Nuclear quadrupole moments of bromine and iodine from combined atomic and molecular data

The multiconfiguration Dirac-Fock (MCDF) and Hartree-Fock (MCHF) models are employed to compute the electric field gradients in the ground states of the bromine and iodine atoms. Combined with experimental electric quadrupole hyperfine interaction constants B, they yield the nuclear quadrupole moments ${Q(}_{35}^{79}\mathrm{Br})$ of 313(3) millibarn (1 ${\mathrm{m}\mathrm{b}=10\mathrm{}}^{\mathrm{-}31}$ ${\mathrm{m}}^2),$ ${Q(}_{35}^{81}\mathrm{Br})$ of 261.5(2.5) mb, and ${Q(}_{\hspace{0.5em}53}^{127}\mathrm{I})$ of $-710\left(10\right)\mathrm{}$ mb. “Molecular” Q values at Douglas-Kroll coupled cluster singles and doubles with perturbative triples [CCSD(T)] level using data on HBr and HI fully support these “atomic” Q values after a “picture-change” correction is applied. In conjunction with several other recent determinations, these results indicate that the previous standard Q values of 331, 276, and $-789$ mb for ${}^{79}\mathrm{Br},$ ${}^{81}\mathrm{Br},$ and ${}^{127}\mathrm{I},$ respectively, should be revised. Q values are also reported for ${}^{129}\mathrm{I}$ and for the Mössbauer states of ${}^{127,129}\mathrm{I}.$