Photo - Fluorescence Decay Times of Organic Phosphors

A method for measuring photo-fluorescence decay times to approximately Observations are made of the phase and modulation of the light from a 7.5 Mc/s air discharge tube, and of the fluorescence excited by this light, using a photomultiplier modulated at 15 Mc/s. The decay times of anthracene, stilbene, terphenyl and diphenylacetylene crystals of various thicknesses have been measured. Due t o the overlap of the emission and absorption spectra, the technical decay time tf of a thick crystal is greater than the molecular decay time (t,), observed for a microcrystalline specimen. For anthracene tf= 14 k 2 mpsec and (t f) o = 3.5 k 1.0 mpsec. This value of tf agrees with that derived from (t,),, and the comparative technical and molecular emission spectra. For each of the compounds studied the molecular decay time (t,),,, in the absence of internal quenching, is of the order of 3-4 mpsec. sec is described. 4 1. INTRODUCTION H E decay time of the fluorescence from the organic crystals and solutions, used in scintillation counting, is of the order of IO-* second. Many T observations have been reported of the decay time tI of the fluorescence excited by ionizing radiations for the more important phosphors. Few direct measurements are available however of the decay time t, of the photo-fluorescence excited by ultraviolet radiation, apart from those of Liebson et al. (1950), who found that t, was greater than t,, and approximately equal to 2tf for most of the organic crystals investigated. Previous measurements by Bowen and Lawley (1949) and Little and Birks (1952) have shown that the spectrum of the photo-fluorescence observed in transmission through an organic crystal is critically dependent on crystal thickness, due t o the overlap of the absorption and emission spectra. Only a fraction of the molecular emission occurs in a spectral region to which the crystal is transparent, the remainder being absorbed. The absorbed radiation will be re-emitted as fluorescence, this process of emission and absorption recurring until all the initial excitation energy either escapes from the crystal as fluorescence in the transparent region, or is dissipated thermally by internal conversion. Due to this 'photon cascade' process, the technical decay times t~ and tf observed for thick crystals would be expected to correspond to the sum of the decay times of several molecular emissions (Birks 1953). This distinction between the technical and molecular decay times appears to have …