In atomic physics, a two-electron atom or helium-like ion is a quantum mechanical system consisting of one nucleus with a charge of Ze and just two electrons. This is the first case of many-electron systems where the Pauli exclusion principle plays a central role.

It is an example of a three-body problem.

The first few two-electron atoms are:

Schrödinger equation

The Schrödinger equation for any two-electron system, such as the neutral Helium atom (He, Z = 2), the negative Hydrogen ion (H⁻, Z = 1), or the positive Lithium ion (Li⁺, Z = 3) is:^[1] For a more rigorous mathematical derivation of Schrödinger's equation, see also.^[1]

$${\displaystyle E\psi =-\hbar ^{2}\left[{\frac {1}{2\mu ))\left(\nabla _{1}^{2}+\nabla _{2}^{2}\right)+{\frac {1}{M))\nabla _{1}\cdot \nabla _{2}\right]\psi +{\frac {e^{2)){4\pi \varepsilon _{0))}\left[{\frac {1}{r_{12))}-Z\left({\frac {1}{r_{1))}+{\frac {1}{r_{2))}\right)\right]\psi }$$

$${\displaystyle |\mathbf {r} _{12}|=|\mathbf {r} _{2}-\mathbf {r} _{1}|}$$

$${\displaystyle \mu ={\frac {m_{e}M}{m_{e}+M))}$$

The cross-term of two Laplacians

$${\displaystyle {\frac {1}{M))\nabla _{1}\cdot \nabla _{2))$$

$${\displaystyle \psi =\psi (\mathbf {r} _{1},\mathbf {r} _{2})}$$

There is no closed form solution for this equation.

Spectrum

The optical spectrum of the two electron atom has two systems of lines. A para system of single lines, and an ortho system of triplets (closely spaced group of three lines). The energy levels in the atom for the single lines are indicated by ¹S₀ ¹P₁ ¹D₂ ¹F₃ etc., and for the triplets, some energy levels are split: ³S₁ ³P₂ ³P₁ ³P₀ ³D₃ ³D₂ ³D₁ ³F₄ ³F₃ ³F₂.^[2] Alkaline earths and mercury also have spectra with similar features, due to the two outer valence electrons.^[2]