Photoexcitation process

Figure 3.2 Jabionski diagram describing photoexcitation process.

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. 

A photoexcited process that, while exhibiting no chemical change, leads to different states of a molecular entity by way of radiation and radiationless transitions. Light absorption is a photophysical process, as is fluorescence. 

In order to determine the feasibility of uniquely promoting a single isotopic species in AHg to the triplet state, we must first examine the hyperfino structure of the 2h.37-A. line of mercury, and other detailed spectroscopic characteristics of this photoexcitation process. 

In the case of lanthanides, the 4f electrons, which are involved in the photoexcitation process, are strongly shielded. Hence one may assume that ... 

To fully understand the photoexcitation processes we should ask ourselves how light is absorbed by a molecular species. Understanding this phenomenon gives insight into the parameters that trigger CT processes in molecules. As we have elucidated in the previous sections, the transfer of electrons from one end of a large molecule to another is based on electronic interactions between the molecular subunits. In fact, this interaction may be induced by the absorption of light. 

In a molecular system which clearly possess a ct LUMO the photoexcitation process may involve promotion of an electron from a n HOMO to the ct LUMO. This process may or may not produce bond cleavage. Direct observation of a 7t — ct electronic transition is often difficult due to the localized nature of the ct molecular orbitals resulting in a low probability for the transition [91]. More likely ait->it electronic transition takes place initially and ET, i.e., a - ct, is required to eventually populate the lower energy ct antibonding molecular orbital. Onium salts are examples of chemical structures that possess a ct LUMO and are expected to behave in this manner (see in Sect. 3.3). 

CIDEP Initial Polarization. The establishment and the development of the photoexcited triplet mechanism in CIDEP of transient radicals in solution had been rather controversial, if not as turbulent and exciting as the photoexcitation process itself. The early objections centered around two very important questions. The first one concerns the uncertainty of whether the spin polarization in the molecular frame can be effectively transferred to the laboratory frame for triplet systems in liquid solution. The second related question involves the fact that the polarized triplet molecules are rotating rapidly with respect to the laboratory axes and the triplet spin lattice relaxation time T x (normally between 10 and lO-- - s) would be too short for the polarization to be retained in the radical pair. The earlier photoexcited triplet mechanism developed by Wong et al. (136,137) is based on a "static model" with the excited triplet molecules being randomly oriented. Such a static model cannot deal satisfac-... 

The apparently slower rate of methoxide decomposition in the presence of oxygen is likewise dependent on the photoexcitation process. As stated above,... 

In a second example the discrete time-reversible propagation scheme for mixed quantum-classical dynamics is applied to simulate the photoexcitation process of I2 immersed in a solid Ar matrix initiated by a femtosecond laser puls. This system serves as a prototypical model in experiment and theory for the understanding of photoinduced condensed phase chemical reactions and the accompanied phenomena like the cage effect and vibrational energy relaxation. It turns out that the energy transfer between the quantum manifolds as well as the transfer from the quantum system to the classical one (and back) can be very well described within the mixed mode frame outlined above. 

Photoelectron spectroscopy is particularly useful for measuring the energy states of electrons in the solid associated with valence orbitals and their absorbate-induced changes. In the presence of an adlayer the spectroscopic data demonstrate the overlap of those substrate and adsorbate orbitals that are involved in chemical bond formation. In the photoexcitation process the incident photons of... 

Near UV and Vacuum UV spectra have been reported for Fe(CO)5 [46,56], The gas phase near-UV optical spectrum of Fe(CO)5 was reported by Kotzian et al. [56], while Marquez and co-workers reported the vacuum UV spectrum of Fe(CO)5, supported by a CASSCF/CI theoretical study of the Rydberg states [46], Semi-quantitative agreement between experiment and theory has been reached although the precise details of the photoexcitation process are still not completely settled. 

Oligothiophenes have been selected as the target molecule, since their photoexcitation processes are well established and they are known as electron donors with large extinction coefficients of the Tj state and radical cation in the D0 state [105-108], The S0 and Tj states of the trimer, tetramer, and... 

The filled circles indicate electrons, and the open circles represent holes. Process 1 indicates photoexcitation process 2 indicates decay of the electronic excited state the dark slanting lines with arrows indicate a hole or electron transfer process (left and right sides, respectively) and the thin lines connecting the conduction band of Ti02 with the hole level in PPV indicate an interfacial recombination process. The state levels are depicted as in this figure, with the holes placed at slightly lower energy than the polymer LUMO. 

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