The combination of three atoms is much more stable than the two-atom helium dimer.

[1][2] Helium-3 is predicted to form a trimer, although ground state dimers containing helium-3 are completely unstable.

[4] In this process, a laser ionizes all three atoms simultaneously, which then fly away from each other due to electrostatic repulsion and are detected.

[5] Interatomic Coulombic decay can occur when one atom is ionised and excited.

However this is much more likely to occur when the atoms are close together, and so the interatomic distances measured by this vary with half full height from 3.3 to 12 Å.