Ion Coulomb crystals: a tool for studying ion processes

Ion processes can be studied by a wide variety of experimental setups ranging from small ion trap devises to large storage rings. In Penning as well as Paul (rf) traps, the effective oscillation frequencies of trapped particles have, e.g., extensively been used to gain information of the charge to mass ratio of produced atomic and molecular ions. For such investigations, space charges due to the simultaneous presence of multiple ions, is a limiting factor, and hence low density or single particle experiments are preferred. The purpose of the present article is to discuss the prospects of using traps in the opposite regime where space charges dominate and so-called ion Coulomb crystals form (at temperatures about similar to10 mK). The experimental results presented below show that in this regime, differences in single particle oscillation frequencies of simultaneously trapped ions lead to a spatial segregation of the ion species which can be used as means to study a large variety of ion processes. Since the detection relies generally on observations of changes in the spatial structure of the Coulomb crystals formed by the cold, trapped ions, it can often be non-destructive, i.e., the product ions of a certain process will stay trapped and be available for further experimentations. It is the intention of the present article to discuss, via some examples, the usefulness of ion Coulomb crystals as a tool in investigations of ion processes more than it is the aim to provide new data. (C) 2003 Elsevier B.V. All rights reserved.