A Coulomb-excitation reorientation-effect measurement using the TIGRESS γ−ray spectrometer at the TRI-UMF/ISAC II facility has permitted the determination of the 2 + 1 ||Ê2 || 2 + 1 diagonal matrix element in 12 C from particle−γ coincidence data and state-of-the-art no-core shell model calculations of the nuclear polarizability. The nuclear polarizability for the ground and first-excited (2 + 1 ) states in 12 C have been calculated using chiral NN N 4 LO500 and NN+3NF350 interactions, which show convergence and agreement with photo-absorption cross-section data. Predictions show a change in the nuclear polarizability with a substantial increase between the ground state and first excited 2 + 1 state at 4.439 MeV. The polarizability of the 2 + 1 state is introduced into the current and previous Coulombexcitation reorientation-effect analyses of 12 C. Spectroscopic quadrupole moments of Q S (2 + 1 ) = +0.053(44) eb and Q S (2 + 1 ) = +0.08(3) eb are determined, respectively, yielding a weighted average of Q S (2 + 1 ) = +0.071(25) eb, in agreement with recent ab initio calculations. The present measurement confirms that the 2 + 1 state of 12 C is oblate and emphasizes the important role played by the nuclear polarizability in Coulomb-excitation studies of light nuclei.

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