The Sn isotopic chain, exhibiting double shell closures at ${}^{100}\mathrm{Sn}$ and ${}^{132}\mathrm{Sn}$, is a key testing ground for theoretical models of the atomic nucleus. It was originally predicted that the transitional matrix elements between the first $2^{+}$ state and the $0^{+}$ ground state for the even-even isotopes in this chain should show a simple dependence on the neutron number. This prediction was, however, disproven experimentally in some of the first experiments with postaccelerated radioactive beams, a situation that has remained unresolved ever since. Subsequent theoretical work has suggested that the explanation can be found in proton excitations across the $Z=50$ shell gap, with an accompanying experimental signature that the first excited $2^{+}$ state in ${}^{110}\mathrm{Sn}$ should have a distinct oblate shape. In this Letter, we present the first measurements of the spectroscopic quadrupole moment of the $2_1^{+}$ state, $B(E2;4_1^{+}\to 2_1^{+})$ and $B(E2;4_2^{+}\to 2_1^{+})$ values for ${}^{110}\mathrm{Sn}$, as well as the $B(E2;2_1^{+}\to 0_1^{+})$ value with significantly improved precision compared to previous results. From the same experiment, half-lives of the $2_1^{+}$ and $4_1^{+}$ states were measured using the Doppler shift attenuation method. Our combined result, $Q(2_1^{+})=20(8){\mathrm{efm}}^2$ for ${}^{110}\mathrm{Sn}$, is the largest positive value known among the Sn isotopes, indicating an oblate shape of the state by more than $2\sigma$. Comparison of the $E2$ transition strengths and quadrupole moments with recent shell model calculations are presented.

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• Coulomb excitation
• Electromagnetic transitions
• Nuclear transition rates
• Unstable nuclei induced nuclear reactions