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

Abstract

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.

Dissociative recombination of protonated methanol

“…29 Another ring experiment, at CRYRING on the H 3 O + ion yielded a branching ratio of 0.67. 23 This channel corresponds to reaction (3b) in the present reaction system, which shows a similar branching ratio of 0.51. The pathway forming H 2 O in the DR of H 3 O + is definitely stronger (0.33 29 and 0.18 23 , respectively, in different storage ring experiments) than the related channel (3e) that forms methanol in the DR of CH 3 OH 2 + , but it is worth noting that flowing afterglow measurements report a much lower value (0.05), [30][31][32] which is similar to our findings with methanol.…”

Experimental studies of the dissociative recombination processes for the dimethyl ether ions CD3OCD$_{2}^{+}$ and (CD3)2OD+

“…The heavy-ion storage ring CRYRING in Stockholm, Sweden is an appropriate tool for investigations of astrophysically relevant DR reactions and has proven successful in doing so for several years, see e.g. Larsson et al (1993Larsson et al ( , 1995; Al-Khalili et al (1998); Neau et al (2000); McCall et al (2003); Hamberg et al (2005); Geppert et al (2005); Thomas (2008).…”

Formation of biomolecule precursors in space?

“…The pulse height of the detector signal was therefore proportional to the total mass. To measure the branching fractions a metal grid with a transmission T = 0.297 ± 0.015 was inserted in front of the detector (Neau et al 2000). In this case the registered DR spectrum splits into a series of peaks with different energies corresponding to the sum of the masses of the particles passing the grid.…”

“…The experimental procedure has been described in detail elsewhere (Neau et al 2000) and is therefore summarized only briefly here. For technical reasons related to the data analysis (better resolution on the detector) fully deuterated compounds were used in the case of protonated acetonitrile and protonated methylacetylene.…”