Excited-State Relaxation Processes

Quantum yields can provide information about the electronic excited state relaxation processes, such as the rates of radiative and nonradiative... 

Vibrationally Equilibrated Excited States Relaxation Processes... 

Any excited-state relaxation process necessarily involves the dissipation of energy from the electronically excited molecular system to the environment. This energy may appear as light, in emission as discussed above, as heat, as electrical energy (in some heterogeneous systems) and/or in some chemical form. The emphasis in this chapter is on the physical processes. 

The low-temperature limit in which 4 8 7 hv < hvi. (The classical limit corresponds to 4 8 7 hv > hv, this limit does not correspond well to observed excited-state relaxation processes). 

Surface plays an important role in excited state relaxation processes. In the ideal case of a three-dimensionally confined exciton, one expects to see strong exciton luminescence due to enhanced overlap of the electron and hole wavefunction. The radiative rate of the exciton should increase with increasing cluster size. In reality, this is generally not observed. Most of the luminescence spectra of semiconductor nanoclusters consist of a stokes-shifted broad luminescence band, usually attributed to emission from surface defects. Sometimes near the band edge, an exciton-like luminescence band can be observed. Various passivation procedures have been used to enhance the exciton luminescence. These are discussed in Section III. 

Topics recently reviewed include photochemical electron-transfer reactions and exciplexes, the design of artificial photosynthetic systems, and photoredox reactions of excited states of phenanthroline complexes. The nonradiative relaxation of charge transfer (CT) excited states to their electronic ground states does not differ in any fundamental sense from other thermal electron-transfer processes. Such excited-state relaxation processes are conveniently viewed as a special class of intervalence transitions. Despite the general relevance of these excited state processes, only a few very pertinent reports are included in this survey. The coupling of CT excited-state relaxation times and CT emission maxima to solvent relaxation times has been noted. " ... 

In photochemical experiments we are generally concerned with the spontaneous luminescence of the excited molecule simultaneous with the deactivation of the molecule to the ground electronic state. The study of parameters relating to luminescence (the energy distribution of emitted light, generally known as the emission spectrum, the emission lifetimes and the frequency of incident light needed to observe emission) provide important information on the excited state relaxation processes. ... 

The microenvironment viscosity may also affect the excited state relaxation process. In fluid solution at ambient temperatures, solvent relaxation occurs much faster than radiative decay, and the probe emits from the solvent-relaxed excited state. However, in viscous solutions radiative decay may compete effectively with solvent relaxation, resulting in a broad emission band containing contributions from both the Franck-Condon and relaxed states. Temperature is also important, since a solution will become more viscous as the temperature is decreased. At very low temperatures, the fluorophore becomes immobilised in a viscous glass, and emission arises from a state very close in energy to the Frank-Condon state. 

In the preparation of zeolite-entrapped CdS, considerable attention has been paid to the ncd/nj ratio. A marked non-stoichiometry of zeolite-hosted metal sulfide particles has been found for CdS nanoparticles by X-ray photoelectron spectroscopy [345]. After sulfidation of a zeolite X sample that is partially ion-exchanged with Cd ions, the binding energies of Cd 3ds/2 electrons decrease by about 0.3 - 0.5 eV in dependence on the diameter of the CdS nanoparticles formed. The shift originates from the replacement of ionic interactions between the Cd + ions and the zeolitic framework oxygen by more covalent (Cd -S ) bonds. However, due to the larger effective masses of the electrons and holes in CdS (m, eff = 0.42 nie, mn, eff = 0.18 m ) [339], the absorption of CdS clusters in the pores of zeohtes is less affected by the zeoUte framework than that of PbS clusters. However, the effect of the zeolite framework on the excited-state relaxation processes, i. e., the luminescence behavior of the CdS clusters, can be very large. 

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