Single electron ions

Single electron ions are any nucleus surrounded by only one electron, examples are He⁺, Li²⁺, Be³⁺, B⁴⁺, …. Energy levels of these ions are calculated like in Hydrogen atom : \(e_n=-\frac{13.6 Z^2}{n^2}\) , where  is the charge of the nucleus. For example He⁺, \(Z=2\), then \(E_1=-54.4 e V\). And this is the second/last ionisation energy of Helium atom (in ultraviolet range).