DETECTION OF HF TOWARD PKS 1830–211, SEARCH FOR INTERSTELLAR H{sub 2}F{sup +}, AND LABORATORY STUDY OF H{sub 2}F{sup +} AND H{sub 2}Cl{sup +} DISSOCIATIVE RECOMBINATION
We report extragalactic observations of two fluorine-bearing species, hydrogen fluoride (HF) and fluoronium (H{sub 2}F{sup +}), 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 × 10{sup 14} cm{sup −2} and 0.18 × 10{sup 14} cm{sup −2}, respectively. H{sub 2}F{sup +} was not detected, down to an upper limit (3σ) of 8.8 × 10{sup 11} cm{sup −2} and an abundance ratio of [H{sub 2}F{sup +}]/[HF] ⩽ 1/386. We also searched for H{sub 2}F{sup +} toward the Galactic sources NGC 6334 I and W51C, and toward Galactic center clouds with the Herschel HIFI spectrometer.{sup 6} The upper limit on the column density was derived to be 2.5 × 10{sup 11} cm{sup −2} in NGC 6334 I, which is 1/68 of that for H{sub 2}Cl{sup +}. In constrast, the ortho transition of H{sub 2}Cl{sup +} is detected toward PKS 1830–211. To understand the small abundance of interstellar H{sub 2}F{sup +}, 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 {sub e}(209 K) = (1.1±0.3)×10{sup −7} cm{sup 3} s{sup −1} and (0.46±0.05)×10{sup −7} cm{sup 3} s{sup −1} for H{sub 2}F{sup +} and H{sub 2}Cl{sup +}, respectively. The difference in the dissociative recombination rates between H{sub 2}F{sup +} and H{sub 2}Cl{sup +} by a factor ∼2 and the cosmic abundance ratio [F]/[Cl] ≈ 1/6 are not enough to explain the much smaller abundance of H{sub 2}F{sup +}. The difference in the formation mechanism of H{sub 2}F{sup +} and H{sub 2}Cl{sup +} in interstellar space would be a major factor in the small abundance of H{sub 2}F{sup +}.