Production of Multiply-Charged Metal-Cluster Anions in Penning and Radio-Frequency Traps

摘要

The number of electrons in an atomic cluster can have a severe influence on its properties, and thus the charge state is a crucial parameter. In the present study, poly-anionic clusters are produced by electron-attachment to cluster mono-anions stored in ion storage devices. The polyanion production is investigated for two different trap types. In a Penning trap (ion-cyclotronresonance, or short ICR trap) charged particles with any mass-to-charge ratio up to a critical value can be stored at the same time. Thus, cluster anions are stored together with electrons, simultaneously, allowing for electron attachment from an electron bath. However, the highest polyanionic charge state that can be produced is limited by two conditions. On the one hand, it is restricted by the upper mass limit of the trap, because a minimum cluster size is required to gain a certain (negative) charge state. On the other hand, it is limited by the Coulomb potential of the cluster anions, because the maximum energy of the attaching electrons in the bath is given by the depth of the trapping potential. In contrast to the Penning trap, the radio-frequency (RF) traps confine particles with only a narrow range of the mass-over-charge ratio values. In particular, there is a lower limit, preventing a simultaneous storage of electrons and clusters. At the same time, the RF trap's mass range can be shifted to large clusters, which are required to reach higher negative charge states. Application of rectangularly shaped RF-voltages, in contrast to sinusoidal ones, facilitates ion storage with respect to variation of the mass range, i.e. cluster size range, to be trapped. While poly-anion production in Penning traps is a well-established technique that has already been applied to several types of clusters, its realization in RF traps is currently being developed. Both schemes are described, including previous and recent experimental results.