The most commonly studied system to date is the  system, consisting of a proton bound with a  ion. The  system was first observed in 2000 by a group at the University of Waterloo in Canada.
The formation of the  ion can be understood classically; as the single electron in a hydrogen atom cannot fully shield the positively charged nucleus, another electron brought into close proximity will feel an attractive force. While this classical description is nice for getting a feel for the interactions involved, it is an oversimplification; many other atoms have a greater electron affinity than hydrogen. In general the process of forming a negative ion is driven by the filling of atomic electron shells to form a lower energy configuration.
Only a small number of molecules have been used to produce heavy Rydberg systems although in principle any atom with a positive electron affinity can bind with a positive ion. Species used include ,  and . Fluorine and oxygen are particularly favoured due to their high electron affinity, high ionisation energy and consequently high electronegativity.