Photoexcitation of the isomers of 19 nuclides was examined in this work. Four accelerators were used as sources of bremsstrahlung to expose the samples and end-point energies covered the range from 0.5 to 11 MeV. No evidence was found for nonresonant processes of excitation. However, more than half the cases showed enhanced channels for the resonant photoexcitation of isomers with integrated cross sections approaching 10 ' cm keV. These results are three to four orders of magnitude larger than values usually characterizing {y, y') reactions.

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Continuing the systematic investigation into the photoexcitation of isomers over wide mass and energy ranges, the production of Se, " Br, and '"Ba 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 (y, y ) 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 Br and the particle-(hole-) core coupling approach for " 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 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.PACS number(s): 25.20. Lj, 27.50.+e, 27.60.+j, 21.90.+f