Vibrational relaxation quantum yield

Donor-acceptor separation Forster distance Rate of reaction at z Dipole orientation parameter Quantum yield of process x Quantum yield of donor emission Quantum yield of emission Quantum yield of fluorescence Quantum yield of luminescence Quantum yield of internal conversion Quantum yield of intersystem crossing Quantum yield of phosphorescence Quantum yield of triplet state formation Quantum yield of vibrational relaxation Quantum yield of singlet oxygen production Lifetime... 

Low temperature experiments have shown the formation of hypso intermediates from several species [99,103,105-107]. The study of early photoconversion processes in squid [108], which also involved the evaluation of the relative quantum yields among the four pigments (squid rhodopsin, squid batho-, hypso- and isorhodopsin) showed that hypsorhodopsin is a common intermediate of rhodopsin and isorhodopsin there is no direct conversion between rhodopsin and isorhodopsin bathorhodopsin is not converted directly to hypsorhodopsin and both rhodopsin and isorhodopsin convert more efficiently to bathorhodopsin than to hypsorhodopsin. While a temperature dependence of the relaxation processes from the excited state of rhodopsin, and an assumption that batho could be formed from one of the high vibrational levels of the ground state hypso have been invoked to explain these findings [108], the final clarification of this matter awaits results from subpicosecond laser photolysis experiments at liquid helium temperature. 

The lack of the triplet-triplet absorption of carbocyanine dyes is due to low values of the intersystem crossing rate constants as compared with the rate constants of competing processes [5, 9]. The dye-DNA interactions lead to an increase in the quantum yield of the triplet state of the dye molecules, since the complexation impedes the processes of photoisomerization and vibrational relaxation (nonradiative deactivation), thus permitting the detection of T-T absorption spectra of the bound dyes upon direct photoexcitation. In the presence of DNA in the solutions, the triplet lifetimes of the dyes comprise himdreds of microseconds [10]. 

Electronic Spectrum. Acetone is the simplest ketone and thus has been one of the most thoroughly studied molecules. The it n absorption spectrum extends from 350 nm and reaches a maximum near 270 nm (125,175). There is some structure observable below 295 nm, but no vibrational and rotational analysis has been possible. The fluorescence emission spectrum starts at about 380 nm and continues to longer wavelengths (149). The overlap between the absorption and the fluorescence spectra is very poor, and the 0-0 band has been estimated to be at - 330 nm (87 kcal/mol). The absorption spectra, emission spectra, and quantum yields of fluorescence of acetone and its symmetrically methylated derivatives in the gas phase havbe been summarized recently (101). The total fluorescence quantum yield from vibrationally relaxed acetone has been measured to be 2.1 x 10 j (105,106), and the measurements for other ketones and aldehydes are based on this fluorescence standard. The phosphorescence quantum yield is -0.019 at 313 nm (105). 

In the same sense, in condensed media, it is often assumed that upper excited electronic states relax rapidly by internal conversion and vibrational equilibration to the lowest excited electronic state of the same multiplicity. Thus, referring to Figure 3, which extends the mechanism of Figure 1, if two excited states, A2 and A, have the same multiplicity, it is likely that the upper state, A, would undergo rapid internal conversion to A2 in solution. The sequence 03-32 could then be considered as the equivalent of step 02, provided that neither of steps 34 or 35 could compete, under these conditions, with step 32. A common diagnostic test (but not proof) of this situation is independence of the quantum yield on wavelength. 

Figure 2a illustrates the concepts of radiative and nonradiative decay, fluorescence quantum yield, and fluorescence decay. A molecule in an excited electronic state can relax by several channels. Molecules excited to a vibrational level in the excited state undergo vibrational relaxation (cooling, yellow arrows in Fig. 2a) to the lowest vibrational levels of the excited state in a... 

Charge injection is slow enough compared with the vibrational relaxation of the dye excited state (k/ kj). In this event, electron transfer would be able to take place only from the lowest excited state v — 0) (Eq. (35)), and the injection quantum yield would be simply controlled by the kinetic competition between the electron injection (Eq. (35)) and the decay of the excited state (Eq. (36)) ... 

The values of the primary quantum yields, found in the photolysis of ketones with y-H atoms, were explained by Brunet and Noyes on the basis of steric effects that diminish the probability of the formation of the cyclic complex. Following the original suggestion of Whiteway and Masson, Martin and Pitts proposed the internal conversion of the cyclic structure to be responsible for the low primary quantum yields observed in the photolysis of ketones capable of forming such a structure. In contrast to other interpretations. Wagner and Hammond explained the low quantum yields by an elementary chemical process, suggesting that the y-H atom transfer is reversible, i.e. that the biradical, after vibrational relaxation, may convert back into the ground state ketone molecule, viz. 

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