Fluorescence and Defect Fluorescence of Anthracene at 4.2°K

Previous measurements of space-charge-limited hole currents have shown that anthracene crystals, even if very pure, contain large numbers of hole traps distributed exponentially in depth. Arguments are presented that these defects, if structural, should also act as singlet excitation states below the singlet exciton band. In order to detect such excitation states, the following experiments were carried out at 4.2°K: (1) observation of fluorescence with ordinary excitation within the singlet absorption band (vexc>v0–0) to check the purity of the anthracene crystals, (2) observation of fluorescence with defect excitation (vexc<v0–0), (3) observation of both kinds of fluorescence after the crystals were annealed at 130°C. Some of the ordinary fluorescence spectra showed sharper lines than spectra previously reported. These lines appeared as doublets. Three kinds of defect fluorescence, which depended on the crystal used and on vexc, were observed: (a) fluorescence without structure in its excitation and emission spectra, (b) fluorescence with structured emission spectrum, (c) strong background emission in the ordinary fluorescence. No indications of chemical impurities were found. The results indicate that disordered regions, point defects, and dislocations play a significant role in the fluorescence process.