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This compound has a high concentration of copper atoms, making it amenable to detailed study. The common feature of HTSCs is their sandwich structure, in which the essential ingredient is a set of parallel copper oxygen planes. One knows from the angular dependence of x-ray absorption that this unoccupied or hole orbital is oriented in the plane with its four lobes pointing toward the four neighboring oxygen atoms, because the electrons are repelled by the negative charge on the oxygen. Owing to quantum-mechanical selection rules, transitions between this and other d states are forbidden by absorption or emission of a single photon and hence are too weak to study by conventional means. However, resonant Raman scattering is a two-photon process that, together with the brightness afforded by the ALS, allows their observation .