Fluorescence oscillations

Figure 2.17. Fluorescence spectra of the anthracene crystal at 5 K, observed from the front (001) face of a thin sample. 20 The main lines cited in the text are as follows Ph 23 = 25082cm Ph46 = 25052cm Ph 61 = 25037cm F ° = 24703cm Flvo+ 46 25052 cm" 1 -390 cm" F 00 = 23692cm" We note the very narrow and strong line (laser emission) of the 0-1400 fluorescence, as well as the fluorescence oscillations around the structure Ph 135.

Laplante and Pottler (1982) observed fluorescence oscillations upon irradiating the 9,10-dimethyl anthracene/chloroform systems continuously. Fluorescence oscillations were found to occur immediately after the reaction was started. However, initial oscillations were usually found to be of horatian nature and periodic oscillations were obtained following a horatian initial period. In some cases the periodic behavior was not reproducible even though the nonperiodic oscillatory behavior could be reproduced. Experiments were carried out to determine the effect of e.g., excitation wavelength, substrate concentration and specificity. The mechanism has not yet been elucidated. 

The fluorescence oscillates in time with frequency 2t>. Of course, since we neglected the radiative width, it seems to oscillate forever. If we reintroduce this width, realizing that each molecular eigenstate in this symmetric case gets y/2, we have... 

In Figs. 1 and 2 (A,B), the fluorescence yields, 80 ms after each flash of a series of flashes and the luminescence intensities, 1ms after each flash were measured under the same flash energy. The two types of inside-out vesicles used (BS and 30S) differ in their functional antenna size. The antenna size of the 30S fraction is smaller than the antenna size of the BS fraction which is about the same as that of the inside-out thylakoids from which it originates [12]. In the 30S fraction, less flash energy is given to the photosystem II centers and the fluorescence oscillations in Fig. 2 present the same change observed when the flash energy is decreased [ 6]. The least... 

Fig. 1 (left). In the BS fraction of inside-out thylakoids, comparison of the luminescence intensity, measured 1 ms after each flash of a series (B), with the predicted flash yields of proton release according to the stoichiometry 0,0,2,2 and 1,0,1,2 (C). The transition parameters are those obtained from the fitting of the oscillation pattern of fluorescence yield under the same conditions. The dark adaptation was 10 min, the flash interval 730 ms and 0.5 mM Fecy was added. 1 pM DBMIB was added in the luminescence experiments only. Experimental data are represented by dashes, theoretical data by dots. The best least squares fitting [8] yields a = 0.01, P = 0 and a progressive decrease of 8% of the number of centers after each flash (z= 0.92). The calculated concentrations of Sq,Sj,S2,S in the dark are in %, 36,51,7.5,5.5 assuming that the fluorescence oscillations arise from the S state. 

Fig. 4. Theoretical oscillation patterns of the S concentration simulating experimental fluorescence oscillations and the corresponding predicted flash yields of proton release according to the stoichiometry 0,0,2,2. The transition parameters are the same, a = 0.05, p = 0, z= 0.9. The apparent ratio S (Sq+ Sj) changes from 15% to 60%. Dashed line 15% after dark adaptation.

Fluorescence decay kinetics also can be measured by exciting the sample with continuous light whose intensity is modulated sinusoidally at a frequency (m) on the order of 1/t, where t again is the fluorescence lifetime. The fluorescence oscillates sinusoidally at the same frequency, but the amplitude and phase of its oscillatirais relative to the oscillations of the excitation light depend on the product of oo and t (Fig. 1.16 and Appendix A4). If mr is much less than 1, the fluorescence amplitude tracks the excitation intensity closely if an is larger, the oscillations are delayed in phase and damped (demodulated) relative to the excitation [28-30]. Fluorescence with multiexponential decay kinetics can be analyzed by measuring the fluorescence modulation amplitude or phase shift with several different frequencies of modulated excitation. 

It is instructive to compare these results with the fluorescence oscillations we discussed in Sect. 10.8 (Eq. 10.79). The electronic coherences we considered there caused the amplitudes of the fluorescence at different frequencies to oscillate in phase. Here the oscillations at different frequencies are out of phase. The difference is that in Sect. 10.8 we considered coherences between two excited electronic states that decay to a single ground state, whereas here we are discussing transitions from a single excited electronic state to various vibrational levels of the ground state. 

Stanley R J and Boxer S G 1995 Oscillations in the spontaneous fluorescence from photosynthetic reaction centers J. Phys. Chem. 99 859-63... 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Figure C 1.5.10. Nonnalized fluorescence intensity correlation function for a single terrylene molecule in p-terjDhenyl at 2 K. The solid line is tire tlieoretical curve. Regions of deviation from tire long-time value of unity due to photon antibunching (the finite lifetime of tire excited singlet state), Rabi oscillations (absorjDtion-stimulated emission cycles driven by tire laser field) and photon bunching (dark periods caused by intersystem crossing to tire triplet state) are indicated. Reproduced witli pennission from Plakhotnik et al [66], adapted from [118].

One characteristic property of dyes is their colour due to absorption from the ground electronic state Sq to the first excited singlet state Sj lying in the visible region. Also typical of a dye is a high absorbing power characterized by a value of the oscillator strength/ (see Equation 2.18) close to 1, and also a value of the fluorescence quantum yield (see Equation 7.135) close to 1. 

Figure 9.45(a) shows fhe resulting fluorescence intensify as a function of time. This is dominated by oscillations wifh a period of abouf 300 fs buf shows an amplifude modulation wifh a period of abouf f 0 ps. The 300 fs period is fhaf of a vibration in fhe u = 7 — f 2 range and fhe modulation is due to fwo neighbouring levels having slighfly differenf frequencies due to fhe vibration being anharmonic. 

Figure 9.45 (a) Oscillations, in the time domain, in the fluorescence intensity from the/state of I2... 

The oscillator strength of the longest wavelength absorption band of BMPC (1.1, [25]) is very similar to those of two previously studied carbocyanines (DOC and DTC) [45] so that we can expect that, for BMPC as well as for EK)C and DTC, the radiative constant (kp) is equal to 2-3x10 s". Combining this value with the fluorescence quantum yield of BMPC in methanol, 4)p= 5.3x10", we can estimate its room-temperarnre fluorescence lifetime to be = 2 ps. 

Certain features of light emission processes have been alluded to in Sect. 4.4.1. Fluorescence is light emission between states of the same multiplicity, whereas phosphorescence refers to emission between states of different multiplicities. The Franck-Condon principle governs the emission processes, as it does the absorption process. Vibrational overlap determines the relative intensities of different subbands. In the upper electronic state, one expects a quick relaxation and, therefore, a thermal population distribution, in the liquid phase and in gases at not too low a pressure. Because of the combination of the Franck-Condon principle and fast vibrational relaxation, the emission spectrum is always red-shifted. Therefore, oscillator strengths obtained from absorption are not too useful in determining the emission intensity. The theoretical radiative lifetime in terms of the Einstein coefficient, r = A-1, or (EA,)-1 if several lower states are involved,... 

Little attention has been devoted to the effects of time-dependent magnetic fields (created by electromagnetic waves) in the absence of a strong magnetic field. Hore and his coworkers [123-125] recently described this effect as the oscillating magnetic field effect (OMFE) on the fluorescence of an exciplex formed in the photochemical reaction of anthracene with 1,3-dicyanobenzene over the frequency range 1-80 MHz. 

---