Dissociative recombination of D_3O~(+) and H_3O~(+): Absolute cross sections and branching ratios

摘要

Dissociative recombination of the polyatomic ions D304 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 x1013 cm2 for D3O~ and 3.3x10’2 cm2 for H3O~ at 0.001 eV). At low energies, the cross sections for D3O~ are energy dependent whereas it is slightly steeper for H3O~. A similar E1 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 a!. [Astrophys. 3. 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 a!. Our absolute measurements are in fairly good agreement with those from Mul et a!., which were first divided by 2 (Mitchell, 1999, private communication) and from Heppner et a!. [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 7.6X i0~ cm3 s~ for H3O~ and to 3.5X i0~ cm3 s’ 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 a a!. 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.