A. Ehlerding

An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+ – e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

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 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.

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.

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.

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.

Dissociative recombination cross section and branching ratios of protonated dimethyl disulfide and N-methylacetamide

Dimethyl disulfide (DMDS) and N-methylacetamide are two first choice model systems that represent the disulfide bridge bonding and the peptide bonding in proteins. These molecules are therefore suitable for investigation of the mechanisms involved when proteins fragment under electron capture dissociation (ECD). The dissociative recombination cross sections for both protonated DMDS and protonated N-methylacetamide were determined at electron energies ranging from 0.001 to 0.3 eV. Also, the branching ratios at 0 eV center-of-mass collision energy were determined. The present results give support for the indirect mechanism of ECD, where free hydrogen atoms produced in the initial fragmentation step induce further decomposition. We suggest that both indirect and direct dissociations play a role in ECD.