Exciton-biexciton transitions

The procedure described in Section 6.4.2, is used to determine the exciton-biexciton transitions. Equation 6.20 is modified to take into account the reduced transition dipole moment involving CS states ... 

Figure 6.23 Exciton-biexdton transitions in a type-II system - Allowed transitions between upper panel, LE exdton and biexciton states lower panel, CS exciton and biexdton states.

PA at l. 48 eV appeal s instantaneously, shows spectral relaxation to the red, and decays on the same timescale of SE, as shown in Figure 8-9. We assign the observed PA to singlet Bu exciton transitions towards higher lying even parity (A ) states. We can speculate on the nature of this state within the proposed model. A possible candidate for the final slate is the inirachain biexciton. However, its energy level is located below the two-exciton stale by an amount equal to the bind-... 

The relatively low values of yss and Ds in quasi-amorphous solids might be underlain by disorder (see Sec. 2.4.3) and/or a contribution of triplet excitons in quenching of fluorescent singlets (cf. Sec. 2.5.1.2). The diffusion coefficient of triplets is expected to be lower than of singlets since both energy donor and acceptor transitions are disallowed. A low value of yss has been found for the triplet-triplet annihilation rate constant from biexcitonic quenching... 

Equation 3.26 shows that efficient MEG, i.e. the predominant generation of multiexcitons, is possible only if the relaxation rate of the biexciton is much faster than that of the exciton (72 yi), because Pi 2 < 1. For strong coupling, the transition probability approaches unity and the population ratio approaches its maximum value of (72/71). This result has a transparent physical meaning for a strongly coupled superposition state, the populations of A bi and A/ex are controlled by the state decay into the two independent thermalisation channels. 

Apparently, in the near future there will be developed (a) a detailed theory of surface excitons not only at the crystal boundary with vacuum but also at the interfaces of various condensed media, particularly of different symmetry (b) a theory of surface excitons including the exciton-phonon interaction and, in particular, the theory of self-trapping of surface excitons (c) the features of surface excitons for quasi-one-dimensional and quasi-two-dimensional crystals (d) the theory of kinetic parameters and, particularly, the theory of diffusion of surface excitons (e) the theory of surface excitons in disordered media (f) the features of Anderson localization on a surface (g) the theory of the interaction of surface excitons of various types with charged and neutral particles (h) the evaluation of the role of surface excitons in the process of photoelectron emission (i) the electronic and structural phase transitions on the surface with participation of surface excitons. We mention here also the theory of exciton-exciton interactions at the surface, the surface biexcitons, and the role of defects (see, as example, (53)). The above list of problems reflects mainly the interests of the author and thus is far from complete. Referring in one or another way to surface excitons we enter into a large, interesting, and yet insufficiently studied field of solid-state physics. 

FIGURE 22.4 Energy levels, optical transitions, and emission bands associated with singlet and triplet excitons and PP excitations, respectively. The symbols lAg, IBu, mA, and BX are the ground state, lowest allowed exciton, most strongly coupled even-parity exciton, and biexciton level, respectively T is the lowest triplet level Full and... 

Figure 6.16 Configuration functions which comprise the ground state (a) exciton and (b) biexciton, organized horizontally by total angular momentum F), and vertically by spin quantum number. Spin and orbital angular momenta are as shown in the inset legend. Bold face labels each configuration state, while smaller labels in brackets show the allowed transitions to single/biexcitonic configuration functions (reproduced from Wong and Scholes ).

The transition dipole moment between an exciton and a biexciton state is calculated using ... 

Transitions between exciton and biexciton CSFs are allowed when all the configurations in the biexciton CSF are reachable from those in the exciton CSF. A biexciton configuration is reachable from an exciton configuration when there is no change in the total spin, that is, AS =0. A graphical representation, Figure 6.17, is most helpful to visualize this. For example, the first... 

Assigning allowed and forbidden transitions between exciton and biexciton configurations - A biexciton configuration that can be obtained in a single excitation step from an exciton configuration is considered allowed (green arrows). Those that cannot be attained in this way are considered forbidden (red dashed arrows). 

After determining the allowed transitions between exciton and biexciton configurations, those between CSFs can be obtained. For example, the transition 64 bi2 is allowed because each exciton configuration in 4 can reach a biexciton configuration in b 2, thus p.4 12 = 1. On the other hand, the transition 64 bii is forbidden because it requires two steps in order to produce the biexciton configuration from each of the exciton ones therefore, /44 U = 0. Transitions between exciton and biexciton states, bx, are allowed when each e, in an allowed transition to a bj in Vbx- Figure 6.18 summarizes... 

Figure 6.18 Optically allowed transitions from ground state to ground single exciton fine structure states to ground biexcitonic fine structure states. The horizontal position of state indicates F. Arrows in green, red, blue denote transitions originating from states with F— 2, 1,0, respectively. Solid, dashed and dotted lines show strong, medium and weak transitions. Dashed levels are dark states (reproduced from Wong and Scholes. ...

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