Half-century old Berkeley idea now finding missing links of nuclear quadrupole moments

Abstract

There are basically two ways to determine precision values for nuclear quadrupole moments (Q): measurements for stable or reasonably long-lived (mostly ground) states by atomic and molecular spectroscopy and measurements for much shorter-lived excited states using nuclear condensed-matter techniques like Mössbauer or perturbed-angular distribution and correlation (PAC) spectroscopy. In all cases, the direct experimental result is the product of the electric-field gradient (EFG) at the nuclear site with Q. The EFG for atomic and simple molecular systems can now mostly be calculated by theory with good accuracy, while the present status of density functional calculations of solid-state systems used for short-lived excited states limits the accuracy, generally to a 10%–20% level. Thus, the EFG of at least one matrix where data for exited states exist must be calibrated by measuring a ground state with known Q using magnetic or quadrupole resonance. This procedure is obviously not applicable to elements having no stable isotope with I > 1/2. For Cd, the problem has now been overcome using a concept proposed in Berkeley half a century ago, measuring isolated free Cd (and Hg) molecules with PAC. A similar project for Pb ongoing at ISOLDE/CERN is sketched, as well as a related one for Sn.