Relaxation of electronic excited state

As mentioned earlier, vibrational relaxation of electronically excited states in solution is very rapid and thus this situation often prevails for solute-solute transfer. In spite of the fact that the coupling is very weak in this case, quantitative theoretical treatments lead to predictions of resonance transfer between appropriate molecules over large distances, i.e., 50-100 A.81... 

A method, femtosecond time-resolved stimulated emission pumping (SEP) fluorescence depletion (FS TR SEP ED), has been developed to study the vibrational relaxation of electronic excited states of molecules (Figures 11.9 and 11.10) [31]. 

The radiative lifetime ranges from 10 to 10 s for atomic transitions and from 10 to 0.1 s for vibrational transitions in molecules. Therefore, radiative decay is entirely negligible in comparison with the other pathways on metal and semiconductor surfaces. However, it can play a role in relaxation of electronically excited states of adsorbates on dielectrics. 

Condensed phase vibrational or vibronic lineshapes (vibronic transitions create vibrational excitations of electronic excited states) rarely provide infonnation about VER (see example C3.5.6.4). Experimental measurements of VER need much more than just the vibrational spectmm. The earliest VER measurements in condensed phases were ultrasonic attenuation studies of liquids [15], which provided an overall relaxation time for slowly (>10 ns) relaxing small molecule liquids. 

Thermoluminescence. Thermoluminescence is a property of some solids in which excitation by light or particle radiation is frozen in as trapped electrons and holes or a crystal defect. Subsequent heating allows relaxation of the excited state and emission... 

It should be noted that no geometrical relaxation of the excited state geometry was considered in our calculations. Such relaxations may well occur to some degree and would then enhance the localization of the electronic excitation. However the relatively narrow spectral bandwidth suggests that the geometrical changes associated with excitation into the first excited singlet state are only minor. 

Different photoreactions can be initiated in structurally related complexes of a metal ion as a result of the intrinsic properties of the LMCT excited state and radical-ion pairs. The excited-state reactions of azido complexes of Co(III) are one example of this chemical diversity.106-109 Irradiation of Co///(NH3)5N2+ aqueous acidic solutions in the spectral region 214 nm < 2exc < 330 nm produces Coin(NH3)4(H20)N, 6 0.6, and Co(aq)2 +, molar ratio.93 The ammonia photoaquation has two sources that also account for the large quantum yield of the photoprocess. One source competes with the formation of Co(aq)2 + from radical-ion pairs. These pairs must be produced with a quantum yield 0.5. The second source is a process unrelated to the Co(aq)2 + production and it has a quantum yield excited state where a Co-NH3 bond has been considerably elongated and where the electronic relaxation of the excited state has been coupled with aquation. A second rationale for the large aquation quantum yield is that a reactive LF excited state is populated by the LMCT excited state. 

If the electron acceptors are in great excess, one can express the relaxation of the excited state population N (t) during and after arbitrary light excitation, through the survival probability of the excited donors after 8 pulse, R(t) as was done in Eq. (3.5). By substituting Eq. (3.415) into Eq. (3.5), we obtain the following for C pulse ... 

As other semiconductors, QDs are characterized by a certain band gap between their valence and conduction electron bands.20 When a photon having an excitation energy exceeding the semiconductor band gap is absorbed by a QD, electron-hole pairs are generated (electrons are excited from the valence to the conduction band) and the recombination of electron-hole pairs (the relaxation of the excited state) results in the emission of the measured fluorescence light. 

The electrons and holes flowing between the cathode and the anode in opposite directions may collide on route and then form an excited state, i.e. an exciton. Upon relaxation of the excited state to the ground state, light is emitted, if the excited state is a singlet. In the singlet excited state the spin states of the... 

Charge injection is fast compared with nuclear relaxation of the excited state (k k,). In this case, interfacial charge transfer would take place from the prepared hot vibronic level (Eq. (34)) and the quantum yield for the primary injection process would be close to unity = 1). For both limiting cases, k[ kr and k[ kr, relation (30) would be relevant, provided electron transfer is nonadiabatic. 

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