Self-trapped excitons

Such renormalization can be obtained in the framework of the small polaron theory [3]. Scoq is the energy gain of exciton localization. Let us note that the condition (20) and, therefore, Eq.(26) is correct for S 5/wo and arbitrary B/ujq for the lowest energy of the exciton polaron. So Eq.(26) can be used to evaluate the energy of a self-trapped exciton when the energy of the vibrational or lattice relaxation is much larger then the exciton bandwidth. 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

Nishimura, H., Yamaoka, T., Hattori, K., Matsui, A. and Mizuno, K. (1985) Wavelength-dependent decay times and time-dependent spectra of the singlet-exciton luminescence in anthracene crystals./. Phys. Soc. Jpn., 54, 4370-4381. Matsui, A. and Nishimura, H. (1980) Luminescence of free and self trapped excitons in pyrene. J. Phys. Soc. Jpn., 49, 657-663. 

Ch. Lushchik and A. Lushchik, Decay of Electronic Excitations into Defects in Solids (Nauka, Moscow, 1989) K. S. Song and R. T. Williams, Self-Trapped Excitons (Springer, Berlin, 1993). 

The optical gain observed in Si-NC embedded in SiC>2 formed by different techniques [24-27] has given a further impulse to these studies. Interface radiative states have been suggested to play a key role in the mechanism of population inversion at the origin of the gain [24,25,28]. However many researchers are still convinced of the pure quantum confinement model and they are focusing their efforts mainly on the self trapped excitonic effects [29,30] in order to explain the differences between their results and the experimental outcomes. 

The electronic properties of RGS have been under investigation since seventies [3-7] and now the overall picture of creation and trapping of electronic excitations is basically complete. Because of strong interaction with phonons the excitons and holes in RGS are self-trapped, and a wide range of electronic excitations are created in samples free excitons (FE), atomic-like (A-STE) and molecular-like self-trapped excitons (M-STE), molecular-like self-trapped holes (STH) and electrons trapped at lattice imperfections. The coexistence of free and trapped excitations and, as a result, the presence of a wide range of luminescence bands in the emission spectra enable one to reveal the energy relaxation channels and to detect the elementary steps in lattice rearrangement. 

K.S. Song, R.T. Williams, Self-Trapped Excitons, Springer-Verlag, Berlin, 1996. 

Figure 4. Transients of DMABI films, (a) Up-conversion PL intensity decays of two self-trapped exciton states of DMABI at excitation energy 3.14 eV.(b) Time-dependence of photoinduced absorption monitored at 1.93 eV for different energy densities. Excitation at energy 2.15 eV. Reprinted with permission from Ref. [16].

Excitons. Localization of the excitons occurs via the process of self-trapping to produce so-called Self Trapped Excitons (STE). For a description of STE s we refer to Figure 2 in which are sketched three typical configurations for STE s in an M+X crystal. Toyozawa (L5) discusses the formation of STE s in which the electron and hole are localized concentrically (STE 1 and STE 2) or eccentrically (STE 3). In types 2 and 3 the hole is trapped on an X2 molecule and the strong coulombic repulsion between it and the trapped electron make this type of STE highly unstable. 

Figure 2. Three typical configurations of Self-Trapped Excitons (STE), named STE 1, STE 2 and STE 3. (Reproduced with kind permission from Ref. 14. Copyright 1986 Gordon and Breach.)...

Fig. 11. Total energy of an exciton in an anisotropic elastic continuum for different Pt-Pt distances Rm). The energy is calculated for Mg[Pt(CN)4] 7 H20 from Eq. (6). a(ai ct ) represents a localization parameter which describes a free exciton (FE) with a = 0 and a localized exciton (self-trapped exciton STE) with a = 1. The exciton binding energy EB is normalized to zero for different R-values...

The interpretation of the described magnetic field effects can be given within the model of localized, self-trapped excitons (STE). A simplified energy level scheme is shown in Fig. 4794). For abbreviation the prefix 1 at the symbols of the low lying self-trapped states A(u and E will be omitted below (see also Fig. 13). 

Highly ordered Ti02 nanowire (TN) arrays were prepared [211] in anodic alumina membranes by a sol-gel method. The TNs are single crystalline anatase phase with uniform diameters around 60 nm. At room temperature, photoluminescence measurements of the TN arrays show a visible broadband with three peaks, which are located at about 425, 465, and 525 nm that are attributed to self-trapped excitons, F, and F+ centers, respectively. 

For concentrated Bi compounds another model yields a similar temperature dependence, viz. mobile excitons with concentration nj and self-trapped excitons with concentration n and an energy difference AE, representing the thermal activation energy for exciton migration through the lattice. Unfortunately it is seldom checked whether the temperature dependence of the decay time in concentrated systems refers to an intrinsic property of an isolated luminescent centre or to an activation energy for migration. This, by the way, holds also for other compounds which have been discussed above. ... 

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).

We mention here also an excimer (originally short for excited dimer) which is a type of small radius self-trapped exciton state. In organic solids this excited state can be considered as a dimeric molecule formed from two molecules, when one of the molecules is in an electronic excited state. Excimers are usually formed between two molecules that would not bond if both were in the ground state and the molecule pyrene is one of the canonical examples of an excimer. The excimer states play an important role in applications and in photochemistry (more information can be found in (30)). 

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