Final-state-resolved mutual neutralization in I+- I- collisions

We have studied the mutual neutralization reaction of atomic iodine ions (i.e., I+ + I- -> I + I) in a cryogenic double electrostatic ion-beam storage-ring apparatus. Our results show that the reaction forms iodine atoms either in the ground-state configuration (I(5p(5) P-2 degrees), similar to 40%) or with one atom in an electronically excited state (I(6s(2)[2]), similar to 60%), with no significant variation over the branching ratios in the studied collision-energy range (0.1-0.8 eV). We estimate the total charge-transfer cross section to be of the order of 10(-13) cm(2) at 0.1 eV collision energy. Ab initio relativistic electronic structure calculations of the potential-energy curves of I-2 suggest that the reaction takes place at short internuclear distances. The results are discussed in view of their importance for applications in electric thrusters.