Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV. The proposed mechanism for this double ionization is that the photon ejects an electron from one atom, and then that electron hits the other helium atom and ionizes that as well.
Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV. The dimer then explodes as two helium ions repel each other with the same speed but opposite directions. That electron hits the other helium atom and ionizes that as well.