Light absorption mechanics

Zhirko Yu.I. (1999) Investigation of the light absorption mechanisms near exciton resonance in layered crystals. N=1 state exciton absorption in InSe. 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

An important application of photochemical initiation is in the determination of the rate constants which appear in the overall analysis of the chain-growth mechanism. Although we shall take up the details of this method in Sec. 6.6, it is worthwhile to develop Eq. (6.7) somewhat further at this point. It is not possible to give a detailed treatment of light absorption here. Instead, we summarize some pertinent relationships and refer the reader who desires more information to textbooks of physical or analytical chemistry. The following results will be useful ... 

PHOTOCHEMICAL KINETICS CONCENTRATIONS, RATES, YIELDS, AND QUANTUM YIELDS For a molecule A undergoing light absorption and reaction in its lowest excited singlet state to form a product P, we can write the following hypothetical mechanism, where A and Af are the lowest excited singlet and triplet states, respectively ... 

Kuhn, H. 1949. A quantum-mechanical theory of light absorption of organic dyes and similar compounds. J. Chem. Phys. 17 1198-1212. 

It has been suggested that the elusive zwitterionic state [75], or a novel nucleophilic addition/elimination mechanism at the central carbon of the exocyclic bridge [79], or solvent-solute H-bonding interactions [76, 80] might play a role in modulating cis-trans interconversion. Cis-trans isomerization gives rise also to a remarkable intrinsic photochromism of HBI, as it can be easily and reversibly induced upon light absorption [74—76, 79, 80]. 

The photoreaction of oxidation of water was discovered in 1927 by Baur and Neuweiler (76) and investigated later by a number of workers. The analysis of experimental results performed by Korsunovsky (65-68) is based on the exciton mechanism of light absorption. The kinetics of the reaction has been investigated by Grossweiner (77). 

It should be noted that excitons can annihilate on surface defects as well, in particular on chemisorbed particles participating in the reaction. This involves a change in the charged state of these particles and, as a result, the chemisorption capacity of the surface with respect to these particles and the rate of the reaction in which these particles participate are also changed. This case requires a special investigation since the quantities p and involved in the theory are of a different form (8) than in the case of the electronic mechanism of light absorption to which our attention was restricted in the present article. 

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

Another important area is the use of photochemistry—chemistry that results from light absorption—to perform transformations that are not otherwise possible. The practical applications of photosynthesis were based on fundamental work to learn the new pathways that light absorption makes possible, but the work on these synthetic methods has also added to our basic understanding of the reaction mechanisms. The important natural process of photosynthesis also inspires some work in photochemistry, where the challenge is one of producing artificial photosynthetic systems that could use sunlight to drive the formation of energetic materials. 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

In this case the initial act of light absorption leads to the formation of an exciton, rather than a free electron or hole. Such an exciton, as it wanders through the crystal, may meet a lattice defect and annihilate on it, the energy of the exciton being utilized to ionize the defect, i.e., to transfer an electron or hole localized on the defect to the free state (the mechanism of Lashkarev-Juze-Ryvkin, see 90, 91). If such a defect is a foreign particle chemisorbed on the crystal surface, the result will be a change in the character of the bond between this particle and the surface. Thus, the interaction of the lattice excitons with the chemisorbed particles may cause a change in the relative content of the different forms of chemisorption and... 

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