Electronic relaxation processes

Collisional Electronic Relaxation Processes, a singlet excited state (S ) of an aldehyde or ketone can be electronically quenched by a collision partner molecule, 

Similarly, the fluorescence quantum yield in the presence of a quencher ( t F) is given by another Stern-Volmer expression, 

The electronic quenching process (eq. 13) may occur through an exoergic electronic-to-electronic (E-E) energy transfer, an electronic-to-vibrational (E-V) energy transfer, an electronic-to-rotation (E-R) transfer, an electronic-to-translation (E-T) transfer, or a chemical reaction. Studies of E-E transfer, 

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J. Tauc, Time-Resolved Spectroscopy of Electronic Relaxation Processes P.E. Vanier, IR-Induced Quenching and Enhancement of Photoconductivity and Photoluminescence... 

The slow-motion theory describes the electron relaxation processes implicitly, through a combined effect of static and transient ZFS, and reorienta-tional and pseudorotational dynamics. This is necessary under very general conditions, but simpler descriptions, appropriate in certain physical limits, can also be useful. In this chapter, we review some work of this type. 

J. Tauc, Time-Resolved Spectroscopy of Electronic Relaxation Processes... 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

The electronic states of the medium must be far enough removed from those of the solute in which the electronic relaxation processes are occurring. For example, in pure crystals the intermolecular coupling does appear to influence the radiationless processes.42"43... 

It thus appears that, in general, the rates of electronic relaxation processes in dense media are of the same order of magnitude as those found for isolated molecules in the gas phase, assuming that the molecule has a sufficiently large number of vibrational degrees of freedom. Therefore it may be concluded that the mechanism which operates in isolated molecules must be responsible for the gross features of the phenomena observed in dense media. 

A phenomenon closely related to electronic relaxation is the existence of diffuseness in the absorption spectra of the higher excited electronic states of molecules. It has been known for some time that very fast electronic relaxation processes occur when the higher excited states of molecules are caused to interact with radiation. It is remarkable then that only in relatively recent work has the association between these fast processes and spectral diffuseness been clearly focused upon. These spectral results provide some of the most definitive features that may be associated with the electronic relaxation mechanisms. First, the results from solid-state spectra 62 ... 

M. Bixon and J. Jortner, Comments on Electronic Relaxation Processes in Molecular Crystals, in Molec. Crystals (in press). 

A special version of UV-vis spectroscopy is the detection of the sample light emission after irradiative molecular excitation. This phenomenon is called luminescence and specified as fluorescence in case of relatively fast electron relaxation processes without changing spin multiplicity (time scale of microseconds and below). 

A particular kind of electronic relaxation process is electron transfer. In this case (see Chapter 16) the electronic transition is associated with a large rearrangement of the charge distribution and consequently a pronounced change of the nuclear configuration, which translate into a large A. Nuclear tunneling in this case is a very low-probability event and room temperature electron transfer is usually treated as an activated process. 

The excited electronic states of pyridazine have attracted interest because of the possibility to explore the interactions of the nonbonding (n) orbitals of the nitrogen atoms and as a model for testing the theory of nonradiative electronic relaxation processes. Several singlet excited states of pyridazine have been examined, and the changes in hydrogen atom positions on electronic excitation of pyridazine were calculated. ... 

Carotenoids are still highly topical systems for research. Both Sj Sq and S2 Sq electronic relaxation process in carotenoids with 7 to 11 conjugated double bonds have been subjected to very comprehensive study . The lifetime of the S2 state of P-carotene in CS2. measured by a femtosecond absorption method, is found to be 200-250 fs at room temperature . Fs time resolved CARS from p-carotene in solution shows the occurrence of ultra-high frequency (llTHz) beating phenomena and sub-ps vibrational relaxation. The same technique has been used to observe solvent effects on the a C=C stretching mode in the 2 Ag excited state of P-carotene and two derivatives . A similar study has been made with several derivatives of P-carotene. ... 

Table II, which is adapted from a previous description (Nenner 1987 Nenner et al. 1988), presents some of the electronic relaxation processes associated with the decay of a core hole. In these equations, c represents a core orbital, v an occupied valence orbital, u an unoccupied valence or Rydberg orbital, and s represents a shape resonance orbital. The term orbital is used simply to mean a one-electron wavefunction. An electron in a continuum orbital free from the influence of the molecular potential is represented as e .

Clearly, much activity may be expected in the specific area of rotational coherence and the more general one of time-resolved and polarization-analyzed fluorescence. One area of interest, not touched upon here, concerns the influence of rotational coherence in electronic relaxation processes. In this we regard it is pertinent to note the polarization-dependent decays reported first by Matsumoto et al.72 in their studies on the nanosecond time scale of singlet-triplet coupling in pyrazine. 

Picosecond Spectroscopy and Dynamics of Electron Relaxation Processes in Liquids... 

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