Magnus af Ugglas

Dissociative recombination of highly enriched para-H-3(+)

The determination of the dissociative recombination rate coefficient of H(3) (+) has had a turbulent history, but both experiment and theory have recently converged to a common value. Despite this convergence, it has not been clear if there should be a difference between the rate coefficients for ortho-H(3) (+) and para-H(3) (+). A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of H(3) (+) in the diffuse interstellar medium, where H(3) (+) has been widely observed. We present the results of an experiment at the CRYRING ion storage ring in which we investigated the dissociative recombination of highly enriched ( approximately 83.6%) para-H(3) (+) using a supersonic expansion source that produced ions with T(rot) approximately 60-100 K. We observed an increase in the low energy recombination rate coefficient of the enriched para-H(3) (+) by a factor of approximately 1.25 in comparison to H(3) (+) produced from normal H(2) (ortho:para=3:1). The ratio of the rate coefficients of pure para-H(3) (+) to that of pure ortho-H(3) (+) is inferred to be approximately 2 at low collision energies; the corresponding ratio of the thermal rate coefficients is approximately 1.5 at electron temperatures from 60 to 1000 K. We conclude that this difference is unlikely to have an impact on the interstellar ortho:para ratio of H(3) (+).

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.

The ionospheric oxygen Green airglow: Electron temperature dependence and aeronomical implications

The laboratory measurement of processes in- volved in terrestrial airglows is essential in developing di- agnostic tools of the dynamics and photochemistry of the upper atmosphere. Dissociative electron recombination of O + in the ionospheric F-region is expected to produce both O( 1 D) and O( 1 S) which are the sources of the 630.0 nm red airglow and the 557.7 nm green airglow lines, respectively. We present both theoretical and experimental evidence, the latter from a heavy ion storage ring technique, that the O( 1 S) quantum yield from O + (v = 0) is a strong function of the electron temperature due to a molecular resonance phenomenon. At present the O + (v = 0) theoretical and laboratory recombination data cannot explain rocket obser- vations of the ionospheric green and red airglows (Takahashi et al. 1990; Sobral et al. 1992).

Investigating the breakup dynamics of dihydrogen sulfide ions recombining with electrons

This paper presents results concerning measurements of the dissociative recombination (DR) of dihydrogen sulfide ions. In combination with the ion storage ring CRYRING an imaging technique was used to investigate the breakup dynamics of the three-body channel in the DR of 32SD2(+). The two energetically available product channels S(3P) + 2D(2S) and S(1D) + 2D(2S) were both populated, with a branching fraction of the ground-state channel of 0.6(0.1). Information about the angle between the two deuterium atoms upon dissociation was obtained together with information about how the available kinetic energy was distributed between the two light fragments. The recombination cross sections as functions of energy in the interval of 1 meV to 0.3 eV in the center-of-mass frame are presented for 34SH2(+). The thermal rate coefficient for the DR of 34SH2(+) was determined to be (4.8+/-1.0) x 10(-7)(T/300)(-0.72+/-0.1) cm3 s(-1) over this interval.

Dissociative recombination of C2H+ and C2H4+: Absolute cross sections and product branching ratios

Dissociative recombination (DR) of the hydrocarbon ions C2H+ and C2H4+ has been examined at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm, Sweden). Absolute DR cross sections were measured for center-of-mass collision energies between 1 meV and 0.1 eV for both ions, giving cross sections at 1 meV of 7.6 × 10−13 cm2 for C2H+ and 1.7 × 10−12 cm2 for C2H4+. The dissociative recombination branching ratios were determined at minimal collision energy, showing that the carbon bond is broken in 57% of the dissociative recombination events for C2H+, while this happens in only 7% of the events for C2H4+. In the case of C2H4+, three-particle breakup into C2H2 and 2H is the dominant product channel with a branching ratio of 66%. The present results are compared with previous DR measurements made at CRYRING for DR of C2H2+ and C2H3+.

Dissociative Recombination of CH4

CH4(+) is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the common destruction mechanisms for molecular ions in these regions, dissociative recombination (DR), is somewhat uncertain. Here, we present absolute measurements for the DR of CH4(+) made using the heavy ion storage ring CRYRING in Stockholm, Sweden. From our collision-energy dependent cross-sections, we infer a thermal rate constant of k(Te) = 1.71(±0.02) × 10(–6)(Te/300)(−0.66(±0.02)) cm3 s(–1) over the region of electron temperatures 10 ≤ Te ≤ 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH4 (0.00 ± 0.00); CH3 + H (0.18 ± 0.03); CH2 + 2H (0.51 ± 0.03); CH2 + H2 (0.06 ± 0.01); CH + H2 + H (0.23 ± 0.01); and CH + 2H2 (0.02 ± 0.01), indicating that two or more C–H bonds are broken in 80% of all collisions.

Dissociative recombination of the acetaldehyde cation, CH3CHO+

The dissociative recombination of the acetaldehyde cation, CH(3)CHO(+), has been investigated at the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm, Sweden. The dependence of the absolute cross section of the reaction on the relative kinetic energy has been determined and a thermal rate coefficient of k(T) = (1.5 ± 0.2) × 10(-6) (T/300)(-0.70±0.02) cm(3) s(-1) has been deduced, which is valid for electron temperatures between ∼10 and 1000 K. The branching fractions of the reaction were studied at ∼0 eV relative kinetic energy and we found that breaking one of the bonds between two of the heavy atoms occurs in 72 ± 2% of the reactions. In the remaining events the three heavy atoms stay in the same product fragment. While the branching fractions are fairly similar to the results from an earlier investigation into the dissociative recombination of the fully deuterated acetaldehyde cation, CD(3)CDO(+), the thermal rate coefficient is somewhat larger for CH(3)CHO(+). Astrochemical implications of the results are discussed.

Investigation into the vibrational yield of OH products in the OH+H+H channel arising from the dissociative recombination of H3O+

The vibrational population of the hydroxyl radical, OH, formed in the OH+H+H channel arising from the dissociative recombination of the hydronium ion, H(3)O(+), has been investigated at the storage ring CRYRING using a position-sensitive imaging detector. Analysis shows that the OH fragments are predominantly produced in the v=0 and v=1 states with almost equal probabilities. This observation is in disagreement with earlier FALP experiments, which reported OH(v=0) as the dominant product. Possible explanations for this difference are discussed.

Complete dissociation branching fractions and Coulomb explosion dynamics of SO2 induced by excitation of O 1s pre-edge resonances

Fragmentation processes of SO2 following excitation of the six main O 1s pre-edge resonances, as well as above the ionization threshold and below the resonances, are studied using a position-sensitive time-of-flight ion imaging detector, and the associated dissociation branching ratios and break-up dynamics are determined. In order to distinguish between the O(+) and S(2+) fragments of equal mass-to-charge ratio, the measurements have been performed with the isotopically enriched S(18)O2 sample. By analysis of the complete set of the fragment momentum vectors, the β values for the fragments originating from the SO(+) + O(+) break-up and the kinetic energy release for fragmentation channels of both SO2 (2+) and SO2 (3+) parent ions are determined. We also present results on the three-body break-up dynamics.