Excitation density

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 phoioexcited material, as would be the case for excitonic mol-... 

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 ... 

To interpret the magnitude of the PIA signal, the rale equation for the photogeneration of states will be discussed. It describes the change of the number of states, n, with respect to time, /, depending on the generation rate kp (A=const., excitation density) and the decay rate In1 /=inverse lifetime, (/reorder of kinetics) and can be written as follows ... 

Fig. 3.16. Left transient reflectivity changes of TTF-CA with double pulse excitation at At = 0.62 ps (=T) for various pump fluences, indicating the super-linear enhancement of the coherent phonons with increasing excitation density. The pump pulses are indicated by grey lines. Id indicates the excitation density of each pulse in the pulse pair in unit of Io = 1 x 1016 photons/cm2. Right Maximum AR/It, (open circles) and the oscillation amplitude (closed circles) as a function of Id/Io-From [44]...

All systems presented in this section show lasing only in the optical pumping mode. There is much interest in electrically pumped devices, but for molecular glasses the difficulties in achieving high excitation densities and low absorption due to charge carriers and electrodes have yet to be overcome. This problem and the related semiconducting polymer lasers that are based on the same principles will not be covered here, but are treated in recent reviews [214-216]. 

Stelmakh and Tsvirko have also proposed the mechanism [b] for the production of S2 state fluorescence in Mg, Zn, and lanthanide-porphyrins complexes (15), and showed the possibility of another mechanism of pumping by triplet-triplet annihilation (16)(mechanism [a]). The latter process was considered to predominate at relatively low values of the excitation densities. However, the contribution of the mecahnism [C] was not determined experimentally. The experimental results in the present work are summarized as follows ... 

The compression that is used to calculate the internal representation consists of a transformation rule from the excitation density to the compressed Sone density as formulated by Zwicker [Zwicker and Feldtkeller, 1967], The smearing of energy is mostly the result of peripheral processes [Viergever, 1986) while compression is a more central process [Pickles, 1988], With the two simple mathematical operations, smearing and compression, it is possible to model the masking properties of the auditory system not only at the masked threshold, but also the partial masking [Scharf, 1964] above masked threshold (see Fig. 1.5). 

Figure 28. The 300 K time-resolved luminescence spectra of Mn2+ ZnS nanocrystals in polyvinyl butyral (PVB) films, collected at the various delay times indicated following a 248-nm excimer laser pulse (pulse width 40 ns, 2 Hz, excitation density = 5.6 mJ/cm2, detection gate width = 2 ps). [Adapted from (133).]...

In the previous photoswitches, a radical unit is placed at each side of the diarylethene photo switching unit and separated by an extended it-conjugated chain. When the jr-conjugated chain length between the radical becomes longer, both photocyclization and cycloreversion reactivities are reduced. This is attributed to the reduced excitation density at the central diarylethene unit [69]. The excitation... 

Optical excitation of metals with intense femtosecond laser pulses can create extreme non-equilibrium conditions in the solid where the electronic system reaches several thousand degrees Kelvin on a sub-picosecond timescale, while the lattice (phonon) bath, stays fairly cold. As illustrated in Figure 3.22, photoexcited hot electrons may transiently attach to unoccupied adsorbate levels and this change in the electronic structure may induce vibrational motions of the adsorbate-substrate bond. For high excitation densities with femtosecond pulses, multiple excitation/deexcitation cycles can occur and may eventually lead to desorption of adsorbate molecules or reactions with co-adsorbed species. After 1-2 ps, the hot electron... 

Im. Z i(E) can be considered as a product of an ionic excitation density of states and an energy-dependent coupling constant. In model calculations one can independently vary the shape and the band with of the denstiy of states and the strength of the coupling constant. In the present case we can only vary these parameters indirectly by changing the atomic number Z. Since the self-energy involves the polarizability of the ionic system there must be an oscillator-strength sum rule such that... 

Finally, we must underline that the above formalism applies to any system of dipoles the only limitation is that of low excitation densities. 

Often referred to as Dicke s superradiance,144 but with a very low excitation density. 

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