The search for «missing links» of nuclear quadrupole moments – where we stand

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 Moessbauer (ME) or perturbed angular distribution (PAD) (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 mostly to the 10 to 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 (NMR) or quadrupole resonance (NQR). A short summary of the still missing relations between the results of Q for short-lived relative to long-lived states for 4sp, 5sp, 6sp elements will be presented. The sketched procedure is obviously not applicable to elements having no stable isotopes with I > 1/2. For Cd we have overcome this problem by measuring isolated Cd (and Hg) molecules with PAC. Similar experiments for Pb are in preparation at ISOLDE/CERN, as well as related ones for Sn. (author)