SI, singlet state

Many other processes in the Si singlet state are also possible they may compete with fluorescence and affect directly the quantum yield, the lifetime, and the... 

Similar considerations apply to the T triplet state as to the Si singlet state. By analogy with the expressions for the lifetimes of Si, the values for Ti are given by ... 

When a molecule is excited to an energy level higher than the lowest vibrational level of the first electronic state, vibrational relaxation (and internal conversion if the singlet excited state is higher than Si) leads the excited molecule towards the 0 vibrational level of the Si singlet state with a time-scale of 10 B-10 11 s. 

A different transition of the exdted electrons from the Si singlet state to the Ti triplet state by inter-system crossing can also take place. This transition is formally forbidden by quantum-mechanical rules, and consequently it occurs only very rarely and the lifetime of the Ti state is unusually long (1 ms up to hours). Decay from the Ti to the S0 state is accompanied by the emission of light called phosphorescence. The energy states and emissions responsible for fluorescence and phosphorescence are summarised in Figure 7-8. 

Another example is the deactivation of high vibrational levels in the So and Si singlet states of dye molecules in organic liquids pumped by a pulsed laser (Fig. 6.98). The laser populates many vibrational levels in the excited Si singlet state, which are accessible by optical pumping on transitions starting from thermally populated levels in the electronic ground state So. These excited levels v )... 

Therefore, in the orddative photodegradation of polyethylene, Trozzolo and Winslow suggested that one of the main causes of chain scission is the unquendied Norrish type II reaction, which mainly occurs via the excited ( , si ) singlet state of the ketone group. 

In Chapter VIII, Haas and Zilberg propose to follow the phase of the total electronic wave function as a function of the nuclear coordinates with the aim of locating conical intersections. For this purpose, they present the theoretical basis for this approach and apply it for conical intersections connecting the two lowest singlet states (Si and So). The analysis starts with the Pauli principle and is assisted by the permutational symmetry of the electronic wave function. In particular, this approach allows the selection of two coordinates along which the conical intersections are to be found. 

The first excited singlet state, 2 Sq, is also metastable in the sense that a transition to the ground state is forbidden by the Af selection rule but, because the transition is not spin forbidden, this state is not so long-lived as the 2 Si metastable state. 

Figure 9.1. A Jablonski diagram. So and Si are singlet states, Ti is atriplet state. Abs, absorption F, fluorescence P, phosphorescence IC, internal conversion and ISC, intersystem crossing. Radiative transitions are represented by full lines and nonradiative transitions by dashed lines...

Figure 11-7. Minimum energy pathways on the Si surface of cytosine connecting the Si minimum to two Si/So conical intersections. The five singlet state energies at the MRCI level are shown. Energies are given in eV with respect to the ground state minimum energy, ciIJ represents conical intersection between states S/, Sj. (From Ref. [157])...

Figure 3 Type I and type II photooxidation processes with a porphyrin sensitizer illustrated with a modified Jablonski diagram. (S0 = ground singlet state, Si = first excited singlet state, S2 = second excited singlet state, T,i— ground triplet state, Ti = first excited triplet state, i.s.c. — intersystem crossing.)...

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