Photolysis, condensed phase process

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

Products. Photolysis of 1,3-cyclohexadiene gives rise to 1,3,5-hexatriene of unknown stereochemistry. During irradiation in the condensed phase initial product is the m-triene which subsequently isomerizes photochemically. The other primary processes which seem to originate directly in 1,3-cydohexadiene are dehydrogenation ... 

The results of the investigations carried out with mixtures of acetone and ace-tone- /6 in the presence of inhibitors clearly indicate the free radical character of the photolysis in the liquid phase , in inert solvents as well as in aqueous media . The available evidence supporting the formation of primary products other than CH3CO and CH3 is not convincing. Although some radicals, which could possibly be formed in the primary process, were detected by the paramagnetic resonance spectra, these studies were made under extreme conditions. Essentially, all the experimental observations on photolysis in the condensed phase can be interpreted by the mechanism derived for the reaction in the gas phase. 

Upon collisional deactivation tra i-azoalkanes isomerize to the cis form. (This reaction is the only known synthetic route to m-azoalkanes.) The process is reversible, but — at least in the liquid phase — occurs only in direct photolysis or singlet sensitized photolysis and not in triplet sensitization. Decomposition in the condensed phase appears to exhibit similar behavior. These kinetic features cannot be rationalized in terms of the two electronic levels predicted by mo calculations and various other alternatives have been suggested as will be discussed below. 

A paper by Suppan draws attention to electrostatic interaction effects on condensed phase photoinduced electron transfer and the need to take account of the fact that solvent is not in reality a uniform dielectric material. Pressure effects on exciplex formation has been exemplified in the pyrene-p-cyanobenzene system. Ternary electron donor acceptor complexes are formed and in the case of anthracene-tetracyanoethylene gives rise to (DO ) dimer radical cations. Laser flash photolysis shows that perylene in acetonitrile undergoes three distinct electron transfer processes, (i) gives pt + MeCNT, (ii) gives... 

In the aromatic chamber experiments aerosol particles are formed and chemistry may take place on the surface of these particles. For example, NO2 could partition to the condensed phase and be reduced to HONO by the surface bound species on the secondary organic aerosol formed in the aromatic chamber experiments. Such heterogeneous processes would provide a sink for NOx and a source of radicals via the photolysis of HONO). 

The condensed phase photolysis of 2,2-dimethyl heptan-3-one, shows both Norrish type I and Norrish type II reactions. The Norrish type I (a-cleavage) occurs from both the excited singlet and triplet states. But the Norrish type II y-Hydrogen transfer) process occurs predominantly from excited singlet state. 

This process uses light irradiation of wavelengths lower than the UV-C, i.e., lower than 190 nm. Generally, Xe excimer lamps (Xexc = 172 nm) are used. The excitation leads, in the majority of the cases, to the homolytic breakage of chemical bonds, degrading OM in condensed and gaseous phases (for example, fluorinated and chlorinated hydrocarbons) [1,3]. However, its application is limited, and the most important use of VUV radiation is in water photolysis (Eq. 8) ... 

The vapor phase photolysis above 2500 A yields mainly Nj, a condensate of empirical formula CH3N, and traces of CH4, C2H4 and C2Hg . The quantum yield of N2 formation is 2. It was concluded that the major primary process is... 

An investigation of the reaction products from the photodissociation of OCS in mixtures with olefins in both liquid and solid solution (102) indicates that 16b continues to participate as a primary process but with reduced quantum efficiency in comparison to the gas phase. It is suggested (102) that inter-system crossing between the singlet excited state of OCS and a repulsive triplet state is more important in condensed than in gas phases. On the other hand, Gollnick and Leppin (105) have studied the photolysis of OCS at 253.7 nm In solution with a variety of solvents, find the quantum efficiency of CO formation to be 0.90 + 0.05 in all solvents, and suggest that process 15b is the only primary process. 

Gas- or solution-phase photolysis is always complicated by the fact that the primary photoproducts diffuse and undergo secondary reactions. To avoid such difficulties the technique of matrix isolation was developed. Here a dilute mixture of, e.g., CH4 in Ng is condensed from the vapor phase onto a transparent window maintained at temperatures of 4-20 K. Then the CH4 can be photolyzed and the resultant primary photoproduct, CH3, studied at leisure. A variant of this technique is to use a reactive matrix. When HBr or HI, condensed in a CO matrix, is photolyzed the primary process is dissociation of the excited HX. The H atom then reacts and HCO is observed. 

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