Structure with the lowest energy observed in the ${}^{10}\mathrm{He}$ spectrum populated in the proton knockout reaction with a ${}^{11}\mathrm{Li}$ beam has a peak at $1.2\text{to}1.5$ MeV. This peak is usually interpreted as a resonant $0^{+}$ ground state of ${}^{10}\mathrm{He}$. Our theoretical calculations indicate that this peak is likely to be a superposition of $1^{-}$, $0^{+}$, and $2^{+}$ excitations with very similar shapes. Moreover, the “soft” $1^{-}$ excitation appears to be the lowest in energy. Such an anomalous continuum response is traced to the halo structure of ${}^{11}\mathrm{Li}$ providing an extreme low energy shift to all the expected continuum excitations. Competition between effects on the spectrum of the initial-state structure and of the final-state interaction and three-body correlations in ${}^{10}\mathrm{He}$ are discussed. The analogous effect of an extreme low-energy shift could also be expected in other cases of $2n$ emitters populated in reactions with halo nuclei. A simplified example of the ${}^{10}\mathrm{He}$ spectrum in $\alpha$ knockout from ${}^{14}\mathrm{Be}$ is given. We also discuss limits on the properties of ${}^9\mathrm{He}$ stemming from the observed ${}^{10}\mathrm{He}$ spectrum.

• Received 6 March 2014
• Revised 25 May 2014

DOI:https://doi.org/10.1103/PhysRevC.90.024610