DETECTION of HF TOWARD PKS 1830-211, SEARCH for INTERSTELLAR H2F+, and LABORATORY STUDY of H2F+ and H2Cl+ DISSOCIATIVE RECOMBINATION

We report extragalactic observations of two fluorine-bearing species, hydrogen fluoride (HF) and fluoronium (H2F+), in the z = 0.89 absorber in front of the lensed blazar PKS 1830-211 with the Atacama Large Millimeter/submillimeter Array. HF was detected toward both southwest and northeast images of the blazar, with column densities > 3.4 x 10(14) cm(-2) and 0.18 x 10(14) cm(-2), respectively. H2F+ was not detected, down to an upper limit (3 sigma) of 8.8 x 10(11) cm(-2) and an abundance ratio of [H2F+]/[HF] <= 1/386. We also searched for H2F+ toward the Galactic sources NGC 6334 I and W51C, and toward Galactic center clouds with the Herschel HIFI spectrometer.(6) The upper limit on the column density was derived to be 2.5 x 10(11) cm(-2) in NGC 6334 I, which is 1/68 of that for H2Cl+. In constrast, the ortho transition of H2Cl+ is detected toward PKS 1830-211. To understand the small abundance of interstellar H2F+, we carried out laboratory experiments to determine the rate constants for the ion-electron recombination reaction by infrared time-resolved spectroscopy. The constants determined are k(e)(209 K) = (1.1 +/- 0.3) x 10(-7) cm(3) s(-1) and (0.46 +/- 0.05) x 10(-7) cm(3) s(-1) for H2F+ and H2Cl+, respectively. The difference in the dissociative recombination rates between H2F+ and H2Cl+ by a factor similar to 2 and the cosmic abundance ratio [F]/[Cl] approximate to 1/6 are not enough to explain the much smaller abundance of H2F+. The difference in the formation mechanism of H2F+ and H2Cl+ in interstellar space would be a major factor in the small abundance of H2F+.