Paper 3.6: "Search for P and T Violations in the Hyperfine Structure of Thallium Fluoride," D. A. Wilkening, N. F. Ramsey and D. J. Larson, Phys. Rev. A29, 425–438 (1984)
Reprinted with permission from The Physical Review. Copyright 1984, The American Physical Society.
Although most of my electric dipole moment experiments have been with neutrons, one was with TlF. E. A. Hinds and P. G. H. Sandars published a report (1980) on their ingenious experiment to observe an electric dipole interaction in the 205Tl nucleus with the nonuniform electric field within the molecule; even though the electric field averaged over the nuclear electric charge distribution had to be zero, it did not have to be so when averaged over the electric dipole moment distribution. By suitable molecular state selections, they were able to observe the difference between the 205Tl resonance frequencies when the effective electric and magnetic fields of the rotating molecule were changed from parallel to antiparallel and from this measurement infer the proton electric dipole moment. To obtain a sharp resonance they constructed an apparatus many meters long and as a result had such a weak resonance that they ran successfully only a few hours during more than several years. As a result, their electric dipole limit was less sensitive than hoped.
Since our electric resonance apparatus described in Paper 2.15 had both high resolution and high intensity, we decided (Paper 3.6) to modify that apparatus to do a more sensitive electric dipole moment search in TlF. The sensitivity of the experiment came up to our expectations. We lowered the limit and even had an exciting few weeks during which we seemed to be obtaining a nonzero result. However, we eventually found that, when we reversed the signs of our focussing electric quadrupole magnets, the sign of the result unexpectedly reversed. Although we never fully understood this spurious effect, we realized that our use of an existing apparatus had forced us to place the two different magnets excessively close together. Since the apparatus would have had to be entirely rebuilt to overcome this problem, we decided to terminate our experiment with our already improved limit and to encourage Hinds in his plans to build a new apparatus at Yale incorporating the best features of our apparatus. In subsequent years, Hinds and his students did construct their new apparatus and did not find an electric dipole moment, but did establish a still lower limit.
