Absolute Quantum Efficiency of Photofluorescence of Anthracene Crystals

Abstract

The fluorescence characteristics of organic crystals differ appreciably from those of the emitting molecules due to self-absorption of the molecular fluorescence. These differences are investigated and properties of the fluorescence of the crystal are related to the corresponding properties of the molecular fluorescence; the equations developed are made the basis of an experimental determination of molecular quantum efficiencies of photofluorescence for the crystalline state. This method avoids the considerable errors involved in a direct measurement which are occasioned by having to integrate numerically over the whole sphere of emission inside an optically anisotropic crystal the fluorescence intensity that is measured directly only within a certain solid angle external to the crystal. The method developed is particularly applicable to crystalline anthracene which has considerable overlap of absorption and fluorescence spectra. Experimental data for this substance are presented. These include the molecular and crystal fluorescence spectra and decay times; the latter are found to be 6 4 ± 0.2 mμ sec and 18.0 ± 0 5 mμ sec respectively. The molecular quantum efficiency of photofluorescence of crystalline anthracene at a temperature of 290°κ is found to be 0.94 ± 0 02 which is reduced, by the effects of self-absorption of fluorescence, to a quantum efficiency for the crystal of 0.80 ± 0.05; efficiencies of photoluminescence are found to be the same within the limits of experimental error.