Role of Defects and Reabsorption in the Decay of Fluorescence of Anthracen from 2–350 ° K

Abstract

Abstract Fluorescence spectra and decay times were measured for anthracene crystals grown from the vapour and from the melt, and for anthracene dissolved in polymethylmethacrylate (PMM). The decay times were corrected for reabsorption. The procedure for determining the correction is described. It was facilitated by recording the spectra in digital form on magnetic tape and using a computer to calculate and plot the area accumulated under the fluorescence spectrum as a function of wavelength. Sets of accumulated area curves not only allowed the reabsorption correction to be made but also indicated the relative number of physical defects present in the crystals. They also showed changes in reabsorption with temperature. The corrected fluorescence decay times of anthracene crystals showed little variation with method of growth or with temperature. They were 5 ± 1 nsec at 2°K and increased by 1–2 nsec as the temperature was raised to 350°K. The variation with temperature was much less than the correction for reabsorption, and the interpretation of the change is therefore difficult. In PMM the decay time decreased as the temperature was raised, from about 10 nsec at 2°K to 5 nsec at 350°K. The lifetime of defect fluorescence—in the spectral region <21300 cm−1 in crystals with many defects—was approximately double that of defect-free regions. A maximum intensity at 23512 cm−1, or 260 cm−1 from the O-O fluorescence band, was found in crystals with many defects. The intensity of this band varied from one crystal to another, and was especially prominent in crystals melt grown under a Xenon atmosphere. Its intensity fell to zero at about 40°K and it was therefore assigned as a trap.