  Using the example of metal clusters, an experimental setup and procedure is
presented, which allows for the generation of size and charge-state selected
polyanions from monoanions in a molecular beam. As a characteristic feature of
this modular setup, the further charging process via sequential electron
attachment within a 3-state digital trap takes place after mass-selection. In
contrast to other approaches, the rf based concept permits to access heavy
particles. The procedure is highly flexible with respect to the preparation
process and potentially suitable for a wide variety of anionic species. By
adjusting the storage conditions, i.e., the radio frequency, to the change in
the mass-to-charge ratio, we succeeded to produce clusters in highly negative
charge states, i.e., Ag$_{800}^{7-}$. The capabilities of the setup are
demonstrated by experiments extracting electronic and optical properties of
polyanionic metal clusters by analyzing the corresponding photoelectron
spectra.
