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 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.

This paper presents the first dissociative recombination (DR) measurement of electrons with rotationally and vibrationally cold H 3 + ions. A dc discharge pinhole supersonic jet source was developed and characterized using infrared cavity ringdown spectroscopy before installation on the CRYRING ion storage ring for the DR measurements. Rotational state distributions ͑T rot ϳ 30 K͒ produced using the source were comparable to those in the diffuse interstellar medium. Our measurement of the electron energy dependence of the DR cross section showed resonances not clearly seen in experiments using rotationally hot ions, and allowed calculation of the thermal DR rate coefficient for ions at interstellar temperatures, ␣ DR ͑23 K͒ = 2.6ϫ 10 −7 cm 3 s −1 . This value is in general agreement with recent theoretical predictions by Kokoouline and Greene [Phys. Rev. A 68, 012703 (2003)]. The branching fractions of the two breakup channels, H+H+H and H+H 2 , have also been measured for rotationally and vibrationally cold H 3 + .

Branching ratios and absolute cross sections have been measured for the dissociative recombination of N 2 H + using the CRYRING ion storage ring. It has been found that the channel N 2 H + + e À ! N 2 + H accounts for only 36% of the total reaction and that the branching into the other exoergic pathway, N 2 H + + e À ! NH + N, consequently amounts to 64%. The cross section of the reaction could be fitted by the expression ¼ (2:4 AE 0:4) ; 10 À16 E À1:04AE0:02 cm 2 , which leads to a thermal reaction rate of k(T) ¼ (1:0 AE 0:2) ; 10 À7 (T =300) À0:51AE0:02 cm 3 s À1 , 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 N 2 H + in the dark clouds but a five-to sevenfold increase of the NH concentration as steady state is reached.

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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.