Electron photo-excitation

This assumption breaks down in many molecules, especially upon photo-excitation, since excited states are often close to each other or even cross one another (i.e. have the same electronic energy at a given nuclear position). Thus, the fiill Scluodinger wavefiinction needs to be considered ... 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

The photogalvanic effect is based on light absorption by a suitable photoactive redox species (dye) in the electrolyte solution. The photo-excited dye subsequently reacts with an electron donor or acceptor process, taking place in the vicinity of an electrode, is linked to the electrode... 

Photo-excitation and de-excitation are basic processes in nuclear reactions. A Japanese-Hungarian cooperation investigating these processes has yielded good results during the past few years [21-26], These studies used weighable amounts of "Tc to look at the (y, y ) reaction that leads to the production of the nuclear isomer "mTc by electron linear accelerator irradiation. 

Using the OPENCORE spectrometer, a research group led by M. Kitagawa in Osaka have developed an experimental setup for dynamic nuclear polarization (DNP) using electron spins in the photo-excited triplet state. This nuclear hyperpolarization technique, called hereafter triplet DNP,... 

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

In the framework of DECP, the first pump pulse establishes a new potential surface, on which the nuclei start to move toward the new equilibrium. The nuclei gain momentum and reach the classical turning points of their motion at t = nT and t = (n + l/2)T. The second pump pulse then shifts the equilibrium position, either away from (Fig. 3.10b) or to the current position of the nuclei (Fig. 3.10c). The latter leads to a halt of the nuclear motion. Because photo-excitation of additional electrons can only shift the equilibrium position further in the same direction, the vibrations can only be stopped at their maximum displacement [32]. 

In ideal situations, optical spectroscopy as a function of temperature for single crystals is employed to obtain the electronic spectrum of a SCO compound. Knowledge of positions and intensities of optical transitions is desirable and sometimes essential for LIESST experiments, particularly if optical measurements are applied to obtain relaxation kinetics (see Chap. 17). In many instances, however, it has been demonstrated that measurement of optical reflectivity suffices to study photo-excitation and relaxation of LIESST states in polycrystalline SCO compounds (cf. Chap. 18). 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly... 

At heart, this greater intensity may be explained as follows. The ease with which an electron may be photo-excited depends on the probability of successful excitation, which itself depends on the likelihood of photon absorption. If the probability of excitation in the woad was 20 per cent, then 20 from every 100 incident photons are absorbed (assuming each absorption results in a successful electron excitation). By contrast, cobalt blue is more intense because it has a higher probability of photon uptake, so fewer photons remain to be seen, and the absorbance increases. 

We used the word simplistic in the previous paragraph because we described the vibrations getting more and more violent, as though there was no alternative to eventual bond cleavage. In fact, there is a very straightforward alternative absorption of a photon (i.e. energy) to X2 will also cause an electron to photo-excite, as follows. 

Following photon absorption, an electron from the HOMO of X2 is excited from the ground to the first excited state. The electronic excitation that occurs on photon absorption is represented on the figure by an arrow from the lower (ground state) Morse curve to the higher (excited state) curve. The time required for excitation of the electron is very short, at about 10-16 s. By contrast, because the atomic nuclei are so much more massive than the electron, any movement of the nuclei occurs only some time after photo-excitation of the electron - a safe estimate is that nuclear motion occurs only after about 10-8 s, which is 108 times slower. 

The Bom-Oppenheimer principle says that the atomic nuclei do not move during the electronic excitation only later will the excited state structure relax to minimize its conformational energy. An arrow represents the photo-excitation from the ground state to the excited state structures. The requirement for the excited-state structure to... 

The second difference caused by photo-excitation is to decrease the bond dissociation energy (see Figure 9.13). We will call the new value of bond dissociation energy A//i ( 1 here because we refer to the first excited state.) This decrease arises from differences in the localization of electrons within the molecular orbitals in the ground and excited states. 

The multiplicity of excitations possible are shown more clearly in Figure 9.16, in which the Morse curves have been omitted for clarity. Initially, the electron resides in a (quantized) vibrational energy level on the ground-state Morse curve. This is the case for electrons on the far left of Figure 9.16, where the initial vibrational level is v" = 0. When the electron is photo-excited, it is excited vertically (because of the Franck-Condon principle) and enters one of the vibrational levels in the first excited state. The only vibrational level it cannot enter is the one with the same vibrational quantum number, so the electron cannot photo-excite from v" = 0 to v = 0, but must go to v = 1 or, if the energy of the photon is sufficient, to v = 1, v = 2, or an even higher vibrational state. 

Alternatively, the ground-state electron could be in v" = 1. In which case it can photo-excite to v = 0, v = 2, etc. but not v = 1, because that would imply no change in v during the excitation process. Such a situation is also shown in Figure 9.16(b). 

The reason why the colour of MnC>4- is so intense follows from the unusual way in which the electron changes its position. There are no restrictions (on a quantum-mechanical level) to the photo-excitation of an electron, so the probability of excitation is high. In other words, a high proportion of the MnC>4- ions undergo this photoexcitation process. Conversely, if a photo-excited charge does not move spatially, then there are quantum-mechanic inhibitions, and the probability is lower. 

Mixed valency of this sort is the cause of the reflective, gold colour of Nao.3W03. In this system, like the MnfTc ion described above, electrons are excited optically following photon absorption from a ground-state electronic configuration to a vacant electronic state on an adjacent ion or atom. The colour is caused by a photo-effected intervalence transition between adjacent WVI and Wv valence sites ... 

The labels A and B merely serve to identify two adjacent atoms. Notice how the atoms do not move spatially, but an electron does move in response to the photo-excitation, going from Wb to Wa. 

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