Evidence for a (sub)-shell closure at $Z=14$ has been observed from the spectroscopy of ${}^{22}\mathrm{Si}$ and ${}^{21}\mathrm{Al}$. Using a fast ${}^{23}\mathrm{Si}$ beam on a ${}^9\mathrm{Be}$ target, several proton-decaying resonances have been populated in ${}^{21}\mathrm{Al}$ and ${}^{22}\mathrm{Si}$, including the first measurement of the $2_1^{+}$ state in ${}^{22}\mathrm{Si}$ with an excitation energy of 2.352(55) MeV. Relative to the known $N=14$ subshell closure in ${}^{22}\mathrm{O}$, the ${}^{22}\mathrm{Si}$ $2_1^{+}$ excitation energy indicates the existence of a weakened $Z=14$ subshell closure. To reproduce the new spectroscopic information and known ${}^{22}\mathrm{Al}$ levels, the energy of the $1s_{1/2}$ orbit must be substantially decreased, which imparts Thomas-Ehrman shifts to all excited states observed in the present Letter and explains the structure of nuclei in the $sd$ shell for this region of the proton drip line.

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• Direct reactions
• Energy levels
• Nuclear structure & decays
• Proton emission
• Rare & new isotopes
• Shell model
• 20 ≤ A ≤ 38
• Radioactive beams