FLUORESCENCE, DEFECT FLUORESCENCE, AND PHOSPHORESCENCE SPECTRA AT 4.2 °K OF UNDOPED AND β-METHYLNAPHTHALENE-DOPED NAPHTHALENE CRYSTALS

Abstract

Fluorescence, defect fluorescence, and phosphorescence spectra were obtained at 4.2 °K from undoped crystals containing unwanted impurities and from crystals deliberately doped with β-methylnaphthalene. The doped crystals included normal and fully deuterated naphthalene and β-methylnaphthalene in all combinations. Fluorescence attributable to physical defects, analogous to that recently reported for anthracene, was found in all crystals. This fluorescence consisted of bands rather than lines, and occurred in the spectral region between fluorescence (singlet emission) and phosphorescence (triplet emission). The excitation spectrum (intensity of the fluorescence as a function of excitation wavelength) showed one of two forms depending on the fluorescence band chosen, which indicated that the defect fluorescence may be excited either directly or by energy transfer, similarly to other modes of luminescence. The decay time of the defect fluorescence was < 0.01 second, indicating that the defect states are probably disturbed naphthalene singlet states.Moderately intense phosphorescence consisting of lines was found in all of the doped crystals. Assignments of the lines could not be made because of the large number of lines present. However, probable values of the triplet 0–0 levels of normal and deuterated naphthalene and β-methylnaphthalene were deduced from the present and related experiments. The undoped crystals showed a few very weak phosphorescence lines superimposed on a weak continuum of fluorescence. Those crystals with the fewest lines were the purest, and showed a decay time of line plus continuum < 0.01 second, confirming that pure naphthalene is not phosphorescent at 4.2 °K. Phosphorescence excitation spectra, decay times, and temperature dependences were found. For the doped crystals, the decay times ranged from < 0.01 to 18 seconds and the phosphorescence intensity at 10–12 °K fell to half the value at 4.2 °K, indicating an activation energy of about 37 cm−1. The phosphorescence excitation spectrum, decay time, and temperature dependence of naphthalene-d8 doped with β-methylnaphthalene were dissimilar to those of the other doped crystals, for reasons which are not yet understood.The fluorescence spectra consisted of lines for both undoped and doped crystals. The results were tabulated and compared with those of other research workers. It was not possible, however, to assign each line, to either a particular material or a particular vibrational mode.