Time-resolved fluorescence spectroscopy modulators

We have already discussed quantum-beat spectroscopy (QBS) in connection with beam-foil excitation (Fig.6.6). There the case of abrupt excitation upon passage through a foil was discussed. Here we will consider the much more well-defined case of a pulsed optical excitation. If two close-lying levels are populated simultaneously by a short laser pulse, the time-resolved fluorescence intensity will decay exponentially with a superimposed modulation, as illustrated in Fig. 6.6. The modulation, or the quantum beat phenomenon, is due to interference between the transition amplitudes from these coherently excited states. Consider the simultaneous excitation, by a laser pulse, of two eigenstates, 1 and 2, from a common initial state i. In order to achieve coherent excitation of both states by a pulse of duration At, the Fourier-limited spectral bandwidth Au 1/At must be larger than the frequency separation ( - 2)/ = the pulsed excitation occurs at... 

Munro, I. H., Sabersky, A. P., Synchrotron Radiation as a Modulated Source for Fluorescence Lifetime Measurements and for Time Resolved Spectroscopy in Ref 10, p. 323... 

In the first section, steady-state spectroscopy is used to determine the stoichiometry and association constants of molecular ensembles, emphasize the changes due to light irradiation and provide information on the existence of photoinduced processes. Investigation of the dynamics of photoinduced processes, i.e. the determination of the rate constants for these processes, is best done with time-resolved techniques aiming at determining the temporal evolution of absorbance or fluorescence intensity (or anisotropy). The principles of these techniques (pulse fluorometry, phase-modulation fluorometry, transient absorption spectroscopy) will be described, and in each case pertinent examples of applications in the flelds of supramolecular photophysics and photochemistry will be presented. 

Time resolved evanescent wave induced fluorescence spectroscopy is a powerful method for the investigation of dye molecules at interfaces. This technique has been used on studies on the popular photosensidzer aluminium phthalocyanine tetra/sulphonate absorbed at fused silica/methanol interfaces . 2nd harmonic detection of sinusoidally modulated two photon excited fluorescence can also be used to obtain luminescence spectra ". ... 

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