Photoselection process

The photoselection process resulting in the preferential distribution of azo-chromophores in their perpendicular orientation has been mainly performed by irradiation with visible (blue) light. It was confirmed by the attenuated total reflection method that polarized pumping UV light also caused the formation of the Z-isomer, again with perpendicular orientation, in a thin film of poly-(L-glutamate) with azobenzene residues in the side chains.86... 

Fluorescence polarization cannot attain the +1 theoretical limits for maximum beam polarization owing to the nature of the absorption and emission processes, which usually correspond to electric dipole transitions. Although the excitation with linearly polarized radiation favours certain transition dipole orientations (hence certain fluorophore orientations, and the so-called photoselection process occurs), a fairly broad angular distribution is still obtained, the same happening afterwards with the angular distribution of the radiation of an electric dipole. The result being that, in the absence of fluorophore rotation and other depolarization processes, the polarization obeys the Lev shin-Perrin equation,... 

Evidently, correlation functions for different spherical harmonic functions of two different vectors in the same molecule are also orthogonal under equilibrium averaging for an isotropic fluid. Thus, if the excitation process photoselects particular Im components of the (solid) angular distribution of absorption dipoles, then only those same Im components of the (solid) angular distribution of emission dipoles will contribute to observed signal, regardless of the other Im components that may in principle be detected, and vice versa. The result in this case is likewise independent of the index n = N. Equation (4.7) is just the special case of Eq. (4.9) when the two dipoles coincide. 

The E/Z-isomerization process is characterized by angular-dependent excitation and leads, therefore, to the photoselection of a preferred azobezene dye orientation. In other words, the dichroic dye units choose an orientation where the electronic transition moment is perpendicular to the light electric vector. It promotes, in turn, the cooperative reorientation of neighboring moieties, which include other fragments of the macromolecule, such as the main chain or photochemically inactive comonomer units, and low molar mass additives. Thus, a macroscopic orientation of the sample arises, and it remains long after the illumination is stopped and all the dye moieties return to the thermodynamically equilibratory -state. 

The principle of an FP measurement is depicted in Fig. 10. Upon illumination with Hnearly polarized light, the fluorophores in solution experience the highest probability of absorbing a photon when they have an orientation parallel to the incoming light vector. If the molecules were stationary or if fluorescence were an instantaneous process, the emission of this photoselected fluorophore ensemble... 

The processes of energy acquisition, storage and disposal in clusters are of considerable interest in their own right and also for the interpretation of similar processes in finite systems. Consider vibrational energy excitation of an intramolecular vibration of a molecule in a cluster, or of a cluster inter-molecular mode(s), which can be accomplished by collisional excitation, photoselective vibrational excitation, electronic excitation followed by intramolecular radiationless transitions or exciton trapping.178 In charged clusters... 

Steady-State Anisotropy Following continuous excitation with vertically polarized light, a distribution of fluorophores whose transition vectors for the absorption process are vertically aligned will be photoselected, creating an excited state population, which possesses a degree of anisotropy (r) or optical order, in an otherwise isotropic distribution of fluorophores. Measurement of the intensity of fluorescence, via an emission polarizer in planes parallel (z n) and perpendicular (zx) to the vertical plane allows estimation of r from... 

In Section I we briefly discuss the relationship between the theoretical parameters and experimental observables in these experiments in terms of the spectroscopy of electrons in liquids. Experimental techniques are considered in more detail in Section II, while the data from electron solvation in pure liquids are reviewed in Section III in the context of the molecular dynamics of the host liquid. Section IV presents current results on electron trapping in very dilute polar systems and leads to speculation on mechanisms of electron localization. In Section V the first direct observations of a photoselective, laser-induced electron-transfer process are presented, following which we summarize as yet unresolved issues and speculate on future directions in the laser spectroscopy of electron-relaxation processes. 

In order to treat these observations and hypotheses in a theoretical framework as successfully as the case of electron localization in helium, we must first probe the dynamical properties of the IR absorptions in the subpicosecond regime. What perhaps is surprising and stimulating for future studies is the wealth of microscopic details that can be obtained on intermolecular interactions and electron transfer in liquids through picosecond spectroscopy, information of fundamental interest to chemical dynamics in the condensed phase. In this vein, we will conclude this chapter by an example of photoselective chemistiy in electron transfer processes that occur following laser excitation of e in the cluster. 

We can indeed claim that this is an example of photoselective laser chemistry. The competition between relaxation and reaction of photoex-cited electrons in clusters represented in Fig. 14(b) is reminiscent of the competition in many laser-induced chemical processes, stimulated by the selective absorption of one or more photons, such as photodissociation, photoionization, isomerization, and so forth in polyatomic molecules, where the coupling of many vibrational modes provides energy randomization and relaxation on picosecond time scales. 

Due to a process called photoselection, to can have a maximum value of 0.4. A measured value for that is significantly less than this strongly suggests that some fast depolarization process is taking place that is beyond the equipment s temporal resolution. Energy transfer, excimer formation, fast local libration and radiationless relaxation between two different electronic states (such as that from lb to la in tryptophan) are some examples of processes that result in a significant decrease in to. 

Methods of photoselective chemistry can be rrsed to pump molecules up and to monitor the collisional processes. It can even be done in the brrlk as long as the pressrrre is low enough that the prrmp process is over in a time that is short... 

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