Energy distributions ofHe+andHe2+ions formed in ultracoldHe(23S1)+He(23P2)collisions

Energy distributions of ${\mathrm{He}}^{+}$ and ${\mathrm{He}}_2^{+}$ ions produced in ultracold $\mathrm{He}\left(2\mathrm{}{}^3{S}_1\right)+\mathrm{He}\left(2\mathrm{}{}^3{P}_2\right)$ collisions are presented. These helium metastable atoms are trapped and cooled in a magneto-optical trap, and have a temperature of $1.1\pm 0.2\hspace{0.5em}\mathrm{mK}.$ Initial kinetic energies of the product ${\mathrm{He}}^{+}$ and ${\mathrm{He}}_2^{+}$ ions are analyzed in energy, using a weak electrostatic extraction field in combination with a time-of-flight technique. A pronounced structure is observed in the ${\mathrm{He}}^{+}$ ion distribution, which can be well explained by a theoretical model that considers the avoided crossings between only three metastable ${}^3{\Sigma }_{\mathrm{g}}^{+}$ states of the transient ${\mathrm{He}}_2$ molecule. The measured ${\mathrm{He}}_2^{+}$ distribution has a width that is consistent with the maximum recoil energy of 1.1 meV.