2016ApJ...822..115K

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 10¹⁴/cm2 and 0.18 x 10¹⁴/cm2, respectively. H₂F⁺ was not detected, down to an upper limit (3σ) of 8.8 x 10¹¹/cm2 and an abundance ratio of [H₂F⁺]/[HF] \leqslant 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 10¹¹/cm2 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)×{10}^{-7} cm³/s and (0.46±0.05)×{10}^{-7} cm³/s for H₂F⁺ and H₂Cl⁺, respectively. The difference in the dissociative recombination rates between H₂F⁺ and H₂Cl⁺ by a factor ∼2 and the cosmic abundance ratio [F]/[Cl] ≃ 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⁺.