Primary photochemical act

The primary photochemical act, subsequent to near-uv light (wavelengths <400 nm) absorption by Ti02 particles, is generation of electron—hole pairs where the separation (eq. 3) into conduction band electrons (e g ) and valence band holes (/lyB ) faciUtated by the electric field gradient in the space charge region. Chemically, the hole associated with valence band levels is constrained at... 

The reaction is the photochemical combination of hydrogen and chlorine. It is clear that chlorine is the only photochemically active constituent. It is, furthermore, very probable that the elementary photochemical act is the dissociation of the chlorine molecule. Rollefson3 has recently studied and discussed the status of this reaction, and has developed the probable mechanism. It will be assumed in what follows that the necessary primary photochemical act is the dissociation of the chlorine molecule. 

Work by Warburg and Bodenstein (1912-1925) clarified earlier confusions between photon absorption and observed chemical change. Molecules which absorb photons become physically excited , and this must be distinguished from becoming chemically active. Excited molecules may lose their energy in nonchemical ways, or alternatively may trigger off thermal reactions of large chemical yield. The socalled law , therefore, rarely holds in its strict sense, but rather provides essential information about the primary photochemical act. 

The second law is the Stark—Einstein law. whose re-statement in current terminology is that the primary photochemical act involves absorption of just one photon by a molecule. This holds true for the vast majority of processes exceptions to it arise largely when very intense light sources, such as lasers, are employed, and the probability of concurrent or subsequent absorption of two or more photons is no longer negligible. 

UV irradiation of low-valent cations (e.g. V2+, Cr2+, Fe2+) in acidic aqueous solution leads to photooxidation of the metal and evolution of H2.110 This reaction, described by equation (47), is thought to involve the formation of the oxidized metal and an electron in the primary photochemical act. Immediate scavenging of the photoelectrons by protons yields hydrogen atoms which then combine to produce H2. Some interest has been shown in this type of system as a component of a photochemical water-splitting cycle.110 111... 

Following the classical observation of Lewis and his school (29, 30, 34) of the photoionization of aromatic molecules in rigid solvents, Land, Porter and Strachan (25) proved such processes in the flash photolysis of aqueous solutions of phenols. It was suggested (7, 15, 25, 38) that the primary photochemical act involves electron ejection. 

Here quantum yield of 3Chl and

radical products from a 3Cbl... A geminate pair in the primary photochemical act. Particular stages of reaction scheme (5)- (9) will be discussed in more detail in Sects. 3 and 4. 

System 11, the primary photochemical act appears to be oxidation of the photosensitizer via reactions of type (6). For System 11, where pheophytin, Phe, is used as the photosensitizer, the primary reaction is its reduction ... 

Flash photolysis experiments have shown that for these systems the primary photochemical act is the transfer of an electron from 3ZnTPP to MV2+, and the intermediate particles involved in the transfer are 3ZnTPP and ZnTPP+ [56, 58, 59]. The whole set of the data obtained makes it possible to suggest the scheme for PET across the membrane which is shown in Fig. 4b. This scheme assumes the electron transfer at the outer boundary of the vesicle membrane as the first act ... 

Within this scheme the quantum yield (O) of PET across the membrane is determined by the product of the yield of the triplet state ( T), fraction of 3Ru(bpy)3+ molecules quenched by C16V2+(cpq), yield of radical-ion products from the geminate pair in the primary photochemical act ([Pg.22]

A common class of photolytic reactions consists of those in which the primary photochemical act is absorption of the quantum by a molecule followed by dissociation of the molecule ... 

Photosensitized reactions make up an important class of photochemical reactions. In these reactions the reactants are mixed with a foreign gas mercury or cadmium vapor are often used. The primary photochemical act is the absorption of the quantum by the foreign atom or molecule. 

In a primary photochemical act there are as many broken down molecules as there are absorbed photons, which mean that the primary act can be written as ... 

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