Fock-space relativistic coupled-cluster calculations of clock transition properties in Pb$^{2+}$

Abstract: We have implemented an all-particle multireference Fock-space relativistic coupled-cluster theory to probe $6s^2{\;^1}S_{0} - 6s6p{\;^3P^o_{0}}$ clock transition in an even isotope of Pb$^{2+}$. We have computed, excitation energy for several low lying states, E1 and M1 transition amplitudes, and the lifetime of the clock state. Moreover, we have also calculated the ground state dipole polarizability using perturbed relativistic coupled-cluster theory. To improve the accuracy of results, we incorporated the corrections from the relativistic and QED effects in all our calculations. The contributions from triple excitations are accounted perturbatively. Our computed excitation energies are in excellent agreement with the experimental values for all the states. Our result for lifetime, $9.76\times10^{6}$ s, of clock state is $\approx$ 8.5\% larger than the previous value using CI+MBPT [Phys. Rev. Lett. {\bf 127}, 013201 (2021)]. Based on our analysis, we find that the contributions from the {\em valence-valence} correlations arising from higher energy configurations and the corrections from the perturbative triples and QED effects are essential to get accurate clock transition properties in Pb$^{2+}$. Our computed value of dipole polarizability is in good agreement with the available theoretical and experimental data.