Vibrationally Equilibrated Excited States Relaxation Processes

Vibrationally Equilibrated Excited States Relaxation Processes... 

For thiones the processes of intramolecular (subpicosecond) and intermolecular (a few ps) vibrational relaxation occur on a time scale that is rather shorter than in at least some solvents (water [84] is exceptional). Thus the relaxation dynamics of the vibrationally equilibrated excited state can be observed in different solvents [54,58,59,63,67,85-87]. 

Chemical lasers are complex nonequilibrium molecular systems governed by an intricate interplay between a variety of chemical, radiative, and collisional relaxation processes. Many of their kinetic properties are reflected by the temporal, spectral, and power characteristics of the out-coupled laser radiation. For example, threshold time measurements and other gain experiments have provided detailed information on vibrational distributions of nascent reaction products. Another, more qualitative, example Single-line and simultaneous multiline operation indicate, respectively, whether the lasing molecules are rotationally equilibrated or not. Besides their practical applications, chemical lasers are widely used as means of selective excitation in state-to-state kinetic studies. On the other hand, many experimental and theoretical studies have been motivated by the wish to understand and improve the mechanism of chemical laser operation. 

In a crystalline host, the potential curves as drawn in Figure 5 describe the total energy, complex and environment. If vibrational relaxation within an electronic state is faster than other competing steps, then photophysical and photochemical processes occur in thermally equilibrated populations. Figure 5 is also applicable for a rigid, noncrystalline medium, but as the solvent melts and solvent relaxation takes place during the excited-state lifetime, a more complex representation is required. 

The next process is the emission of a photon. As already mentioned, the emission of photons in a macroscopic system of fluorophores proceeds on the nanosecond timescale. This means that most photons are usually emitted and detected from excited states with fully relaxed solvate shell. Because the excited state population decays exponentially, fast measurements enable detection of hot photons from non-relaxed states at early times. When discussing the relaxation at the level of a single molecule, we have to consider different timescales. An isolated single emission event is as fast as the absorption, and the electronic transition takes less than 10 s. It is evident that the fluorophore does not reach the relaxed ground state immediately after the emission of a photon. What follows, is a cascade of processes that resemble the mirror image of the above-described relaxations. Firstly, the vibrational relaxation occurs and, finally, the solvent equilibrates corresponding to... 

These complexes also demonstrate a change in the excited state character between a Frank-Condon (vibrationally hot ) electronically excited state and the vibrationally relaxed, lowest excited state. Resonance Raman (rR) spectra show that the vibrationally hot Franck-Condon states of [RuI(Me)(CO)2(iPr-DAB)] have virtually pure XLCT character [55]. However, the TRIR data indicate that thermally equilibrated, vibrationally-relaxed excited state has a mixed MLCT-XLCT character [6]. Hence, combining the results from resonance Raman and TRIR data allows one to obtain insight into charge redistribution processes in the excited state on a very short timescale. 

In condensed phase, vibrational relaxation is a very fast process (10 to 10 s), so that the electronically excited states involved in bimolecular processes are thermally equilibrated species. This means that these reactions can be dealt with in the same way as any other chemical reaction, i.e. by using thermodynamic and kinetic arguments. 

For this reason it equilibrates its temperature in a short time in the condensed phase. It deactivates without radiation to the lowest vibrational state of the actual electronic state (marked in Fig. 1.1 as te). By isoelectronic internal conversion (ic) it can pass over to a very high vibrational state of the next lower electronic energy state. These deactivations normally take place until the molecule has reached the vibrational ground state of the first excited electronic state S,. This overall deactivation process is called thermal relaxation (sd), which can be divided into the isoenergetic deactivation internal conversion and the non-isoenergetic thermal equilibration . Its time scale is 10 ° to 10 s. The process can be symbolised by... 

Figure 6 shows deactivation as well as excitation processes. After excitation to a higher level, the molecule usually relaxes to the vibrational ground state of the 5, level by thermal equilibration (te). This 5i level is the starting point for several competitive processes, the preferred one depending on the type of molecule, temperature, and its environment [29]. These include (1) radiationless deactivation (rd), which consists of internal conversion (ic) followed by thermal equilibration (te) (2) intercombination or intersystem crossing (isc) (3) spontaneous emission (F, P) and (4) photochemical processes (see survey in... 

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