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 × 10¹⁴ cm^-2 and 0.18 × 10¹⁴ cm^-2, respectively. H₂F⁺ was not detected, down to an upper limit (3σ) of 8.8 × 10¹¹ 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 × 10¹¹ 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) ×10^-7 cm³ s^-1 and (0.46 ± 0.05) ×10^-7 cm³ s^-1 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⁺.