It was shown by Brown and Henneaux that the classical theory of gravity on AdS 3 has an infinite-dimensional symmetry group forming a Virasoro algebra. More recently, Giveon, Kutasov and Seiberg (GKS) constructed the corresponding Virasoro generators in the first-quantized string theory on AdS 3 . In this paper, we explore various aspects of string theory on AdS 3 and study the relation between these two works. We show how semi-classical properties of the string theory reproduce many features of the AdS/CFT duality. Furthermore, we examine how the Virasoro symmetry of Brown and Henneaux is realized in string theory, and show how it leads to the Virasoro Ward identities of the boundary CFT. The Virasoro generators of GKS emerge naturally in this analysis. Our work clarifies several aspects of the GKS construction: why the Brown-Henneaux Virasoro algebra can be realized on the first-quantized Hilbert space, to what extent the free-field approximation is valid, and why the Virasoro generators act on the string worldsheet localized near the boundary of AdS 3 . On the other hand, we find that the way the central charge of the Virasoro algebra is generated is different from the mechanism proposed by GKS.

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The photo-induced depopulation of the quasistable isomer (t 1/2 у1.2ϫ10 15 yr) in 180 Ta with angular momentum and parity J ϭ9 Ϫ at an excitation energy E x ϭ75 keV was studied at the new bremsstrahlung irradiation facility installed at the Stuttgart 4.3 MV DYNAMITRON accelerator in the energy range of bremsstrahlung end point energies between E 0 ϭ0.8-3.1 MeV. The onset of the isomer depopulation could be observed starting at an end point energy of E 0 Ϸ1 MeV, i.e., at an intermediate state of 180 Ta at or below that energy. Higher-lying intermediate states were found at 1.22, 1.43, 1.55, 1.85, 2.16, 2.40, 2.64, and 2.80 MeV. The extracted integrated cross sections show a remarkably strong depopulation of the 180 Ta m isomer by photoexcitation. The results are compared with previous experiments and recent quasiparticle-phonon model calculations. Implications of the results for a possible nucleosynthesis of 180 Ta in the s process and the neutrino process are discussed.

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