Extraordinary branching ratios in astrophysically important dissociative recombination reactions.

Abstract

Branching ratios of the dissociative recombination reactions of the astrophysically relevant ions DCO+, N2H+ and DOCO+ (as substitute for HOCO+) have been measured using the CRYRING storage ring at the Manne Siegbahn Laboratory at the University of Stockholm, Sweden. For DCO+, the channel leading to D and CO was by far the most important one (branching ratio 0.88), only small contributions of the CD + O and OD + C product pathways (branching ratios 0.06 each) were recorded. In the case of N2H+ the surprising result of a break-up of the N-N bond to N and NH (branching ratio 0.64) was found with the branching ratio of the N2 + H product channel therefore displaying a branching ratio of only 0.36. In the case of DOCO+, the three-body break-up into D + O + CO dominated (branching ratio 0.68), whereas the contribution of the CO2 + H channel was only minute (0.05). The remaining share (branching ratio 0.27) was taken by the pathway leading to OH + CO. For the dissociative recombination of N2H+ and DOCO+ also absolute reaction cross sections were obtained in the collisional energy range between 0 and 1 eV. From these cross sections it was possible to work out the thermal rate constants, which were found to be k(T) = 1.0+/-0.1 x 10(-7) (T/300 K)(-0.51+/-0.02) cm3 s(-1) and k(T) = 1.2+/-0.1 x 10(-6) (T/300 K)(-0.64+/-0.02) cm3 S(-1) for N2H+ and DOCO+, respectively.