Conversion IC

The theory of radiationless transitions (or electronic relaxation) based on the BOA approximation as a basis set was originally proposed by Huang and Rhys [29] and applied to color centers and later modified and extended by Lin and Bersohn [30] to molecular systems in photochemistry and photophysics. Notice that for the IC a b, the IC rate constant is given by 

In most cases, the second terms in Eq. (62) are smaller than the first ones. In this approximation, Eq. (63) becomes 

The vibrational modes in radiationless transitions have been classified by Lin and Bersohn [30] into promoting modes like Q in Rha(f) and accepting modes, other vibrational modes participating in accepting the electronic energy fty)a [30-34]. In Eq. (64) for simplicity it is assumed that there is only one promoting mode involved in IC. Suppose 

If all the accepting modes (usually the totally symmetric modes) are displaced oscillators [24], then 

This indicates that the electronic energy gap is reduced by to, one quantum of the promoting mode i.e., the promoting mode can accept at least one vibration quantum. From Eq. (65) one can see that the symmetry argument can be used to determine the promoting mode. For example, for formaldehyde [35 42] the n- ti transition corresponds to Aj A2 for the C2v symmetry in this case the promoting mode for the IC A2- A should possess the A2 symmetry. 

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Just as above, we can derive expressions for any fluorescence lifetime for any number of pathways. In this chapter we limit our discussion to cases where the excited molecules have relaxed to their lowest excited-state vibrational level by internal conversion (ic) before pursuing any other de-excitation pathway (see the Perrin-Jablonski diagram in Fig. 1.4). This means we do not consider coherent effects whereby the molecule decays, or transfers energy, from a higher excited state, or from a non-Boltzmann distribution of vibrational levels, before coming to steady-state equilibrium in its ground electronic state (see Section 1.2.2). Internal conversion only takes a few picoseconds, or less [82-84, 106]. In the case of incoherent decay, the method of excitation does not play a role in the decay by any of the pathways from the excited state the excitation scheme is only peculiar to the method we choose to measure the fluorescence (Sections 1.7-1.11). 

Internal conversion (IC) is the intramolecular crossing of an excited molecule from one state to another of the same multiplicity without the emission of radiation. As seen in Fig. 3.9, the horizontal wavy line (IC), represents internal conversion from the lowest excited singlet state S, to high vibrational levels of the ground state S this is generally followed by vibrational deactivation to y" = 0. 

We focused here on triplet-state reactions occurring after photoexcitation. We have not treated the ISC event rigorously, and believe, subject to the evidences provided above, that the ISC event itself is of minor interest in these systems. However, there exist systems where the ISC itself plays an important role. Recent advances in method developments have provided a tool for treating intersystem crossing events. Worthwhile to mention is the work of Gonzalez (SHARC) [71] and Thiel and coworkers [72] in treating internal conversion (IC) and ISC events on the same footing. 

However, whether the kcs (rate constant for CS) -AGCS relation could be reproduced satisfactorily by this equation in nonpolar or less polar solvents was not clear. On the other hand, it is important for the photochemistry of the higher excited state to elucidate the underlying mechanisms of their competing or associated processes (S2 — S, internal conversion (IC), charge recombination (CR), etc.) leading to the lower energy states. 

The wavy arrows in the Jablonski diagram of Figure 3.23, p. 50, correspond to the non-radiative transitions of internal conversion (ic) and the short arrows to intersystem crossing (isc) the former are spin allowed, as they take place between energy states of the same multiplicity the latter are spin forbidden and are therefore much slower. The rate constants of ic and isc span extremely large ranges because they depend not only on the spin reversal (for isc) but also on the energy gap between the initial and final states. 

Nuclear electromagnetic decay occurs in two ways, y decay and internal conversion (IC). In y-ray decay a nucleus in an excited state decays by the emission of a photon. In internal conversion the same excited nucleus transfers its energy radia-tionlessly to an orbital electron that is ejected from the atom. In both types of decay, only the excitation energy of the nucleus is reduced with no change in the number of any of the nucleons. 

Internal conversion (IC) is a competing process to 7-ray decay and occurs when an excited nucleus interacts electromagnetically with an orbital electron and ejects it. This transfer of the nuclear excitation energy to the electron occurs radiationlessly (without the emission of a photon). The energy of the internal conversion electron, Eic, is given by... 

The quantum yield of the primary act of spectral sensitization is limited by competitive processes fluorescence (fl), thermal deactivation of the excited dye molecule by internal conversion (ic), and intersystem crossing to the triplet manifold (isc). The sum of the quantum yields of sensitization and all competitive processes is one ... 

Brightener molecules that have been excited to a higher electronic state such as S2 or into a higher vibrational level of Si relax by a nonradiative process within ca. 10-12 s to the vibrational ground state of S (internal conversion, IC), which has a lifetime of ca. 10 9 s. This time is sufficient for the brightener geometry to adapt to the electron distribution in the Si state. 

An internal conversion (IC) is observed when a molecule lying in the excited state relaxes to a lower excited state. This is a radiationless transition between two different electronic states of the same multiplicity and is possible when there is a good overlap of the vibrational wave functions (or probabilities) that are involved between the two states (beginning and final). 

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