Photoexcitation dynamics

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. 

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 ... 

An accurate description of the photoexcitation dynamics of adsorbates or of biomolecular chromophores over times of the order of picoseconds or longer must account for two different mechanisms of relaxation electronic and vibrational relaxation. The former one dominate at short times while the latter one dominate at longer times. The present treatment describes both processes within a single formulation, and should be applicable to a wide variety of physical systems. 

Magnetic resonance also has an effect on photoexcitation dynamics as the absorption of microwaves will equalize the populations of the two coupled spin sublevels. Consider two triplet sublevels X and Y with sublevel populations generation rates and recombination rates 7 ,= l/r,. Under steady state con-... 

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... 

The primary photoexcitation dynamics in PDPA solutions and films in the fs to ps time domain using transient PM spectroscopy were extensively studied [182]. The PDPA polymer used was a disubstituted biphenyl derivative of frans-polyacetylene, where one of the hydrogen-substituted phenyl groups was attached to a butyl group, which is referred to as PDPA-mBu (Figure 22.25 inset) [181]. The polymer films were cast on sapphire substrates from a toluene solution the same solution was used for measuring the photoexcitation dynamics in a PDPA-mBu solution. 

Sakamoto, A. and Takezawa, M. (2009) Picosecond time-resolved infrared absorption study on photoexcited dynamics of regioregular poly(3-hexylthiophene). Synth. Met., 159, 809-812. 

Photoexcitation dynamics of fuiierenes have been also widely investigated. Nowadays, the excitation-relaxation processes of Qg and C70 have been well established [10-15]. Singlet and triplet properties have been investigated by using pico- and nanosecond laser flash photolysis techniques. One of the important photophysical properties of and C70 (Fig- 1) is almost quantitative triplet generation, which results in effective photochemical bimolecular reactions [10]. 

In this chapter, we review the photoexcitation dynamics of fuiierenes including OjQ, higher fuiierenes, endohedral metallofullerenes, and fullerene ohgomers. Furthermore, photoinduced processes in the fuUerene-donor linked molecules have been also reviewed, since they will serve as important molecular devices. 

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