Slow electrons triplet state excitation

If we substitute amplitude (4.32) into formula for the differential excitation cross section (4.9) and integrate it, making the substitution (1/27r) dil = q dqlk0kn, we will get a factor (hk0)6 in the total cross section. Thus, while the direct scattering cross section decreases as 1IE with decrease of the energy, the exchange cross section behaves as 1/E3, meaning that the triplet states can be excited only by the slow electrons. 

As a rule, the lowest excitation level in molecules is a triplet level. Since such states are efficiently excited by slow electrons (see Section IV.B.3), it is the energy of the lowest triplet level that one should take as a boundary energy for subexcitation electrons rather than the energy of the first singlet excitation level, as was done in Ref. 23. 

Phosphorescence is emission of light from triplet excited states, in which the electron in the excited orbital has the same spin orientation as the ground state electron. Transitions to the GS are forbidden and the emission rates are slow, so that phophoresence life times are typically milliseconds to seconds. 

In conclusion, time-resolved excitation spectroscopy or, more correctly, excitation spectroscopy with time-resolved detection of emission, opens access to studies of intra- and inter-system crossing paths, i.e. of relaxation paths within or between hypersurfaces of different triplet substate systems. This method -applied for the first time in our investigation [60] for transition metal complexes - complements other measurements of pico-/subpico-second time resolution. In particular, it is shown that after an excitation of a vibrational state of an excited electronic triplet substate, the relaxation proceeds within the same triplet substate system downwards to the zero-point vibrational level. Subsequently, an inter-system crossing to a different sublevel system occurs in a relatively slow process by spin-lattice relaxation. This result fits well to the concept that a spin-flip is usually slower than the process of intra-state relaxation. 

Photoinduced electron transfer from dilferent electron donors to the triplet excited states of Ceo and C70 occurs efficiently and is typically associated with a small reorganization energy [18, 19, 21-27]. Consequently, the occurrence of fast electron-transfer events involving the fullerene excited states has been well established as giving rise to small intrinsic barriers. In contrast with the fast electron-transfer reactions of the triplet excited state of Ceo, an extremely slow electron-transfer reaction has been reported for the reaction of Ceo in its ground state with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to produce Ceo in benzonitrile. The latter can be followed even by conventional Vis-NIR spectroscopy [28]. In this instance, however, it is not clear whether the generation of Ceo is directly related to electron transfer from DBU to Ceo, or if Ceo evolves from the produet of a secondary reaction. 

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