Radiationless decay channel

Zgierski MZ, Patchkovskii S, Lim EC (2005) Ab initio study of a biradical radiationless decay channel of die lowest excited electronic state of cytosine and its derivatives. J Chem Phys 123 081101... 

It is on the fac (product) side that we find a non-adiabatic radiationless decay channel back to the ground state via a sloped conical intersection (Fig. 8) that connects the excited and ground state fac species. To investigate this region we have used CASSCF. The shear system size here presents difficulties... 

Figure 3 Opening of a fast radiationless decay channel in all-trans octatetraene in (a) matrix-isolated conditions and (b) expanding cool jet. (From Ref. 14.)...

Figure 4 Opening of a fast radiationless decay channel via conical intersection for (a) a barrier controlled reaction, (b) a barrierless path, and (c) an uphill path without transition state (sloped conical intersection). M" is an excited state intermediate and FC is a Franck-Condon point.

As a conclusion we mention an old problem for the solution of which one of us (A. L.) spent some time (with scarce success), when he was a young student, as a part of its work for obtaining the degree in Chemistry, under the guide of the person to whom this volume is dedicated. This concerns the striking different photophysical behavior of two similar species, which can be both considered as derivatives of the triphenil-methane the phenolphtalein (HI) and the fluorescein dianions (V) [47]. As is well documented in the literature, the fluorescence quantum yield of the first excited singlet of the fluorescein ion is 1, while that of the phenolphtalein is essentially 0. We firmly believe that the very different efficiency of the radiationless decay channel of such molecules can be explained... 

Modern experimental measurements and the new computational techniques just discussed are now providing results that can rationalize issues such as the efficiency of 1C at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and the structure of the photoproducts. Experiments on isolated molecules in cold-matrices or expanding-jets have revealed the presence of thermally activated fast radiationless decay channels. For example, Christensen et al. have proposed that (under isolated conditions in a cool-jet) trans — cis motion in all-tra 5-octa-1.3,5,7-tetraene (all-trow -OT) induces the opening of an efficient nonadiabatic radiationless deactivation channel on Si (2Ag). We now discuss this experiment and complementary theoretical results that illustrate the way in which theory and experiment can be used in concert. 

The mechanisms of luminescence decay from an optical center are of critical importance. In particular we have to know if there are any processes internal to the center or external to it, which reduce the luminescence efficiency. It is possible to define two decay times, ir, the true radiative decay time which a transition would have in absence of all non-radiative processes, and r, the actual observed decay time, which maybe temperature dependent, as will usually occur when there are internal non-radiative channels, and which may also be specimen dependent, as when there is energy transfer to other impurities in the mineral. The quantum yield may be close to unity if the radiationless decay rate is much smaller than the radiative decay. 

With the analyses of the previous sections we have prepared the background for a discussion of the time evolution of states undergoing radiationless transitions. For the present the radiative decay channel will be ignored. The simultaneous influence of radiative and radiationless transitions on the time evolution of a state will be considered in Section X. 

Another process which has to be considered is the conformation change from the nonrelaxed nonplanar to the planar excited state, which could be connected with a fast increase of absorption. The most important radiationless decay process in oligothiophenes is ISC. Therefore, a fast channel of ISC was suggested, which is effective before the lowest vibronic state of Si is occupied. The fast decay of the AD(f) kinetics is caused by an effective formation of triplets. 

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 the photochemistry of benzene, the so-called channel 3 represents a well-known decay route along which fluorescence is quenched above a vibrational excess of 3000 cm [57], The decay takes place through a prefulvenic conical intersection characterized by an out of plane bending [52,58] and results in the formation of benzvalene and fulvene. The purpose of this study is to find distinct radiationless decay pathways that could be selected by exciting specific combinations of photoactive modes in the initial wavepacket created by a laser pulse. For this, we carry out quantum dynamics simulations on potential energy surfaces of reduced dimension, using the analysis outlined above for the choice of the coordinates. 

We consider three decay channels for D in addition to injection Fluorescence (rate constant k ), intramolecular radiationless decay (rate constant k ), and energy transfer quenching within the adsorbed layer (rate constant kg) ... 

Oclkrug et al. could not find any evidence of singlet oxygen in 5T aggregates, in contrast to solution [ 152]. Therefore a radiationless deactivation via internal conversion or sudden polarization (see Section 4.2) is concluded. The latter intermolecular electron-hole pair creation as decay channel is also proposed by the group... 

The radiationless decay of a quasidiscrete excited state of an atom or molecule into an ion and electron of the same total energy is called autoionization. The quasidiscrete state must, of course, lie above the first ionization potential of the atom or molecule. The occurrence of autoionization may be inferred from the appearance of absorption spectra or ionization cross-section curves which exhibit line or band structure similar to that expected for transitions between discrete states. However, in the case of autoionization the lines or bands are broadened in inverse proportion to the lifetime of the autoionizing state, as required by the uncertainty principle. In the simple case of one quasidiscrete state embedded in one continuum, the line profile has a characteristic asymmetry which has been shown to be due to wave-mechanical interference between the two channels, i.e., between autoionization and direct ionization. In an extreme case the line profile may appear as a window resonance, i.e., as a minimum in the absorption cross section. 

As seen in Sec. 2.3, conical intersections that mediate unsuccessful chemical reactions have been shown to provide the decay channel associated with processes that are usually thought to occur through a photophysical mechanism (e.g. controlled by the Fermi Golden Rule ) such as the radiationless deactivation and/or quenching processes. Furthermore, important organic chromophores have been demonstrated to undergo either photochemical reactions or internal conversion processes on an extremely fast (usually sub-picosecond) time scale (i.e. emission is not observed from the excited state, since the time scale of the reaction is faster than the radiative lifetime). 

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