Identification of the one-quadrupole phonon [formula omitted] state of 204Hg.

One-phonon states of vibrational nuclei with mixed proton–neutron symmetry have been observed throughout the nuclear chart besides the mass A ≈ 200 region. Very recently, it has been proposed that the 2 2 + state of 212 Po is of isovector nature. This nucleus has two valence protons and two valence neutrons outside the doubly-magic 208 Pb nucleus. The stable isotope 204 Hg, featuring two valence-proton and valence-neutron holes, with respect to 208 Pb, is the particle-hole mirror of 212 Po. In order to compare the properties of low-lying isovector excitations in these particle-hole mirror nuclei, we have studied 204 Hg by using the projectile Coulomb-excitation technique. The measured absolute B ( M 1 ; 2 2 + → 2 1 + ) strength of 0.20 ( 2 ) μ N 2 indicates that the 2 2 + level of 204 Hg is at least the main fragment of the 2 1 , m s + state. For the first time in this mass region, both lowest-lying, one-quadrupole phonon excitations are established together with the complete set of their decay strengths. This allows for a microscopic description of their structures, achieved in the framework of the Quasi-particle Phonon Model. [ABSTRACT FROM AUTHOR]