Dissociative recombination of D3O+ and H3O+: Absolute cross sections and branching ratios

Dissociative recombination of the polyatomic ions D₃O⁺ and H₃O⁺ with electrons have been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Absolute cross sections have been determined from 0.001 eV to 0.25 eV center-of-mass energy for D₃O⁺ and from 0.001 eV to 28 eV for H₃O⁺. The cross sections are large (7.3×10^-13 cm² for D₃O⁺ and 3.3×10^-12 cm² for H₃O⁺ at 0.001 eV). At low energies, the cross sections for D₃O⁺ are E^-1 energy dependent whereas it is slightly steeper for H₃O⁺. A similar E^-1 energy dependence was also observed by Mul et al. [J. Phys. B 16, 3099 (1983)] with a merged electron-ion beam technique for both H₃O⁺ and D₃O⁺ and by Vejby-Christensen et al. [Astrophys. J. 483, 531 (1997)] with the ASTRID storage ring in Denmark, who presented relative cross sections for H₃O⁺. A resonance has been observed around 11 eV for H₃O⁺. It reflects an electron capture to Rydberg states converging to an excited ionic core. A similar structure was reported by Vejby-Christensen et al. Our absolute measurements are in fairly good agreement with those from Mul et al., which were first divided by 2 (Mitchell, 1999, private communication) and from Heppner et al. [Phys. Rev. A 13, 1000 (1976)] for H₃O⁺. Thermal rates were deduced from the measured cross sections for electron temperatures ranging from 50 K to 30 000 K. At 300 K, the thermal rate is equal to 7.6×10^-7 cm³ s^-1 for H₃O⁺ and to 3.5×10^-7 cm³ s^-1 for D₃O⁺. Complete branching ratios for all the possible product channels have been determined from 0 eV to 0.005 eV center-of-mass energy for D₃O⁺ and at 0 eV for H₃O⁺, using a well-characterized transmission grid in front of an energy-sensitive surface-barrier detector. No isotope effect was observed within the experimental uncertainties. The three-body break-up channel OX+X+X (where X stands for H or D) is found to occur for 67%-70% of the dissociations. Water or heavy water is produced with an 18%-17% probability and the production of oxygen atoms is negligible. These results support the three-body break-up dominance already found by Vejby-Christensen et al. for the DR of H₃O⁺ in a similar heavy-ion storage ring experiment. However, even if the general trend is the same for both storage rings, significant differences have been observed and will be discussed.