Spectroscopy in the Second Minimum of the Potential Energy Surface ofPu239

Conversion electrons from the decay of a spin isomer in the secondary minimum of ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ have been measured. They are interpreted as transitions feeding and depopulating a rotational band built on the fission isomeric ground state with spin $\frac{5}{2}$. Rotational parameters of $A=3.36\mathrm{}\pm 0.10$ keV and $B=4\mathrm{}\pm 3$ eV have been deduced. The observed upper limit on the $\frac{M1\mathrm{}}{E2\mathrm{}}$ mixing ratio of 1.4 suggests an antiparallel coupling between orbital angular momentum and spin vectors for the wave function of the fission isomer.