Incoherent electronic excitation

There has been increasing interest in recent years in using incoherent electronic excitation transport as a probe of molecular interactions in solid state polymer systems. The macroscopic properties of such systems arise from the microscopic interaction of the individual polymer chains. The bulk properties of polymer blends are critically dependent on the mixing of blend components on a molecular level. Through the careful adjustment of the composition of blends technological advances in the engineering of polymer materials have been made. In order to understand these systems more fully, it is desirable to investigate the interactions... 

In this section we develop a microscopic quantum model which accounts both for positional and orientational disorder in such a system. Using the bosonic Hamiltonian for the system, we find the structure of the eigenstates (i.e. the weights of the electronic excitations on different molecules of the disordered medium) in the intervals where the wavevector of the cavity polaritons is a good quantum number. These weights will be used in Ch. 13 in consideration of the upper polariton nonradiative decay and also for estimations of the rate transition from incoherent states to the lowest energy polariton states. 

Consider the fluorescence of a fluorophore in the proximity of a solid-state particle. Both the molecule and the particle are assumed to be suspended in solution. The process may be regarded as occurring in a sequence of three incoherent steps absorption of the light, followed by the rapid relaxation of the excited molecule to some lower electronically excited state, followed by radiative emission. Light of incident intensity Jo and angular frequency coi impinges upon the system. The intensity that the molecule experiences is... 

Just as above, we can derive expressions for any fluorescence lifetime for any number of pathways. In this chapter we limit our discussion to cases where the excited molecules have relaxed to their lowest excited-state vibrational level by internal conversion (ic) before pursuing any other de-excitation pathway (see the Perrin-Jablonski diagram in Fig. 1.4). This means we do not consider coherent effects whereby the molecule decays, or transfers energy, from a higher excited state, or from a non-Boltzmann distribution of vibrational levels, before coming to steady-state equilibrium in its ground electronic state (see Section 1.2.2). Internal conversion only takes a few picoseconds, or less [82-84, 106]. In the case of incoherent decay, the method of excitation does not play a role in the decay by any of the pathways from the excited state the excitation scheme is only peculiar to the method we choose to measure the fluorescence (Sections 1.7-1.11). 

The theoretical model developed to explain these experiments is based on inelastic tunneling of electrons from the tip into the 2ir adsorbate resonance that induces vibrational excitation in a manner similar to that of the DIMET model (Figure 3.44(b)). Of course, in this case, the chemistry is induced by specific and variable energy hot electrons rather than a thermal distribution at Te. Another significant difference is that STM induced currents are low so that vibrational excitation rates are smaller than vibrational de-excitation rates via e-h pair damping. Therefore, coherent vibrational ladder climbing dominates over incoherent ladder climbing,... 

Both qualitative and quantitative applications of Eq. (8.8) are possible. Qualita- tively, for example, a traditional scheme where the ground electronic state of L and D are incoherently excited to bound levels of an excited state, gives 5 = 0. This is because all processes connecting the initial and final Lt) and D,) states, that is, 5 J contributions to the matrix elements in Eq. (8.8), are even in the amplitude of the. Xj electric field. Hence, propagation under E and —E are identical. By contrast, -) consider the four-level model scheme in Figure 8.1, and discussed in detail in Section 8.2. When 0( ) 0 there exist processes connecting the initial and final L) and D) states that are of the form L) - 1) - 2) D), and hence there are... 

Figure 11.3 Incoherent interference control (IIC) scheme and potential energy curves fori, Na2 - Na + Na(3d), Na(4s), Na(3p). In this scheme an (on + a), photon excitation to the continuum interferes with an co2 photon from an initially unpopulated state. Two-photons absorption proceeds from an initial state, 0) (in Na2 it is taken to be the v = 5, 7 = 37 state), via the ]is1) (u = 35, 7 = 36,38) intermediate resonance belonging to the interacting, S /3n electronic states. The oj2 photon couples the continuum to the (initially unpopusf lated) E2) (v = 93, J — 36 or u = 93, 7 = 38) level of the lSll/3ril, electronic state j (Taken from Fig. 1, Ref. [201].). feg...

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