Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H-atom production in the dissociative recombination of some protonated ions

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H-atom contributions, f_H, to the product distributions for the dissociative recombination of a series of protonated ions (N₂H⁺, HCO⁺, HCO⁺₂, N₂OH⁺, OCSH⁺, H₂CN⁺, H₃O⁺, H₃S⁺, NH⁺₄, and CH⁺₅ ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H-atom number density using vuv absorption spectroscopy at the L_α (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO₂→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of f_H varying from ∼0.2 (for OCSH⁺ ) to 1.2 (for CH⁺₅ ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO⁺₂/OCSH⁺ and H₃O⁺/H₃S⁺ showed that the f_H values were very different. Possible reasons for these differences are discussed. Comparison is also made with the available theory.