Phenomenology of recombination luminescence from trapped electrons and tetramethyl- p-phenylenediamine cation (TMPD +) in methyltetrahydrofuran glass at 77 K

Abstract

The recombination luminescence from trapped electrons (e t −) and N, N, N', N'- tetramethyl- p-phenylenediamine cation (TMPD +) in methyltetrahydrofuran glass at 77 K following polarized photoionization of TMPD has been studied. Both fluorescence (F) and phosphorescence (P) have been detected during U.V. excitation, after U.V. excitation (isothermal luminescence), and after U.V. excitation with additional stimulating light (stimulated luminescence) which detraps e t −. Polarized emission has been measured following polarized excitation and the results have been compared with theoretical polarization ratios. The TMPD triplet lifetime is 2·8 s. The initial lifetime of the isothermal luminescence depends on the excitation energy and indicates that larger average e t −, TMPD + distance distributions are produce for higher energy excitation. The F/P ratio from stimulated luminescence depends strongly on the stimulating wavelength; only phosphorescence is observed at λ > 900 nm. This differs from previous results in less polar alkane matrices. At low stimulating light energy, it is suggested that recombination occurs by tunnelling from an excited state of e t − which only has sufficient energy to produced triplet TMPD. At high stimulating light energy, e t − is excited to a conduction state from which it can recombine with TMPD + to produce both singlet and triplet TMPD. In alkane matrices it appears that only the second process is observed in the accessible stimulating wavelength range.