Photochemistry pp 79-118 | Cite as

# Electronic Excitation and Decay

## Abstract

This chapter deals with the optical excitation processes that bring about transitions between electronic states and with some of the dynamical processes that may follow. The excited states created by photon absorption can be nonstationary under two basic aspects: first, they can undergo radiationless electronic transitions (nonadiabatic dynamics), and second, internal motions can occur in the new potential energy surface as the nuclear wavefunction or “wavepacket” evolves in time (adiabatic dynamics). After presenting the basic aspects of optical excitation, in this chapter we shall consider the slow radiationless transitions between electronic states caused by nonadiabatic or spin–orbit couplings. The adiabatic dynamics, i.e., the nuclear motion in a single potential energy surface, will be dealt with in the next chapter. Finally, in Chap. 5 we shall tackle the ultrafast nonadiabatic transitions that occur when two or more PESs are close in energy. In such events, the nonadiabatic dynamics and the nuclear motion are inextricably coupled. To begin with, we shall introduce and make use of some important formalisms, such as the time-dependent perturbation theory or the relationship between autocorrelation functions and spectra. In presenting such concepts and tools, we shall focus rather on their physical meaning and their applicability to real phenomena, than on the mathematical formalism.

## Keywords

Rabi oscillations Time dependent perturbation theory Autocorrelation function Franck-Condon factors Fermi golden rule Quasi-continuum## References

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