The spectroscopic quadrupole moment of the first excited 21+ state, QS(21+), at 1.970 MeV in Ar36 was determined at energies well below the Coulomb barrier—where nuclear interference effects are negligible—using the Pt194(Ar36,Ar*36)Pt*194 Coulomb-excitation reaction at 134.2 MeV. Particle-gamma coincidence data were collected using the AFRODITE array—composed of eight high-purity germanium clover detectors—and an upstream double-sided silicon detector at iThemba LABS. A large diagonal matrix element of 〈21+∥E2̂∥21+〉=0.163(42) eb was determined, which yields a more accurate value of QS(21+)=+0.12(3) eb as compared with previous work, QS(21+)=+0.11(6) eb, in agreement with modern beyond mean-field and large-scale shell-model calculations. This value is consistent with the ratio of electric quadrupole moments found for other A=4n self-conjugate nuclei extracted from the reorientation effect and the rotor model, which are surprisingly equivalent to those observed in good rotors in the mass A≈160–180 region.Received 22 September 2021Accepted 28 November 2021DOI:https://doi.org/10.1103/PhysRevC.104.L061305©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsCollective modelsElectromagnetic transitionsHydrodynamic modelsNuclear structure & decaysPhotonuclear reactionsShell modelSpectroscopic factors & electromagnetic momentsProperties20 ≤ A ≤ 38TechniquesRadiation detectorsNuclear Physics