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 + .

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.

A laser probing method has been used in an ion storage ring to measure the lifetimes of the metastable and levels in . The lifetimes obtained were s for and s for . The most accurate results from ion trap measurements differ by 10% from each other. Our value for supports the longer lifetime value reported from Werth's group. For the level, the new measurement is consistent with previous results.