Time-resolved photofragmentation of stored silver clusters ${\mathrm{Ag}}_n^{+}$ $(n=8\mathrm{}–21)$

The time-resolved decay of silver clusters ${\mathrm{Ag}}_n^{+}$ $(n=8\mathrm{}-21)$ has been observed after excitation by photons with energies 1.5–4 eV. Clusters were found to decay by emission of neutral atoms or dimers with lifetimes in the range 100 $\mu$s to 15 ms. Separation energies were calculated from the lifetimes assuming a statistical unimolecular decay. As a function of cluster size, the resulting values increase towards the bulk cohesive energy of silver. They show a pronounced odd-even alternation and an indication of a shell closure at $n=9\mathrm{}$. The separation energies for $n=8,9\mathrm{}$ are in good agreement with configuration-interaction ab initio calculations.