Dissociative recombination of the polyatomic ions D3O+ and H3O+ with electrons have been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Absolute cross sections have been determined from 0.001 eV to 0.25 eV center-of-mass energy for D3O+ and from 0.001 eV to 28 eV for H3O+. The cross sections are large (7.3×10−13 cm2 for D3O+ and 3.3×10−12 cm2 for H3O+ at 0.001 eV). At low energies, the cross sections for D3O+ are E−1 energy dependent whereas it is slightly steeper for H3O+. A similar E−1 energy dependence was also observed by Mul et al. [J. Phys. B 16, 3099 (1983)] with a merged electron-ion beam technique for both H3O+ and D3O+ and by Vejby-Christensen et al. [Astrophys. J. 483, 531 (1997)] with the ASTRID storage ring in Denmark, who presented relative cross sections for H3O+. A resonance has been observed around 11 eV for H3O+. It reflects an electron capture to Rydberg states converging to an excited ionic core. A similar structure was reported by Vejby-Christensen et al. Our absolute measurements are in fairly good agreement with those from Mul et al., which were first divided by 2 (Mitchell, 1999, private communication) and from Heppner et al. [Phys. Rev. A 13, 1000 (1976)] for H3O+. Thermal rates were deduced from the measured cross sections for electron temperatures ranging from 50 K to 30 000 K. At 300 K, the thermal rate is equal to 7.6×10−7 cm3 s−1 for H3O+ and to 3.5×10−7 cm3 s−1 for D3O+. Complete branching ratios for all the possible product channels have been determined from 0 eV to 0.005 eV center-of-mass energy for D3O+ and at 0 eV for H3O+, using a well-characterized transmission grid in front of an energy-sensitive surface-barrier detector. No isotope effect was observed within the experimental uncertainties. The three-body break-up channel OX+X+X (where X stands for H or D) is found to occur for 67%–70% of the dissociations. Water or heavy water is produced with an 18%–17% probability and the production of oxygen atoms is negligible. These results support the three-body break-up dominance already found by Vejby-Christensen et al. for the DR of H3O+ in a similar heavy-ion storage ring experiment. However, even if the general trend is the same for both storage rings, significant differences have been observed and will be discussed.

We present experimental data on the dissociative recombination ͑DR͒ and the dissociative excitation ͑DE͒ of O 2 ϩ in its electronic and vibrational ground state using a heavy ion storage ring. The absolute DR cross section has been determined over an electron collision energy range from 1 meV to 3 eV. The thermal DR rate coefficient is derived; ␣(T e )ϭ2.4ϫ10 Ϫ7 (300/T e ) 0.70Ϯ0.01 cm 3 s Ϫ1 , for TϾ200 K. The threshold for DE was observed near its energetic threshold of 6.7 eV. The DE cross section curve has a maximum of 3ϫ10 Ϫ16 cm 2 near 15 eV. We have determined the branching fractions to the different dissociation limits and present atomic quantum yields for the DR process between 0 to 300 meV collision energy. The quantum yield of O( 1 D) is found to be 1.17Ϯ0.05, largely independent of the electron energy. Arguments are presented that the branching fraction to O( 3 P)ϩO( 1 S) is negligible. The branching fraction to the O( 1 S)ϩO( 1 D) is smaller than 0.06 and varies strongly as a function of collision energy. The O( 1 S) quantum yield is a strong function of electron temperature. Hence, the relative strength of the green, O( 1 S), and the red, O( 1 D), airglows may be used as a measure of the electron temperature of the upper atmosphere. A qualitative explanation is given of the consequences of nonadiabatic interactions in the dissociation step of the DR process.

Branching ratios in dissociative recombination of C2H2+ molecular ions with electrons were measured using the CRYRING heavy-ion storage ring. We have determined complete branching ratios for C2H2+ at collision energies between 0 and 7.4 meV. We found dissociative recombination of C2H2+ to be dominated by the two-body C2H+H and three-body C2+H+H channels, with branching ratios of 0.50±0.06 and 0.30±0.05, respectively. The branching to CH+CH was measured to be 0.13±0.01, whereas two other energetically allowed channels were found to be almost negligible.

Rate coefficients and absolute cross sections for center-of-mass energies between 0.0001 and 1 eV are reported for both resonant ion-pair formation and dissociative recombination in electron collisions with HF ϩ ions. The heavy-ion storage ring CRYRING in Stockholm was used for these measurements. Notable is the fact that the dissociative recombination cross section is substantially smaller than that for most diatomic molecular ions. The recombination seems to have an underlying E Ϫ1 energy dependence characteristic of the direct process in dissociative recombination, but both cross sections show structure, which may be attributed to contributions from different indirect processes. The cross sections have no observable energy thresholds. The ratio of the cross section for resonant ion-pair formation to that for dissociative recombination is about 0.25 at 10 Ϫ3 eV, with the ratio depending on the interaction energy, so the competition of the ion-pair process is much stronger than for other ions so far studied. The HF ϩ ion is unique in the fact that the electron affinity of F, the binding energy of HF ϩ , and energy of the atom pair ͓H(nϭ2)ϩF( 2 P 3/2 )͔ are the same within the rotationalenergy spread of the HF ϩ target. The resonant ion-pair formation process, eϩHF ϩ →H ϩ ϩF Ϫ , has some similarities to the photon process, hϩHF→H ϩ ϩF Ϫ , and we discuss comparisons. We deduce thermal rate coefficients from our measurements and discuss them in the context of rate coefficients for other diatomic ions available in the literature.

Experimental data are presented from three different heavy-ion storage rings ͑ASTRID in Aarhus, CRY-RING in Stockholm, and TSR in Heidelberg͒ to assess the reliability of this experimental tool for the extraction of absolute rate coefficients and cross sections for dissociative recombination ͑DR͒. The DR reaction between HD ϩ and electrons has been studied between 0 and 30 eV on a dense energy grid. HD ϩ displays two characteristic local maxima in the DR rate around 9 and 16 eV. These maxima influence the data analysis at smaller collision energies. We conclude that resonant structures in the DR cross sections are reproduced among the experiments within the collision energy resolution. The absolute cross sections agree within the systematic experimental errors of 20% related to the measurement of the ion currents. Absolute thermal rate coefficients for HD ϩ ions are given for an electron temperature range of 50-300 K. Results for the DR cross section and the thermal rate coefficients are compared to recent theoretical calculations including rotational effects, finding satisfactory agreement.