Branching fractions in dissociative recombination of CH2+

The absolute cross section and branching ratios for dissociative recombinat ion of CH2+ with electrons have been measured by means of the heavy-ion sto rage ring CRYRING. Contrary to what has been previously believed, recombina tion of CH2+ is dominated by the three-body channel C + H + H (63%), wherea s breakup into the CH + H and C + H-2 channels occurs with branching ratios of 25% and 12%, respectively. The thermal rate coefficient for dissociativ e recombination at 300 K is 6.4 x 10(-7) cm(3) s(-1), which is higher by a factor of 2.5 than the value used in modeling dark molecular clouds. The lo w CH production and the high production of energetic carbon atoms could be favorable factors for the turbulence model to explain the large abundance o f interstellar CH+. The cross section for dissociative excitation was also measured and found to be in good agreement with results from a crossed elec tron-ion beam experiment.