Storage ring measurements of the dissociative recombination rate of rotationally cold H + 3

We present dissociative recombination measurements, using the CRYRING ion storage ring, of H3 ions produced in a supersonic expansion discharge source. Before and after the CRYRING measurements, the ion source was characterized in Berkeley using infrared cavity ringdown spectroscopy, and was found to exhibit a typical rotational temperature of ∼ 30 K. Our measurement of the dissociative recombination cross section using this ion source revealed resonances that had not been observed clearly in previous experiments that used rotationally hot ion sources. Based on the present measurements, we infer a thermal dissociative recombination rate coefficient for ions at interstellar temperatures of ∼ 2.6 × 10−7 cm3s−1. Our results are in general agreement with theoretical calculations of the dissociative recombination cross section by Kokoouline and Greene. We will review the enigma of the abundance of H3 in the diffuse interstellar medium, and discuss the impact of these experiments, especially in the context of the recent observation of H3 towards ζ Persei.