Delayed luminescence polarization

Thus, BNZ-doped polystyrene appears to possess two structural features which influence the characteristics of the delayed luminescence. One of these involves clusters of BNZ molecules consisting of relatively densely populated regions along with others which are much less densely populated. The other consists of molecular pairs which are capable of trapping excitons by virtue of kinetic barriers as opposed to energetic ones. Although it might seem that features of this sort would be common to all molecularly doped polymers, a search for them in N-ethylcarbazole (NEC) doped polystyrene revealed no indication of their presence. In particular, there was found to be no time dependence of the delayed luminescence polarization on the millisecond time scale(8). With NEC, however, one does find a dependence of the delayed luminescence polarization on dopant concentration. That is, the polarization decreases monotonically as the concentration increases. 

Figure 1. Delayed Luminescence Anisotropy (DIA). A) The sample (lined sphere) is excited with polarized light (vector. The emission is observed at 90" through an analyzer set at an angle corresponding to vector %). B) The ground state G is excited to the singlet state S (square) which decays directly (fluorescence f) or indirectly (phosphorescence p or delayed fluorescence by thermal reactivation to S) through the triplet state T. The circled states are long-lived and the hollow line connecting T and G denotes a slow decay process. Non-radiative modes are omitted (see text).

Fluorescence applied to oil identification has been an active field, with 17 papers presented on the subject at the last three Pittsburgh Conferences. A number of interesting developments for fluorescence and low-temperature luminescence (LTL) are described by Eastwood et al. (58). These include synchronous scanning, difference spectrofluorometry, synchronous difference spectroscopy, derivative spectroscopy, and total luminescence (or contour) spectroscopy and combinations of these techniques. In a recent presentation, Eastwood and Hendrick (59) reported an extension of their low-temperature luminescence studies to include polarized excitation and emission spectroscopy, and time-resolved phosphorescence. Preliminary studies of polarization effects indicate that differences exist in low-temperature polarized luminescence spectra of oils, which may aid in oil identification. In the time-resolved phosphorescence spectra of oils, the most significant difference observed was enhancement of the vanadyl porphyrin signal at approximately 700 nm for short delay times (20 fxsec). 

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