Exciton relaxation process

In a metal, there are excited states for electrons that lie below the ionization energy. This can be conceived as an electron in a "conduction band" and a "hole" that interact so that the combination is neutral but not of lowest energy. Such an excited state is called an exciton. Excitons may move by diffusion of the electron-hole pair or by transfer of a molecular exciton to another molecule. Reversion of the exciton to a lower energy state may be slow enough for the lifetime to be longer that of lattice relaxation processes. 

For Ag, the decay time values were found similar to those reported in ref. [1, 2] providing information about the electron-phonon scattering. For Fe203, several other phenomena could cause the OD changes at the ultrafast time scale. The sub-picosecond and picosecond decay times allow to take into account hot electron thermalization [4] and subsequent fast relaxation processes such as exciton formation or surface traps filling [6]. 

In Section I, the spectra of e"(ai) consist of Dirac 5 peaks (1.79). In a real crystal these peaks are broadened by static disorder, thermal fluctuations, and excitation-relaxation processes. Discarding for the moment the static disorder, we focus our attention on broadening processes due to lattice phonons, which may be described alternatively in terms of fluctuations of the local energies of the sites, or in terms of exciton relaxation by emission and absorption of phonons. These two complementary aspects of the fluctuation-dissipation theorem64 will allow us to treat the exciton-phonon coupling in the so-called strong and weak cases. The extraordinary (polariton) 0-0 transition of the anthracene crystal will be analyzed on the basis of these theoretical considerations and the semiexperimental data of the Kramers-Kronig analysis. 

A QD Hamiltonian includes both Coulomb and electron-phonon interactions. Apparently, the phonon modes (denoted as QD) in the quantum dot are different from the semiconductor ones. The electron-phonon interaction determines relaxation processes in quantum dot (hot electrons or excitons). Thus, the QD Hamiltonian yields... 

Figure 15 The kinetic scheme illustrating the interplay between exciton (S) and charge carrier (q) trapping by crystal defects (S0t)-The PL spectrum of the crystal contains the excitonic emission (kr, hvm) and the trap center emission (kj., hi ). the latter being controlled by the number of the defect sites available for excitation. The exciton capture process (yst) competes directly with charge carrier trapping (yqt). The defects filled with charge reduce the emission resulting from radiative relaxation of the excited states produced at defect sites. For further explanations, see text.

Cyclobutanones in acetic acid undergo a regiospecific photoconversion into 2-acetoxy-5-alkoxytetrahydrofurans with retention of configuration at the migrating a-position, and an investigation of ultrafast relaxation processes in A, A -dimethylaminobenzylidene indan-l,3-dione as a molecular film has shown that formation and vibronic relaxation of the exciton states occurs in less than 100 fs. Equilibration of the two trapped exciton states seems to occur within 20 ps. 

Fig. 3.3. Ground state potential and asymmetric double-well potential associated with the phenomenon of exciton self-trapping, as a function of the coordinate rj that undergoes a strong displacement upon self-trapping. F is the bottom of the free-exciton band, in which the lattice is not distorted (77 = 0), S denotes the lowest self-trapped exciton state, and U is the barrier height. The luminescence from the self-trapped state is red-shifted relative to the free-exciton luminescence. Upon photoexcitation of the system, two pathways towards the self-trapped state occur. The first possibility is that the created excitons first relax towards the bottom of the free-exciton well, after which they may further relax to the self-trapped state through tunneling or a thermoactivated process. This pathway is indicated by the filled arrows. The second possibility is that high-energy (hot) excitons relax directly to the self-trapped state, as indicated by the open arrow. Reprinted with permission from Knoester et al. (47). Copyright Elsevier (2003).

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. 

Consider as a specific example KCI, a very simple compound indeed. In Chapter 2, its lowest optical absorption band was mentioned to be due to the 3p -> 3p 4s transition on the Cl ion. The excited state can be considered as a hole on the Cl ion (in the 3p shell) and an electron in the direct neighbourhood of the Cl ion, since the outer 4s orbital spreads over the K inns. Now we consider what happens after the absorption process. The hole prefers to bind two Cl" ions forming a Vk centre this centre consists of a C " pseudomolecule on the site of two O" ions in the lattice. The electron circles around the Vk centre. In this way a self-trapped excitnn is formed. An exciton is a state consisting of an electron and a hole bound together. By the relaxation process (Cl" Vk.c) the exciton has lowered its energy and is now trapped in the lattice. 

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