74Rb is one of the few odd-proton, odd-neutron nuclides which violates the normal predictions of the nuclear shell model by having a spin-zero (0+) ground state. Its beta-decay product, 74Kr, also has a 0+ ground state, and in fact it has been shown by gamma-ray spectroscopy that many of the low-lying 74Rb excited nuclear states are analogues to 74Kr states. Despite this, electron spectroscopy has observed a second “intruder” 0+ state in 74Kr, whose energy barely exceeds that of the ground state, to which no analogue has been found for 74Rb. Nevertheless, two theoretically possible 0+ configurations exist for 74Rb, and it is unknown which one makes up its ground state, or whether it regularly exists as a superposition of the two. Since both states are spin-zero they must have no static deformation; however, theoretical models predict differing dynamic deformations, one state being oblate and the other prolate. This results in different predicted RMS charge radii for the two states.
Precise measurement of the isotope shift of 74Rb with laser spectroscopy will allow its charge radius to be determined accurately. This should make it possible to determine which of the above possibilities for the ground state describes its true nature. Additionally, an accurate measurement of this quantity will be valuable in ongoing TRIUMF studies involving the superallowed beta-decay transition from 74Rb to 74Kr, which seek to test the unitarity of the CKM matrix. In the two theoretical models used for these studies, one uses the charge-radius of 74Rb as a direct input, while the other provides it as an output. A precise measurement is therefore useful in both cases; to provide an adequate starting point for the former and a validity check for the latter.
Since an isotope-shift can only be measured with respect to another isotope, a spectroscopic analysis of the more easily produced 78Rb has already been preformed. This serves as a reference with which to compare the 74Rb spectrum, but additionally provides a proof-of-concept for the bunched-beam spectroscopy procedure. By taking measurements with a DC beam (top of figure) and a bunched beam (bottom), the massive reduction of background is clearly visible in the bunched case. The measured spectrum of 78Rb shows the presence of an unsplit energy level, surrounded by hyperfine-split peaks. This indicates the previously known fact that the 78Rb ground state, which is itself a 0+, is mixed with a higher-spin state of almost the same energy. The ground state of 74Rb is 0+, so its spectrum will probably contain only a single unsplit peak. However, it is possible that a previously unknown higher-spin state may exist at low energy for this isotope as well, which would lead to a similar multi-peak spectrum.