Intermediate structure in the photoexcitation ofSem77,Brm79, andBam137

Continuing the systematic investigation into the photoexcitation of isomers over wide mass and energy ranges, the production of ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}_{}^{\mathit{m}}{}_{}{}^{}$, ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}_{}^{\mathit{m}}{}_{}{}^{}$, and ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}_{}^{\mathit{m}}{}_{}{}^{}$ was studied with the bremsstrahlung facility at the superconducting Darmstadt linear accelerator. These isomers have half-lives on the order of seconds. Excitation functions were measured for the (γ,γ’) reactions populating the metastable states for energies of 2–7 MeV and the important intermediate states were identified. Nuclear structure calculations with the quasiparticle-phonon model for ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$ and the particle- (hole-) core coupling approach for ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}$ gave satisfactory descriptions for the strength and position of the dominant mediating levels. Admixtures of fragmented outershell single-particle strength shifted to low energies were identified as essential features of the wave functions of those states. Intermediate states in ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}$ displayed very large strengths compared to other isomers in the same mass region, providing further support for the correlation between integrated cross sections and ground state deformations recently discovered in the A=160–200 mass region. Such an enhancement would considerably improve the feasibility of a gamma-ray laser based on the sudden deexcitation of isomeric populations in deformed nuclei.