Nuclear structure of light thallium isotopes as deduced from laser spectroscopy on a fast atom beam

The neutron-deficient isotopes ${}_{}{}^{189\mathrm{-}194}{}_{}{}^{}\mathrm{Tl}$ have been studied using collinear fast atom beam laser spectroscopy with mass-separated beams of 7×${10}^4$ to 4×${10}^5$ atoms per second. By laser excitation of the 535 nm atomic transitions of atoms in the beam, the 6$s^2$7s ${}_{}{}^2{}_{}{}^{}S_{1/2}^{}{}_{}{}^{}$ and 6$s^2$6p ${}_{}{}^2{}_{}{}^{}P_{3/2}^{}{}_{}{}^{}$ hyperfine structures were measured, as were the isotope shifts of the 535 nm transitions. From these, the magnetic dipole moments, spectroscopic quadrupole moments, and isotopic changes in mean-square charge radii were deduced. A large isomer shift in ${}_{}{}^{193}{}_{}{}^{}\mathrm{Tl}$ was observed, implying a larger deformation in the ${(9/2\mathrm{}}^{\mathrm{-}}$ isomer than in the ${(1/2\mathrm{}}^{+}$ ground state. The ${}_{}{}^{189}{}_{}{}^{}$,191,193${\mathrm{Tl}}^{\mathrm{m}}$ isotopes have deformations that increase as the mass decreases. A deformed shell model calculation indicates that this increase in deformation can account for the drop in energy of the ${(9/2\mathrm{}}^{\mathrm{-}}$ bandhead in these isotopes. An increase in neutron pairing correlations, having opposite and compensating effects on the rotational moment of inertia, maintains the spacing of the levels in the ${(9/2\mathrm{}}^{\mathrm{-}}$ strong-coupled band. Results for ${}_{}{}^{194}{}_{}{}^{}\mathrm{Tl}_{}^{\mathrm{m}}{}_{}{}^{}$ differ from previously published values, but are consistent with the ${}_{}{}^{190}{}_{}{}^{}$,192${\mathrm{Tl}}^{\mathrm{m}}$ data.