PAC measurements on free molecules: experiment report

by Heinz Haas (University of Aveiro (PT))

Wednesday 6 Feb 2019, 14:15 → 15:15 Europe/Zurich

The Perturbed Angular Correlation (PAC) technique has over the last 60 years proceeded from a method of nuclear spectroscopy to an established tool in condensed matter studies. Though the application of the method to gases has been discussed even in the earliest reports, essentially no experiments with clear results have been performed as yet. In particular no measurements of free molecules have been successful to date, early attempts in Berkeley and Bonn having failed due to experimental difficulties. We have now been able to overcome these technical problems by using the ISOLDE facilities at CERN and studied the nuclear quadrupole interaction at Cd and Hg in some simple linear halides. In linear molecules the electric field gradient (EFG) is by symmetry along the molecular axis and the rotational angular momentum J is perpendicular to this for all states. The component of the EFG tensor along J is thus equal to -1/2 times that of the molecular value Vzz. The nuclear spin I being much smaller than J for practically all states leads to a total F=J*I essentially aligned with J. The resulting perturbation function is then very similar for all J states, resembling that well known for a randomly oriented axially symmetric EFG.
In recent years it has become apparent that the determination of the nuclear quadrupole moment Q for the 245 keV excited state of 111Cd by comparing solid state values of eQVzz/h with standard density functional calculations of Vzz was unreliable, the application of hybrid techniques resulting in a much reduced value of Q. Since quantum chemical calculations of Vzz for free molecules are considered more accurate, we have lately used the molecule Cd-dimethyl for such an attempt [1]. Unfortunately, here only eQVzz/h  for the molecular solid was available, requiring a correction for the (weak) intermolecular interaction.
The prime objective of the present project was thus to determine a reliable value of Q by combining precise values of eQVzz/h for free molecules with state of the art quantum chemical calculations of Vzz. We have precision data for CdI2 and CdBr2 as well as preliminary ones for CdCl2 that confirm the latest result [1], but with considerably higher precision. Our PAC measurements for HgCl2 and HgI2 using 199Hg (Q known) reproduce the previously published theoretical Vzz values.
[1] H. Haas, S.P.A. Sauer, L. Hemmingsen, V. Kelloe, and P.W. Zhao, EPL, 2017, 117, 62001.