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Knowledge of the abundance of H(3)(+) is needed in interstellar and planetary atmospheric chemistry. An important destruction mechanism of H(3)(+) is low-energy electron impact followed by dissociation, but estimates of the reaction rate span several orders of magnitude. As an attempt to resolve this uncertainty, the cross section for dissociative recombination of vibrationally cold H(3)(+) has been measured with an ion storage ring down to collision energies below 1 millielectron volt. A rate coefficient of 1.15 x 10(-7) cubic centimeters per second at 300 kelvin was deduced. The cross section scaled with collision energy according to E(-1.15), giving thee rate a temperature dependence of T(-0.65).

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5) IThe cross section for neutral-channel dissociative recombination of vibrationally cooled H3 at energies between 0.0025 and 30 eV has been measured by utilizing the electron cooler at the storage ring CRYRING. The molecular ions were stored at an energy of 32.6 MeV and phase-space cooled by the electron cooler for about 8 s prior to data taking. A peak in the cross section at 9.5 eV is interpreted as a direct adiabatic neutral-channel recombination through the 2 A1 resonant state. The cross section below 1 eV is in rather good agreement with single-pass data.PACS numbers: 34.80.Kw Few cross sections in molecular physics have received such attention as that for the process of dissociative recombination (DR) of H3"*" (cf. Ref. 1). The reason for this is twofold. First, H3 + is the most important molecular ion for the chemistry in interstellar clouds, where it is formed from reactions between neutral and ionized hydrogen and can readily protonize oxygen, carbon, and other heavy atoms. This is the beginning of the formation of nearly one hundred different molecules. The dominant loss process of H3"*" in this environment is dissociative recombination with low-energy electrons. H3 + has also been observed in other astrophysical objects such as the Jovian atmosphere [2] and supernova 1987A [3], The H3 + abundance in these sources depends critically on the DR cross section. Second, there is a 4 orders of magnitude spread between different laboratory measurements of the DR destruction rate [4][5][6][7][8][9][10][11][12][13][14][15]. Theoretical calculations favor a small recombination rate [16,17]. Figure 1 shows potential curves of H3 + and H3 relevant for the process of dissociative recombination.The merged-beam technique used by Mitchell and coworkers [1,6,7,12] offers the cleanest conditions so far for studying collisions between electrons and H3 4 " at welldefined, low center-of-mass energies (0.01-1.0 eV). There is clear experimental and theoretical evidence that the DR cross section of H3 + depends critically on the initial vibrational state distribution [1,12,17,18]. In the most recent merged-beam work the ions were therefore produced in an ion-trap source with a residence time of several milliseconds before extraction [12,18]. Calculations of Einstein A coefficients of low vibrational levels of H 3 + show that lifetimes in the range 100-1000 ms are expected for some levels [19]. It would therefore seem that a confinement time of milliseconds [12] may be insufficient to guarantee a complete vibrational relaxation; however, provided that the source pressure is sufficient, collisional deexcitation is the dominant mecha- nism.Heavy-ion storage rings have been used extensively during recent years for atomic physics experiments. The applicability of these facilities for studies of DR of molecular ions was recently pointed out [20] and investigated quantitatively in a case study [21]. It was shown that the main advantages with the storage ring technique for DR measurements are control over internal modes of excitatio...