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

Dissociative recombination of the polyatomic ions D3O+ and H3O+ with electr ons have been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Absolute cross sections have been deter mined from 0.001 eV to 0.25 eV center-of-mass energy for D3O+ and from 0.00 1 eV to 28 eV for H3O+. The cross sections are large (7.3x10(-13) cm(2) for D3O+ and 3.3x10(-12) cm(2) for H3O+ at 0.001 eV). At low energies, the cro ss sections for D3O+ are E-1 energy dependent whereas it is slightly steepe r for H3O+. A similar E-1 energy dependence was also observed by Mul [J. Ph ys. B 16, 3099 (1983)] with a merged electron-ion beam technique for both H 3O+ and D3O+ and by Vejby-Christensen [Astrophys. J. 483, 531 (1997)] with the ASTRID storage ring in Denmark, who presented relative cross sections f or H3O+. A resonance has been observed around 11 eV for H3O+. It reflects a n electron capture to Rydberg states converging to an excited ionic core. A similar structure was reported by Vejby-Christensen Our absolute measureme nts are in fairly good agreement with those from Mul , which were first div ided by 2 (Mitchell, 1999, private communication) and from Heppner [Phys. R ev. A 13, 1000 (1976)] for H3O+. Thermal rates were deduced from the measur ed cross sections for electron temperatures ranging from 50 K to 30 000 K. At 300 K, the thermal rate is equal to 7.6x10(-7) cm(3) s(-1) for H3O+ and to 3.5x10(-7) cm(3) s(-1) for D3O+. Complete branching ratios for all the p ossible 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 tr ansmission grid in front of an energy-sensitive surface-barrier detector. N o isotope effect was observed within the experimental uncertainties. The th ree-body break-up channel OX+X+X (where X stands for H or D) is found to oc cur 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. T hese results support the three-body break-up dominance already found by Vej by-Christensen for the DR of H3O+ in a similar heavy-ion storage ring exper iment. However, even if the general trend is the same for both storage ring s, significant differences have been observed and will be discussed. (C) 20 00 American Institute of Physics. [S0021- 9606(00)01229-0].