Endoergic charge transfer via A + X, -A+ + X,-in single collisions is used to study electron attachment to weakly bound molecular clusters. Two supersonic beams are employed, one to produce fast alkali atoms (usually A = Rb), the other to produce molecular clusters (typically X = Cl, or SO,). From threshold energies for ion-pair formation, nominal electron affinities as a function of cluster size are derived for (ClJ" C1(C12),-1, and (SO,)" with n = 1-4,2-4, and 1-5, respectively. The

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... l charge-transfer process which forms the negative cluster ions occurs on a subpicosecond time scale. The results are interpreted in terms of an impulsive-collision model, with incomplete vibrational relaxation for cluster ions larger than dimers.