Evidence for a reaction window in double‐electron‐capture reactions in collisions involving the molecular target CH ₃ Br

Double‐electron‐capture reactions in which singly charged projectile ions A + convert to negative ions A − in collisions with molecules M are endoergic. Non‐zero cross‐sections for the reactions occur when the endoergicities fall within a ‘reaction window’. The existence of a window for reactions with CH 3 Br molecules is demonstrated in this investigation by altering the population of ground and excited states of the resulting CH 3 Br 2+ ions using three projectile ions, S + ˙, OH + and F + which markedly alters the endoergicities of the double‐electron‐capture reactions. Analysis of the energy‐loss spectra indicates that the reaction window for CH 3 Br is between endoergicities of about 8 and 22 eV. Using F + as the projectile ion, it is shown that the population of the more highly excited states of CH 3 Br 2+ is enhanced.