Time- and frequency-resolved fluorescence

A. Mokhtari, J. Chesnoy and A. Laubereau, Femtosecond time-and frequency-resolved fluorescence spectroscopy of a dye molecule, Chem. Phys. Letts, 155, 593-598 (1989). 

The second major section (Section III), comprising the bulk of the chapter, pertains to the studies of IVR from this laboratory, studies utilizing either time- and frequency-resolved fluorescence or picosecond pump-probe methods. Specifically, the interest is to review (1) the theoretical picture of IVR as a quantum coherence effect that can be manifest in time-resolved fluorescence as quantum beat modulated decays, (2) the principal picosecond-beam experimental results on IVR and how they fit (or do not fit) the theoretical picture, (3) conclusions that emerge from the experimental results pertaining to the characteristics of IVR (e.g., time scales, coupling matrix elements, coupling selectivity), in a number of systems, and (4) experimental and theoretical work on the influence of molecular rotations in time-resolved studies of IVR. Finally, in Section IV we provide some concluding remarks. 

Suppose, now, that the molecule, initially in the ground-state vibrational level gt), is excited by a delta-function pulse of light to the excited state manifold, whereupon time- and frequency-resolved fluorescence to the ground-state vibrational level /> is detected. The signal38 in such an experiment is... 

Figure 22. Time- and frequency-resolved fluorescence of jet-cooled d, -anthracene (top) and d10-anthracene (bottom). The d, decay corresponds to excitation toS, + 1411cm-1 and detection of the 388 cm-1 band in the fluorescence spectrum. The dl0 decay corresponds to excitation to St + 1452 cm-1 and detection of the 372 cm-1 band in the spectrum. Quantum beats are clearly present in both decays.

Fig. 3. Time and frequency resolved fluorescence signal ( ) dispejggj emission spectra (for two probe carrier frequencies, 0J2 = and ojj g...

Fig. 14. Impulsive time- and frequency resolved fluorescence spectra as obtained for (a) the two-mode model and (b) the 25-mode model of rhodopsin. The prominent recurrences of the emission reflect quasiperiodic wave-packet motion on coupled adiabatic potential-energy surfaces.

Fig. 15. Impulsive time- and frequency resolved fluorescence spectra as obtained for the 25-mode model of rhodopsin coupled to a harmonic bath.

Spencer, C.F., Loring, R.F. (1996). Dephasing of a solvated two-level system A semiclassical approach for parallel computing. J. Chem. Phys. 105 6596-6606 Loring, R.F, Yan, Y.Y., Mukamel, S. (1987). Time-and frequency-resolved fluorescence line shapes as a prohe of solvation dynamics. Chem. Phys. Lett. 135 23-29. 

We show how one can image the amplitude and phase of bound, quasibound and continuum wavefunctions, using time-resolved and frequency-resolved fluorescence. The case of unpolarized rotating molecules is considered. Explicit formulae for the extraction of the angular and radial dependence of the excited-state wavepackets are developed. The procedure is demonstrated in Na2 for excited-state wavepackets created by ultra-short pulse excitations. 

The role of vibrational relaxation and solvation dynamics can be probed most effectively by fluorescence experiments, which are both time- and frequency-resolved,66-68 as indicated at the end of Sec. V. We have recently developed a theory for fluorescence of polar molecules in polar solvents.68 The solvaion dynamics is related to the solvent dielectric function e(co) by introducing a solvation coordinate. When (ai) has a Lorentzian dependence on frequency (the Debye model), the broadening is described by the stochastic model [Eqs. (113)], where the parameters A and A may be related to molecular... 

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. 

Fluorescence is unique among spectroscopic techniques due to its inherently multidimensional nature, the emission process containing a wealth of orthogonal information that is related to the fluorophore and its surroundings. Time- and frequency-domain fluorescence methods are instrumentally sophisticated, but they improve both the sensitivity and selectivity of fluorimetry. Any dye that is used for steady-state fluorescence detection can be used for time-resolved detection as well. Most of these fluor-ophores display lifetimes from 1 to 10 ns, which requires fast electronics for time-domain lifetime measurements or modulation frequencies from 10... 

Time- and frequency-domain fluorescence methods have been used in different fields for several purposes, such as distinguishing sample constituents whose fluorescence spectra overlap one another, to distinguish the fluorescence of an analyte from background scattering or luminescence of other sample constituents, in combination with fluorescence resonance energy transfer (FRET) assays to combine the benefits of FRET and time-resolved fluorescence. A listing of some selected applications is given in Table 3. 

Shapiro, M., Imaging of wave functions and potentials from time-resolved and frequency-resolved fluorescence data, /. Chem. Phys., 103, 1748-1754,1995. 

Patterson MS, Pogue BW. Mathematical model for time-resolved and frequency-domain fluorescence spectroscopy in biological tissue. Applied Optics 1994, 33, 1963-1974. 

The fluorescence and phosphorescence of quinazoline, 6-chloro-4-phenyl-and 6-chloro-l-methyl-4-phenylquinazolin-2(lH)-one were recorded in ethanol containing 1% of concentrated sulfuric acid. The luminescence of these compounds on thin-layer chromatography (TLC) plates saturated with ethanol was reported. 4-Morpholino- and 4-piperidino-6-methoxy-2-phenyl-quinazoline also have luminescent properties, and the ultraviolet fluorescence in the crystals and in hexane or benzene solution was discussed. The time and wavelength resolved emission from quinazoline vapor at low pressures was studied with a pulsed frequency double-dye laser and were compared with those of quinoxaline and cinnoline. ... 

A general discussion of the use of least-squares fitting in fluorescence measurements may be found in (28). The global analysis of fluorescence data is discussed in (29). Commercially available time-resolved fluorimeters are typically sold with data analysis software included. Available stand-alone packages include the Globals Unlimited suite, which is capable of analysing both time- and frequency-domain data, stopped-flow kinetics, etc. The Center for Fluorescence Spectroscopy at the University of Maryland (USA) also offers software for frequency- and time-domain fluorescence lifetime analysis. 

A method less common for lifetime measurements is the so-called pump-probe or double-pulse approach. Like time- and frequency-domain detection, the technique originates in non-spatially-resolved fluorescence spectroscopy [ 19]. In this technique, two very short excitation pulses follow each other. The first pulse excites fluorochromes inside the detection volume to full or partial saturation. The second pulse, or probe pulse, arrives at a variable (ns) time delay. If the time delay between the pulses is short compared to the fluorescence lifetime, most of the fluorochromes will still be in the excited state when the second pulse arrives so that the second pulse cannot excite additional fluorochromes and thus does not lead to additional fluorescence. If the time delay is long, most fluorochromes will have relaxed back to their ground state, so that the second pulse leads to... 

Dr can be determined by time-resolved fluorescence polarization measurements, either by pulse fluorometry from the recorded decays of the polarized components I l and 11, or by phase fluorometry from the variations in the phase shift between J and I as a function of frequency (see Chapter 6). If the excited-state lifetime is unique and determined separately, steady-state anisotropy measurements allow us to determine Dr from the following equation, which results from Eqs (5.10) and (5.41) ... 

There are two widely used methods for measuring fluorescence lifetimes, the time-domain and frequency-domain or phase-modulation methods. The basic principles of time-domain fluorometry are described in Chapter 1, Vol.l of this series(34) and those of frequency-domain in Chapter 5, Vol. 1 of this series.<35) Good accounts of time-resolved measurements using these methods are also given elsewhere/36,37) It is common to represent intensity decays of varying complexity in terms of the multiexponential model... 

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