Fluorescence spectroscopy of opal from the Shikaribetsu area, Shikaoi, Hokkaido

Abstract

Opal from the Shikaribetsu area, Shikaoi, Hokkaido, Japan exhibits visible photoluminescence. The opal is characterized using powder X-ray diffraction (XRD) measurements, fluorescence spectroscopy, and fluorescence lifetime measurements. The XRD pattern of the powdered sample indicates that the opal is amorphous opal-A. The dried powdered opal exhibits broad emission spectra ranging from 450 to 750 nm with the maximum at approximately 590 nm. The emission decay curve is fitted with the multi-exponential decay function. The average emission lifetime is shorter than 10 ns, and the emission is ascribed to fluorescence. The d-d transition of transition metal ions and the f-f transition of lanthanide ions are forbidden according to the Laporte selection rule, and their emission lifetime should be much longer than that observed in this study. Therefore, the transition metal ions or lanthanide ions are not the origin of the photoluminescence of the opal. The fluorophore can be extracted from the opal to organic solvents, and the fluorescence color changes from orange to bright cyan in the solvent. The fluorescence and fluorescence excitation spectra show the narrow bands which can be ascribed to the vibronic transition of organic molecules. The typical fluorescence lifetime is approximately 8 ns. These results suggest that the fluorophore in the opal is polyaromatic hydrocarbon molecules. It is also demonstrated that the solvent extraction is a useful method to study the origin of the fluorescence and to analyze ingredients in the opal.