Light absorption and photodissociation

The microscopic theory of light-matter interaction relates o oj) to a matrix element of the electric dipole operator d of the molecule, 

Equation (2.2) is at the center of spectroscopy and photodissociation. It applies equally to bound-bound, bound-free, and free-free transitions. Although almost every textbook on quantum mechanics contains the derivation of (2.2) (see, for example, Merzbacher 1970 ch.l8 Loisell 1973 ch.5 Cohen-Tannoudji, Diu, and Laloe 1977 ch.XIII Loudon 

1983 ch.2), we shall derive it in some detail in order to distinguish clearly the two different time scales inherent to photodissociation the time dependence of the absorption process and the evolution of the subsequent fragmentation in the excited electronic state. 

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Light absorption and photodissociation 2.2 The absorption cross section... 

The light available to a molecule in air for absorption and photodissociation includes both direct and scattered and reflected radiation coming from all directions as described earlier and depicted in Fig. 3.16. The term actinic flux or spherically integrated actinic flux, denoted by F( A), is used to describe the total intensity of this light and is the quantity of interest in calculating kp. 

In contrast to the photo physical processes just described, photochemical processes produce new chemical species. Such processes can be characterized by the type of chemistry induced by light absorption photodissociation, intramolecular rearrangements, photoisomerization, photodimerization, hydrogen atom abstraction, and photosensitized reactions. 

In effect, kp takes into account the intensity of available light that the molecule can absorb, the intrinsic strength of light absorption in that region by A, i.e., the absorption cross section cr, and the quantum yield for photodissociation, < . 

The second group of electronically excited atoms consists of metastable atoms such as O( D) and Of S). These metastable atoms cannot be produced directly from ground state atoms by light absorption. However, they are often formed in photodissociation of molecules. The production of metastable atoms from photodissociation has been known by the different reactivities of the metastables from those of corresponding ground state atoms. 

For the discussion of the formation and destruction of ozone in (In stratosphere it is convenient to define the photodissociation coefficient generally denoted by J (in units ofsec 1). J is the probability of dissocial mn of a molecule per second by light absorption. 

Even if direct light absorption as above does not occur, polymerization can still be initiated if photosensitizers are present that produce free radicals when they absorb ultraviolet or visible light. The same substances that are used for thermal initiation are often used for photosensitization. For example, azo compounds and peroxides are photosensitizers, and the photoinitiation reaction is the same as is the thermal initiation process, described earlier in this chapter. However, the photoinitiation can take place at much lower temperatures than in the thermal initiation by the same initiators. Moreover, many initiators can be used as photosensitizers even though they do not dissociate thermally at convenient rates or temperatures to be useful as thermal initiators. For example, azoisopropane does not dissociate sufficiently rapidly below 180°C to be useful thermal initiator. However, it photodissociates even at low temperatures when irradiated with near-ultraviolet light ... 

The quantum yield of a given pathway following the light absorption is defined as the number of reactant molecules which decompose along this pathway relative to the number of photons absorbed. The sum of quantum yields for all possible pathways is then unity. In this section, we focus on photodissociation processes (also called photolysis), as described by Reactions (2.75) and (2.76a). 

The formation of the benzyl radical by photolysis of toluene in benzene at 4.2 and 77 K has been investigated as a function of light intensity and shown to be unimolecular and biphotonic, and to occur via the lowest excited triplet state. The gas-phase photodissociation of [2,2]paracyclophane (71) into two molecules of /7-quinodimethane and analogous dissociations of two methyl-substituted derivatives have been shown to be efficient two-photon processes.A hot molecule formed by internal conversion from the initially formed singlet electronic excited state is proposed as an intermediate, a mechanism which is completely different from that of the two-photon dissociation of (71) in low-temperature matrices, which involves a triplet state. The photolyses of [2,2]para-cylcophane and [2,2]paracyclophane-l-ene have also been studied in THF and hexane matrices at 77 K, using detection by both luminescence and absorption spectroscopy. The products from both these compounds in THF matrices included alkyl derivatives of rra/15-stilbene, and in hexane matrices alkylphenan-threnes. Cycloreversion of so-called cyclodimers of naphthalene derivatives with benzene or furan, e.g. (72), occurs efficiently upon UV irradiation. ... 

A hot thermal ensemble of Li atoms will also typically include some Li2 molecules (e.g., in thermodynamic equilibrium at 1 atm and 2033 K [5], the Li2 pressure is 0.01 atm). Most of these molecules are in the strongly bound X state, but some are in the weakly bound fl E+ state as well. Molecules in these two states can likewise absorb light to form bound excited molecules (bound bound absorption) or free excited molecules (which dissociate to Li(2j9) + Li(2i) bounds free absorption). An alternative name for bounds free absorption is photodissociation. 

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