Surface aligned ion-molecule reaction: direct observation of initial and final ion momenta.

An ion-molecule reaction has been studied by measuring the momentum of both the reactant and the product ions. This is carried out in an ordered molecular film of CD3I where electron stimulated desorption causes the reaction CD+3+ CD3I--> C2D+5+DI. The close similarity of the normal momentum of CD+3 and C2D+5 indicates that a sticky collision occurs in which, to within 10%, the momentum of the reactant ion is transferred to the momentum of the product ion. The measurement represents the first use of molecularly aligned species to study momentum effects in an ion-molecule reaction.