Multiply negatively charged aluminium clusters -: Production of Al2-n in a Penning trap

The first observation of gas-phase aluminium dianions, Al-n(2-) stop, is reported. Singly negatively charged aluminium clusters are converted to dianions by exposure to low-energy electrons in a Penning trap. Attachment of such secondary electrons to the monoanions leads to the formation of dianionic clusters, as earlier observed for the noble metals, copper, silver and gold. The present work extends the `electron-bath technique' to the trivalent aluminium. The observation of Al-n(2-) stop as a function of cluster size is compared to estimates of the second electron affinities of Al-n as given by a combination of first electron affinities and the charged conducting-sphere model. This simple approach is found to describe the appearance size (size of smallest Al-n(2-) observed) very well.