Photophysical process quantum yield

PHOTOPHYSICAL PRIMARY QUANTUM YIELD the quantum yield of a photophysical primary process. 

For photochemical reactions and photophysical processes the efficiency is determined by the quantum yield d>, which is defined as the number of molecules undergoing a particular process divided by the number of quanta of light absorbed ... 

Chapter 3 is devoted to the characteristics of fluorescence emission. Special attention is paid to the different ways of de-excitation of an excited molecule, with emphasis on the time-scales relevant to the photophysical processes - but without considering, at this stage, the possible interactions with other molecules in the excited state. Then, the characteristics of fluorescence (fluorescence quantum yield, lifetime, emission and excitation spectra, Stokes shift) are defined. 

The relative efficiencies of the various photophysical and photochemical primary processes are described in terms of quantum yields, . The primary quantum yield, , for the ith process, either photophysical or photochemical, is given by Eq. (I) ... 

By definition, the sum of the primary quantum yields for all photochemical and photophysical processes taken together must add up to unity, i.e.,... 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

These photophysical processes often decide the photochemical behaviour of a molecule and reduce the quantum yield of a photochemical reaction to much less than unity. A molecule in the singlet state is a different chemical species from that in the triplet state and may initiate different chemistry. Therefore, for a complete understanding of a photochemical reaction, a clear knowledge of various photophysical processes, that isj how the absorbed quantum is partitioned into different pathways is essential. This account keeping of the absorbed quanta, so to say, may help modify a given chemical reaction if it is so desired. We shall discuss each of these processes one by one. 

The quantum yield of photochemical processes can vary from a low fractional value to over a million (Section 1.2). High quantum yields are due to secondary processes. An initially excited molecule may start a chain reaction and give rise to a great number of product molecules before the chain is finally terminated. For nonchain reactions, the quantum yields for various competitive photophysical and photochemical processes must add up to unity for a monophotonic process if the reaction occurs from the singlet state only ... 

Constants or lifetimes of the excited states are important parameters since the reactivities of these energy states depend on them. Rate constants of various photophysical and photochemical processes can be adduced from quantum yield data only if the mean radiative lifetimes (t0) are known. The defining relationships are (Section 5.3)... 

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