3. Photoluminescence Measurement The sample was put in an unsilvered quartz Dewar vessel and was placed in front of the monochrometer. The luminescence spectra were recorded under the excitation light from a Xe lamp passed through filters. For the measure- ments of stimulation spectra, the excitation and observation were made through the monochro- meter and filters, respectively. The lifetime of emission was determined by using a combination of Xe flash lamp, filters, photomultiplier and oscilloscope. Absorption spectra were recorded with a con- ventional recording spectrophotometer.