Helium chemistry: a survey of the role of the ionic species

The ion chemistry of helium currently embraces a large variety of species, mostly cationic, which range from simple small-size molecular fragments or complexes to large-size cluster ions and encapsulation products. These species are usually observed under the isolated conditions of the gas phase or theoretically investigated by high-level of theory calculations. In the current years, the highest interest is focused on classical topics such as the interaction between helium atoms and monoatomic, diatomic, and simple polyatomic ions as well as on research themes which have more recently emerged, such as the study of ionized helium clusters and the fullerene-helium chemistry. The aim of the present article is to give an account of selected recent advances in these fields and to put them in the perspective of the previous most relevant acquisitions concerning the ion chemistry of helium. In particular, the literature covered here concerns the ionised helium clusters He-n(+) and He-n(-) (n greater than or equal to 2) the hydrogen-containing helium clusters HenH+ and HenH- (n greater than or equal to 1), the adducts of one or more helium atoms with a variety of singly- and multiply-charged monoatomic, diatomic, and polyatomic ions, and cage compounds such as He @ C-60(+) and other related cationic and anionic species. (C) 2004 Elsevier B.V. All rights reserved.