Self trapped exciton

S.L. Dexheimer, in Coherent Vibrational Dynamics of Exciton Self-Trapping in Quasi-One-Dimensional Systems, ed. by S. De Silvestri, G. Cerullo, G. Lanzani. Coherent Vibrational Dynamics (CRC, Boca Raton, 2007), p. 223... 

M-STE is the main channel of exciton self-trapping in solid Xe, Kr, and Ar. In addition to the GS-mechanism, we proposed an ES-mechanism of M-STE to Frankel-pair conversion [20] which consists of three stages (Fig.7). The process is supposed to occur by (stage 1) self-trapping of an exciton (Fig.7a- b) with a subsequent displacement (stage 2) of M-STE from the centrosymmetric position in the <110> direction (Fig.7b—>c) followed by (stage 3) reorientation to the <100> direction (Fig.7d) to... 

Increase of the stable Frenkel-pair concentration under irradiation of the samples is saturated (Fig.6) when the trapping of excitons at defects exceeds the exciton self-trapping in the perfect lattice. Further long-time irradiation of the samples results in an aggregation of vacancies and interstitials, which results in decrease of intensity of defect subbands (Fig.6e). 

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

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...

K. Timpmann, Z. Katiliene, N.W. Woodbury, A. Ereiberg, Exciton self trapping in one-dimensional photosynthetic antennas. J. Phys. Chem. B 105, 12223-12225 (2001)... 

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).

Katrich, G.S., Kemnitz, K, Malyukin, Yu.V., Ratner, A.M. (2000). Distinctive features of exciton self-trapping in quasi-one-dimensional molecular chains (J-aggregates). /. Luminesc. Vol. 90. pp. 55-71... 

J. Du, L. Rene Corrales, K. Tsemekhman and E. J. Bylaska. Electron, hole and exciton self-trapping in germanium doped silica glass from DPT calculations with self-interaction correction. Nucl. Instrum. Methods Phys. Res. B 255, 2007, 188. 

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. 

A self-trapping mechanism for singlet and triplet excitons, ascribed to... 

Therefore the lack of an observable bleach can only be explained by the cancellation of all contributions to the pump-probe signal, which is the case for a perfect harmonic state. It can be shown that the anharmonicity of a vibrational exciton is a direct measure of its degree of delocalization [5]. Thus, we conclude that the free exciton state is almost perfectly delocalized at 90 K. As temperature increases, a bleach signal starts to be observed, pointing to a non-complete cancellation of the different contributions of the total pump-probe signal. Apparently, thermally induced disorder (Anderson localization) starts to localize the free exciton. The anharmonicity of the self-trapped state (1650 cm 1), on the other hand, originates from nonlinear interaction between the amide I mode and the phonon system of the crystal. It... 

Fig.1. (a) Absorption spectra and (b) 2D-IR pump probe spectra of the C=0 mode of crystalline ACN. 2D-IR spectra record the absorption change as a function of probe frequency and the center frequency of a narrow band pump pulse. The contour intervals represent a linear scale. Response of the amide I band upon selective excitation of the self-trapped states (c) and the free exciton peak (d) for two different delay times. The arrows indicate the position of the narrow band pump pulse. 

We used short broadband pump pulses (spectral width 200 cm 1, pulse duration 130 fs FWHM) to excite impulsively the section of the NH absorption spectrum which includes the ffec-exciton peak and the first three satellite peaks [4], The transient absorbance change signal shows pronounced oscillations that persist up to about 15ps and contain two distinct frequency components whose temperature dependence and frequencies match perfectly with two phonon bands in the non-resonant electronic Raman spectrum of ACN [3] (Fig. 2a, b). Therefore the oscillations are assigned to the excitation of phonon wavepackets in the ground state. The corresponding excitation process is only possible if the phonon modes are coupled to the NH mode. Self trapping theory says that these are the phonon modes, which induce the self localization. 

In a second experiment, narrow band pump pulses (spectral width 30 cm 1, pulse duration 250 fs FWHM) were used to selectively excite individual sub-levels of the NH band (Fig. 2e, g) [4]. On the sub-picosecond time scale, the free-exciton and the lower lying self-trapped states behave distinctly differently. When exciting the free-exciton (Fig 2e), a strong bleach and stimulated emission signal is observed which recovers on a 400 fs time scale. Simultaneously, population is transferred into lower lying self-trapped states. On the other hand, when pumping one of the self-trapped states directly (Fig. 2g), population within all self-trapped states equilibrates essentially instantaneously, but the free exciton peak is not back-populated. This is the direct observation of ultrafast self-trapping Excitation of the free-exciton leads to an irreversible population of self-trapped states, but not vice versa. 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

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. 

The commonly used scheme of energy relaxation in RGS includes some stages (Fig.2d, solid arrows). Primary excitation by VUV photons or low energy electrons creates electron-hole pairs. Secondary electrons are scattered inelastically and create free excitons, which are self-trapped into atomic or molecular type centers due to strong exciton-phonon interaction. 

This suggests that the subband M2 is emitted by the excitons which are self-trapped in the regular lattice (M2-centers) while the component Mi is emitted by the centers (Mi-centers) which are populated during trapping that occurs with the lattice imperfections involved. 

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. 

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