Excited-state dynamics, time-resolved electronic relaxation

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

A time-resolved ion yield study of the adenine excited-state dynamics yielded an excited-state lifetime of 1 ps and seemed to support the model of internal conversion via the nn state along a coordinate involving six-membered ring puckering [187]. In order to determine the global importance of the tict channel, a comparison of the primary photophysics of adenine with 9-methyl adenine will be useful, as the latter lacks a tict channel at the excitation energies of concern here. The first study of this type revealed no apparent changes in excited-state lifetime upon methylation at the N9 position [188] a lifetime of 1 ps was observed for both adenine and 9-methyl adenine. This was interpreted as evidence that the tict is not involved in adenine electronic relaxation. 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

The relaxation time for this new dynamic equilibrium varies from femtoseconds to picoseconds. The fast reorientation of solvent molecules causes a fast solva-tochromic shift in the fluorescence band of the organic chromophores. Solvation dynamics is measured in terms of (8v (0) 8v (/)), where the fluctuating frequency v(t) is the difference in solvation energies between the two electronic states involved, i.e., v(t)= sE(t)/h [110]. In time-resolved emission spectroscopy the time dependence of the excited-state distribution is monitored via the frequency shift of the emission... 

To experimentally probe the electronic and thermal consequences of flash photolysis, a femtosecond time-resolved near-IR study of photoexcited Mb was undertaken (22). This study probed the spectral evolution of band III, a weak ( max 100 M-1 cm-1) near-IR charge transfer transition (14) centered near 13, 110 cm-1 that is characteristic of five-coordinate ferrous hemes in their ground electronic state (S = 2). Because band III is absent when the heme is electronically excited, the dynamics of its reappearance provides an incisive probe of relaxation back to the ground electronic state. Moreover, because the spectral characteristics of band III (integrated area center frequency line width) correlate strongly with temperature (23-26), the spectral evolution of band III also probes its thermal relaxation. 

The vibronic spectra of Do — Di — D2 electronic states recoded by da Silva Filho et al. [45] revealed resolved vibrational structures of the Do and D2 electronic states and a broad and structureless band for the Di state. A slow ( 3-20 ps) and fast k, 200 fs) relaxation components are estimated for the Dq D2 transition in a (femto)picosecond transient grating spectroscopy measurements [16]. The fast component is attributed to the Do D2 transition and a nonradiative relaxation time of 212 fs is also estimated from the cavity ringdown (CRD) spectroscopy data [42]. Electronic structure results of Hall et al. [107] suggest that the nonradiative Do D2 relaxation occurs via two consecutive sloped type CIs [66,108]. We developed a global model PESs for the Do — Di— D2 electronic states and devised a vibronic coupling model to study the nuclear dynamics underlying the complex vibronic spectrum and ultrafast excited state decay of N +[20]. 

Fig. 3 The outline of fast dynamic processes that proceed at rates comparable with that of the radiative depletion of the excited state and can be studied by time-resolved fluorescence techniques (TRF) the rate of the polymer chain dynamics (vibrational motion and relaxation) strongly overlaps that of electronic relaxation and can be studied by TRF. Adapted from Springer, Self Organized Nanostructures of Amphiphilic Block Copolymers I, 241, 2011, 187-249, figure 3, [2], Copyright 2011. With kind permission from Springer Science and Business Media...

Two-photon time-resolved photoemission (TPTRP) spectroscopy has been developed to directly study the dynamics of optically excited electrons at metal and semiconductor surfaces. This technique has been applied to direct measurement of hot electron relaxation in noble and transition metals [27, 28], surface-state dynamics on clean and adsorbate-covered metal surfaces [29, 30], as well as charge carrier dynamics in semiconductors, where much work has been performed. 

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