Fluorescence bands decay

For chromium containing lilac chlorite two types of luminescence were observed at 15 K phosphorescence at 14,518 and 1,547 cm with a decay time of 60 ps and a fluorescence band at about 13,850-13,500 cm with a decay time of several microseconds. For green chlorite weak fluorescence at 13,900 cm and phosphorescence at 14,320 and 14,665 cm were observed at room tern-... 

No. Glass composition, mole % (nominal) Sag point, °C Specific gravity Fluorescence of 1,06-fi band Decay Halftime, width, ( sec) A ... 

The strong dependence of the Si/ICT lifetime on solvent polarity revealed first by transient absorption experiments in the visible region by Bautista et al. [8] was further confirmed by measurement of Si/ICT fluorescence using a streak-camera [11]. The Si/ICT state fluorescence kinetics of peridinin taken at 730 nm in solvents of different polarity are shown in Fig. 2b. The lifetime changes more than one order of magnitude, from 156 ps in n-hexane to 10.5 ps in methanol. In the middle-polarity solvents tetrahydrofuran and 2-propanol, the observed lifetimes are 77 and 54 ps, respectively. In all solvents, kinetic traces could be fitted by a single exponential decay independent of detection wavelength over nearly the entire fluorescence band (650 - 850 nm). The same decay times were also observed in transient absorption [11,12]. 

Flow does the occurrence of two fluorescing states for MK fit into the dynamic picture developed in Section IV The observed temperature dependence of the fluorescence quantum yield of MK in ethanol206 yields direct evidence that in this case, also, EBA < Ev. Recent time-resolved measurements at the Berlin Electron Storage Ring for Synchrotron Radiation (BESSY)207 support this argument The viscosity dependence of the decay of the short-wavelength fluorescence band in ethanol is consistent with an apparent value BA — 0.5Ev. Moreover, the decay is nonexponential, as would be expected for a barrierless relaxation. The lifetime of the TICT state (exponential decay) is 0.65 ns in acetonitrile at room temperature, that is, it is unusually short. 

The results for 6T1 (dodecylsexithiophene) after one-photon excitation are very similar to that of 5T. At first, the transient absorption A0 was observed. The maximum of the excited-state absorption A was found by picosecond spectroscopy at 900 nm with a decay time t = HOOps. With excitation by two photons (7,exc = 616 nm, 80 fs), a broad Ao band appears first followed by the A and fluorescence bands. A0 is seen for about 500 fs later, the fluorescence predominates. 

The situation changed, however, with two advances. The first advance was the discovery that in the S, - S0 spectrum of jet-cooled anthracene a second band exists (at S, + 1420 cm-1), the excitation of which gives rise to quantum beat-modulated fluorescence decays.40 Besides indicating a somewhat more global importance to the beat phenomenon in anthracene, the characteristics of these new beats provided very strong evidence that they arose as a manifestation of IVR. In particular, the beats were shown to have phases and modulation depths dependent on the fluorescence band detected. Such behavior, which... 

Now, the preceding treatment predicts specifics about fluorescence decays that arise from two coupled vibrational levels within the same electronic-state manifold. One might ask what these decays have to do with IVR. It turns out, in fact, and this is the particular utility of time-resolved fluorescence in the study of IVR, that the decay of a given fluorescence band is a direct picture of the energy flow in to and out of the zero-order state that gives the band its emission intensity. To see this, consider first the excited state that exists instantaneously after the delta-function excitation of the two-level system ... 

The case of N = 1 is trivial in a dynamics sense in that it corresponds to no IVR. A fluorescence spectrum belonging to this case consists entirely of vibrationally unrelaxed (u-type) bands. Each of these bands decays in the same manner. In most situations, these flecays are unmodulated, single exponentials, although quantum beats and multiexponential decays arising from couplings other than those associated with IVR are possible. 

Finally, Fig. 10 shows the decays of a third group of fluorescence bands in the 61 spectrum. It is apparent from the figure that all four bands decay in a similar manner. Fourier analysis of the decays confirms that this is indeed so. Figure 11 shows the Fourier spectrum of the decay at the top of Fig. 10. One notes that the same three beat components that are present here are present in the other two decay-types. Moreover, one notes (1) two — 1 beat phases and one +1 phase, (2) that the phase behavior in Fig. 11 is different from that in Fig. 9, and (3) that the sum of beat modulation depths is —0.70. 

Figure 13 top shows the decay of the fluorescence band at vd = 390 cm-1. Fourier analysis of the decay (Fig. 14 top) reveals three prominent beat components at a>/2n = 1.0,9.7, and 10.7 GHz. [Note that these three form a triplet of the form defined by Eq. (3.15).] All of these components have +1 phases. The vd = 390 cm-1 decay is representative ofa number of other decays, those of the vd = 0, 780, 1168, and 1480 cm-1 bands. All of these bands are assignable in terms of intervals associated with optically active modes. Based... 

Figure 13. Representative decay types for fluorescence bands in the = 1420 cm 1 spectrum of anthracene. The wavenumber shifts of the bands from the excitation energy are given in the figure. From top to bottom R - 16.0.1.6, and 1.6 A.

Other excitation energies Other than the ones at S, + 1380 and S, + 1420 cm-, there are three prominent bands in the intermediate region of jet-cooled anthracene s excitation spectrum. Time- and frequency-resolved measurements subsequent to excitation of these bands have also been made. Without going into any detail concerning the results of these measurements, we do note that all three excitations give rise to quantum beat-modulated decays whose beat patterns (phases and modulation depths) depend on the fluorescence band detected.42 Figure 16 shows an example of this behavior for excitation to S, + 1514 cm-1. The two decays in the figure correspond to the detection of two different fluorescence bands in the S, + 1514 cm-1 fluorescence spectrum. 

In contrast to other vibrational levels in its vicinity, excitation to the level at E ib = 663 cm-1 gives rise to fluorescence bands, the decays of which are beat modulated. This is evident from Fig. 26 lower left, which shows the decay of the va = 800 cm-1 band in the vib = 663 cm-1 spectrum. (The spectrum appears in Fig. 24 lower left.) Fourier analysis reveals that this decay is modulated by a 780-MHz beat component having a +1 phase. Significantly, at least one other band in the same spectrum (va = 585 cm-1) is modulated at 780 MHz, but with the beat component having a — 1 phase. Therefore, unlike levels of similar energy, it appears that the t-stilbene level undergoes restricted IVR. 

Figure 26. Quantum beat-modulated fluorescence decays observed for excitation of various bands (the excess S, vibrational energies are given in cm-1 in the figure) of jet-cooled c-stilbene. The particular fluorescence band detected for each decay is given by an asterisk in the appropriate spectrum in Fig. 24. All decays were obtained with 80 psec temporal resolution except the ones corresponding to the S, + 852 and 860 cm-1 excitations, which were measured with 300 psec resolution. R for the decays was 1.6 A, except the S, +821 and 987 cm 1 decays for which R = 3.2 and 16.0 A, respectively.

Figure 28. Fluorescence decays and double exponential fits corresponding to the 20S cm"1 bands in the (from top to bottom) vlb = 1237, 1241, 1246,1249, and 1332 cm-1 spectra of jet-cooled c-stilbene. Given in the figure are the best-fit parameters for both the fast and slow lifetimes, and the ratio (F/S) of preexponential factors of fast versus slow fluorescence. All decays were obtained with R = 3.2 A. 

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