Abstract

Since the first attempts to calculate the helium ground state in the early days of Bohr–Sommerfeld quantisation, two-electron atoms have posed a series of challenges to theoretical physics. Despite the seemingly simple problem of three charged particles with known interactions it took more than half a century after quantum mechanics was established to describe spectra of two-electron atoms satisfactorily. The evolution of the understanding of correlated two-electron dynamics and its importance for doubly excited resonance states is described in this overview.