Raman spectra of untwinned YBaiCu.^O? with T, =91 K indicate a 2.2-eV resonance for incident light polarized in the CuO chain direction. The resonance is characterized by the appearance of nominally forbidden phonon modes at 232 and 597 cm~'. Below 100 K the forbidden modes can be optically bleached if the photon energy exceeds 2.2 eV. The resulting optically induced low-temperature state is metastable; upon warming the crystal to T > 200 K the forbidden modes reappear as before.
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