Electronically excited molecule primary processes

The primary products of the Hg(3P,) atom sensitized reaction with olefins are electronically excited molecules. For ethylene the process is the production of electronically excited ethylene, C2HJ, probably in the triplet state r/3Bi at about 3.56 eV (16). 

A tendency to use the term "primary photochemical process" with a narrower and more specific meaning than that of Noyes et al. is evident in most of the recent photochemistry books. Turro (1), Calvert and Pitts (2), Leermakers (4), Wayne (7), and Simons (8) more or less explicitly consider as a primary photochemical process any chemical act of electronically excited molecules. 

The result of this process, however, would be simply a reduction in the rate of production of CO and O from the primary ions (without affecting the species produced directly from electronically excited molecules) and a consequent reduction in the measured steady-state concentrations of CO and 02. Since the experimental data show exactly the opposite behavior, we conclude tentatively that SF6 must be interfering with the production of a negative ion which can oxidize CO efficiently. Moreover, since the measured values[G(CO) — 0.9 and G(02) — 0.4] in the presence of SF6 are significantly lower than the initial values [G(CO) = 4.5 0.5 and G(02) = 2.2 0.2] measured in other ways (4), some species other than the negative ion must be capable of oxidizing CO under these conditions with a yield, G(-CO) — 3.5. 

The primary photochemical processes performed by electronically-excited molecules. They can be divided further into photophysical processes and photochemical reaction processes. The former include luminescent processes and nonradiative deactivation. 

Many good reviews have appeared dealing with photochemical primary processes. The general nature of these processes was considered by Forster Hochstrasser and Porter, Etienne, and Bergmann and McLean dealt more specifically with photo-oxidation. The photochemistry of ketones was reviewed by Noyes, Porter, and Jolley. Simons presented a clear account of the behavior of electronically excited molecules in solution. 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

Different photoreceptor pigments may reasonably be expected to undergo different primary photoprocesses upon light absorption, aside from possessing different spectral characteristics. Specifically, light absorption in flavins fairly easily leads to electronic excitation of the molecule to the triplet state, while this process does not readily occur in carotenoids87). 

The primary process consists of raising of the electronic quantum level of molecule by absorption of energy from photon. The excited molecule may then behave in different ways. The energy of the photon is transformed into heat and temperature of absorbing system is raised but the excited molecule may behave in other ways resulting in a chemical change. 

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

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