The Emission Process in m-LPPP

In the previous section we have shown that for an excitation density above the PL line-narrowing threshold, the shape of the SE spectrum remain unchanged. This demonstrates that the spontaneous emission spike is not due to a new transition existing in the photoexcited material, as would be the case for excitonic 

It has been demonstrated that the whole photoexcitation dynamics in m-LPPP can be described considering the role of ASE in the population depletion process 

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The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

In this section experimental results are described, which are obtained by applying the conventional pump-probe technique to m-LPPP films kept in vacuum at the temperature of liquid nitrogen [25], These results allow the identification of the primary excitations of m-LPPP and the main relaxation channels. In particular, the low and high excitation density regimes are investigated in order to get an insight into the physical processes associated with the emission line-narrowing phenomenon. 

It has been demonstrated that the whole photoexcitation dynamics in m-LPPP can be described considering the role of ASE in the population depletion process [33], Due to the collective stimulated emission associated with the propagation of spontaneous PL through the excited material, the exciton population decays faster than the natural lifetime, while the electronic structure of the photoexcited material remains unchanged. Based on the observation that time-integrated PL indicates the presence of ASE while SE decay corresponds to population dynamics, a numerical simulation was used to obtain a correlation of SE and PL at different excitation densities and to support the ASE model [33]. The excited state population N(R.i) at position R and time / within the photoexcited material is worked out based on the following equation ... 

Therefore one can propose that the field-induced annihilation of the emitting species is the most dominant process for quenching, which also explains the spectrally nonuniform PL quenching that is observed above a magnitude of the applied electric field of 2 MV/cm (see Figs. 30.26 and 30.27). The intensity of the dominant PL peak at 461 nm and the area under the peak decrease in a less pronounced fashion than the intensity of the peak at 491 nm and the broad emission peak at approximately 530 nm. To interpret this phenomenon by field-induced annihilation, we discuss how the PL emission process in the m-LPPP polymer evolves. Time-resolved PL spectroscopy is a powerful method for studying the nature and dynamics of emission processes. Streak camera measurements on the m-LPPP polymer (for details see Ref. 144) reveal that two excited species with different lifetimes contribute to the PL emission (a behavior that is also observed in Ref. 142). 

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