Time- resolved phosphorescence spectra

Fig. 28. Time-resolved phosphorescence spectra of quinoxaline in durene host observed at 1.38 K and at (a) 30 msec, (b) 450 msec, and (c) 1500 msec after excitation cutoff. The ordinate scale is normalized with respect to the 0 - 0" band. The numbers shown in (c) represent the vibrational frequencies (in wavenumber unit) measured from the 0 - 0" band (21639 cm r). The arrows indicate the bands whose relative intensities are remarkably enhanced at later times after the excitation cutoff. (From Yamauchi and Azumi, Ref. >)...

Time resolved phosphorescence spectra of vitreous benzophenone... 

Figure 2. Time resolved phosphorescence spectra of PVCA in solution at 77 K using a monochromator band pass of 2 nm. Delay times of 400 msec (upper) and 800 msec (lower) were used.

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). 

Time resolved phosphorescence of PAcN (At = 0, 1.0, 3.0 sec), P2VN ( At = 0, 0.5, 1.0 sec) plus biacetyl (left) (from ref. C.3, Table 2) and PVCz plus biacetyl (right) (from ref. f. Table 2), with time increments indicated (bottom-most spectrum is for At = 0). 

Some other important characteristics of the emission are the rate of the deactivation of the excited state and the rate of the radiative deactivation. If we measure a time-resolved emission spectrum of the emission, we will observe that the emission spectrum loses some intensity as a function of time after a pulsed excitation. This emission decay is usually monoexponential and corresponds to the rate constant of the deactivation of the excited state, or observed deactivation rate constant kobs- It is important here not to confuse this rate constant with the rate constant of the radiative deactivation (in Figure 8, k and for the fluorescence and phosphorescence rate constants of the ligand, respectively, for the radiative rate constant of the lanthanide). Despite the fact that this method measures the decay of the emission, between each time step, the nonradiative processes (the k deactivation rate constants in Figure 8) also deactivate the excited state. To better visualize the decay rates, some equations are helpful. 

The information obtained from the phosphorescence microwave double resonance (PMDR) spectroscopy nicely complements the results deduced from time-resolved emission spectroscopy. (See Sect. 3.1.4 and compare Ref. [58] to [61 ].) Both methods reveal a triplet substate selectivity with respect to the vibrational satellites observed in the emission spectrum. Interestingly, this property of an individual vibronic coupling behavior of the different triplet substates survives, even when the zero-field splitting increases due to a greater spin-orbit coupling by more than a factor of fifty, as found for Pt(2-thpy)2. 

Since 1997, we have been using in our laboratory an intensified charge-coupled device (ICCD, Oriel model Instaspec V, with a minimum temporal gate of 2.2 ns) in a daily basis for time resolved luminescence studies. The detector has 512x128 pixels in a maximum spectral range of 200 to 900 nm. With a single laser pulse, a fluorescence or a phosphorescence spectrum can be instantaneously obtained, since the combined use of the delay unit and time gate enables one to separate prompt from delayed emissions. 

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