH3OH2+ ion indicate dominating bond breaking between the heavy atoms which is conversely to this experiment. For CD2OD2+ CO-bond breaking was observed for 57% of the reaction flux.