A 20 ps pulsed hard X-ray source (4 -15 keV) was utilized for exciting microcrystals of anthracene, 9,10-dichloroanthracene, 9,10-diphenylanthracene tris (8-hydroxyquinoline) aluminum, and bis(8-hydroxyquinoline) zinc, and the generation process of these fluorescence states was studied. It was found that the fluorescence rise time by hard X-ray excitation was much slower than the rise time by UV excitation for all samples. In addition, there was a tendency that molecules including heavier atoms show faster fluorescence rise time and lower fluorescence intensity. It is considered that because the X-ray absorption occurs at the heaviest atom in a molecule to yield a photoelectron which further generates ionized tracks consisting of electron-hole pairs, the amount of the electron-hole pairs leading to fluorescence states would depend on the initial photoelectron energy. The observed fast rise for the molecules including heavy atoms is attributed to the rapid recombination of electron- hole pairs, which may be affected by the initial distance distribution of electron-hole pairs, the density of electron-hole pairs, and the crystal structures.
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