Seite - 60 - in Advanced Chemical Kinetics

than the singlet electronic excited state of lower energy. Since spin changes are forbidden in electronic transitions, this is a process that occurs only if relaxation in the spin selection rule occurs,providedbyspin-orbit couplingderived fromthecouplingof theelectronspinmotion with its orbitalmotion.Due to that prohibition, this is a very slowprocess, taking from10 6 seconds tominutesorevenhours tooccur [2]. 2.4.Vibrational relaxation Theprocessof releasingtheenergygivenbytheabsorptionofaphotonaskineticenergyis the vibrational relaxation. It involves the conversion of a vibrationalmodewithin an electronic Figure 2. Franck-Condon vibronic absorption from the electronic ground state to an excited state: From the lowest vibrational state (v0) in the ground state toA) the lowest vibrational state (v0) in the excited electronic state andB) to a highervibrational state (V4) in theexcitedelectronic state. Advanced Chemical Kinetics60