Direct Observation of Charge-Exchange Collisions between Mass-selectedNan+Clusters and Cs Atoms

We have measured total charge-exchange cross sections in collisions between mass-selected $\mathrm{Na}_n^{}{}_{}{}^{+}$ cluster ions ($1\le n\le 21$) and atomic cesium. Cross sections for clusters ranging from the monomer to the 21-mer lie between 40 and 10 ${\mathrm{\AA }}^2$, decreasing slowly with increasing mass. Fast neutral products of charge exchange are photoionized and mass analyzed in a secondary time-of-flight spectrometer. Charge-transfer products present a narrow distribution of at most three masses ${\mathrm{Na}}_n$, ${\mathrm{Na}}_{n-1\mathrm{}}$, and ${\mathrm{Na}}_{n-2\mathrm{}}$, with the most probable product being ${\mathrm{Na}}_n$ or ${\mathrm{Na}}_{n-1\mathrm{}}$.