Electron excitation channels, decay

The fluorescence quantum yield of 448 is 0.14, a sixfold increase relative to that of the parent. In comparison, the fluorescence quantum yield of 449 (0.01) is comparable to that of the parent compound. The phosphorescence emission quantum yield of 449 is 0.56 in a frozen matrix as expected as a result of the intramolecular heavy atom effect. In contrast, the phosphorescence is effectively shut off in the anti-isomer where the quantum yield is only 0.04. This observation suggests that the electronic excited state structures and nonradiative decay channels very considerably with constitution of the isomers. The optical gap energy was 3.1 (3.3) eV for 448 (449). 

Kim et al. observed a very fast ion pair formation (below their detection limit of about 1 ps) from transient absorption spectra of fullerenes in the presence of aromatic amines such as /V,/V-dimcthyl- or /V,/V-dicthyl-anilinc, corresponding to a rate > 1 X 1012 M-1 s-1. An explanation for such extremly fast electron transfer is most likely a ground-state complex of fullerene and amine. Excitation leads to the neutral aminc/ C 0 contact pair followed by electron transfer. The decay of the both transient absorption from Cfo and Qo/amine occurs with the same rate suggesting that charge recombination is the major nonradiative relaxation channel [138],... 

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

In spite of these developments. It Is important to realize that ETS provides no Information on the decay channels of temporary anions. Further progress toward a detailed understanding of resonances In complex molecules will require careful studies of the vibrational levels and electronically excited states of the neutral molecule which are formed upon electron detachment. These measurements provide essential data related to the electronic configuration of the anion and the distortion It undergoes during its lifetime. 

While fragmentation is the dominant chemical reaction induced by core electron excitation of molecules in the gas phase, other reactions such as rearrangements can be expected, and this possibility needs to be investigated. The extent of a Coulomb explosion in a large molecule is not known, and the role of Coulomb localization in the chemistry of isolated molecules needs to be examined further. Electron-multiple ion coincidence experiments are essential in the study of the chemistry because it is necessary to relate specific electronic decay channels to particular fragmentation patterns as identified by the several ions that are produced. 

A much harder calculation of coherenf fime-dependent excitation and decay in a polyelectronic, open-shell system was published in 2007 [118]. Specifically, as a prototypical application of the SSEA to a complex system, we chose the problem of the time-resolved coherent excitation and decay of the 2s-hole ls 2s2p 3s 3p P° Auger states of Aluminum, where the dominant channels representing one- as well as two-electron continua are taken into account. In the following paragraphs, we explain how these computations were done. 

Fig. 1. The principle of pumjvprobe spectroscopy by means of transient two-photon ionization A first fs-laser pulse electronically excites the particle into an ensemble of vibrational states creating a wave packet. Its temporal evolution is probed by a second probe pulse, which ionizes the excited particle as a function of the time-dependent Franck Condon-window (a) shows the principle for a bound-bound transition, where the oscillative behaviour of the wave packet will appear (b) shows it for a bound-free transition exhibiting the exponential decay of the fragmentizing particle, and (c) shows the process across a predissociated state, where the oscillating particle progressively leads into a fragmentation channel.

In molecular reactions, electronic excitations as a consequence of the chemical transformation are often associated with light emission called chemiluminescence. As outlined in Section 3.1, with metal surfaces the relaxation times of electronic excitations (from delocalized band states) are much shorter ( r j 10 s) than those for photons s), so this decay channel will be operat-... 

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