Excition annihilation

Trinkunas, G., J. L. Herek, T. Polfvka, V. Sundstrom, and T. Pullerits. 2001. Exciton delocalization probed by excitation annihilation in the light-harvesting antenna LH2. Phys. Rev. Lett. 86 4167 4170. 

BAL 08] Baluschev S., Yakutkin V., Miteva T. et al, A general approach for non-coherently excited annihilation up-conversion transforming the solar-spectmm , New Journal of Physics, vol. 10, p. 013007, 2008. 

S-S annihilation phenomena can be considered as a powerful tool for investigating tire exciton dynamics in molecular complexes [26]. However, in systems where tliat is not tire objective it can be a complication one would prefer to avoid. To tliis end, a measure of suitably conservative excitation conditions is to have tire parameter a< )T < 0.01. Here x is tire effective rate of intrinsic energy dissipation in tire ensemble if tire excitation is by CW light, and T = IS tire... 

Here ak a ) is the annihilation (creation) operator of an exciton with the momentum k and energy Ek, operator an(a ) annihilates (creates) an exciton at the n-th site, 6,(6lt,) is the annihilation (creation) operator of a phonon with the momentum q and energy u) q), x q) is the exciton-phonon coupling function, N is the total number of crystal molecules. The exciton energy is Ek = fo + tfcj where eo is the change of the energy of a crystal molecule with excitation, and tk is the Fourier transform of the energy transfer matrix elements. 

Many transition metal systems are open-shell systems. Due to the presence of low-energy excited states, it is very common to experience problems with spin contamination of unrestricted wave functions. Quite often, spin projection and annihilation techniques are not sufficient to correct the large amount of spin contamination. Because of this, restricted open-shell calculations are more reliable than unrestricted calculations for metal system. Spin contamination is discussed in Chapter 27. 

The decomposition of dioxetanone may involve the chemically initiated electron-exchange luminescence (CIEEL) mechanism (McCapra, 1977 Koo et al., 1978). In the CIEEL mechanism, the singlet excited state amide anion is formed upon charge annihilation of the two radical species that are produced by the decomposition of dioxetanone. According to McCapra (1997), however, the mechanism has various shortfalls if it is applied to bioluminescence reactions. It should also be pointed out that the amide anion of coelenteramide can take various resonance structures involving the N-C-N-C-O linkage, even if it is not specifically mentioned. 

The Brueckner-reference method discussed in Section 5.2 and the cc-pvqz basis set without g functions were applied to the vertical ionization energies of ozone [27]. Errors in the results of Table IV lie between 0.07 and 0.17 eV pole strengths (P) displayed beside the ionization energies are approximately equal to 0.9. Examination of cluster amplitudes amd elements of U vectors for each ionization energy reveals the reasons for the success of the present calculations. The cluster operator amplitude for the double excitation to 2bj from la is approximately 0.19. For each final state, the most important operator pertains to an occupied spin-orbital in the reference determinant, but there are significant coefficients for 2h-p operators. For the A2 case, a balanced description of ground state correlation requires inclusion of a 2p-h operator as well. The 2bi orbital s creation or annihilation operator is present in each of the 2h-p and 2p-h operators listed in Table IV. Pole strengths are approximately equal to the square of the principal h operator coefiScient and contributions by other h operators are relatively small. 

Fig. 9.34 Monitoring of inelastic excitations by nuclear resonant scattering. The sidebands of the excitation probability densities for phonon creation, S(E), and for annihilation, S —E), are related by the Boltzmann factor, i.e., S(—E) = S E) tTvp —Elk T). This imbalance, known as detailed balance, is an intrinsic feature of each NIS spectrum and allows the determination of the temperature T at which the spectrum was recorded...

Cramer, C. J., Dulles, F. J., Giesen, D. J., Almlof, J., 1995, Density Functional Theory Excited States and Spin Annihilation , Chem. Phys. Lett., 245, 165. 

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

Much less is known about excited-state dynamics of carotenoid J-aggregates, as only zeaxanthin J-aggregates have been studied to date. Only two decay components of -5 and 30ps were needed to fit the kinetics recorded at the maximum of the Sj-S band, Figure 8.8. Since no annihilation studies were carried out, the origin of these components is not known. It is likely that the 5ps lifetime is due to annihilation whereas the 30 ps component corresponds to the. S, lifetime, which is even longer than that of the H-aggregates. 

Back electron transfer takes place from the electrogenerated reduc-tant to the oxidant near the electrode surface. At a sufficient potential difference this annihilation leads to the formation of excited ( ) products which may emit light (eel) or react "photochemical ly" without light (1,16). Redox pairs of limited stability can be investigated by ac electrolysis. The frequency of the ac current must be adjusted to the lifetime of the more labile redox partner. Many organic compounds have been shown to undergo eel (17-19). Much less is known about transition metal complexes despite the fact that they participate in fljjany redox reactions. 

In 1978 Wrighton and his group showed that the complex Re(o-phen)-(CO)-Cl undergoes eel from its lowest excited state which lies about +2.3 eV above the ground state (34). The annihilation is energy sufficient. The oxidation of the neutral complex occurs at E. - 1.3 V vs. SCE while the reduction takes place at -1.3 V. 

The potential difference for reduction and oxidation (Ae 2.6 V) provides sufficient energy to generate an excited Ir complex in the annihilation reaction. At an ac voltage of 4 V and 10 Hz we observed a weak eel of Ir(ppy) in acetonitrile. The following reaction sequence may explain this observation ... 

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