Photodissociation product probability

The partial cross section gives the probability of absorbing light and producing a particular final product quantum state. The total photodissociation cross section is clearly given by the sum over all partial photodissociation cross sections ... 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

Very little has been reported on the photodissociation dyamics of HCN leading to the production of the CN X and A state fragments, despite the fact that this is the simplest triatomic cyanide compound. This is probably because the absorption above 150 nm is very weak and structured so that a laser in the VUV region is needed. Nevertheless, some results from a recent study on the photodissociation dynamics of HCN at 193 and 157 nm have been presented at the Sixteenth Informal Conference on Photochemistry (131). 

The photodissociation process takes place most frequently at excitation of the molecule to a non-bonded state, with subsequent dissociation into products. Since the angular part of the transition probability, according to Chapter 2, is still dependent on the mutual orientation of the E-vector of the initiating light beam and on the transition dipole moment d, one may expect spatial anisotropy of angular momenta distribution both in the dissociation products and in the set of molecules which remains undestroyed. 

In nonaqueous solvents, such treatment with dry hydrohahc acids is the only way to cleave the /r-oxo dimer nonoxidatively. However, the /x-oxo dimers of water-soluble porphyrins are readily cleaved by Tewis bases such as hydroxide, imidazole, histidine, and pyridine. Both the equilibria and kinetics of such reactions have been reported. In addition, /x-oxo dimers of water-insoluble Fe porphyrins in dichloromethane can be oxidatively cleaved to yield PFe p2 and (probably) an Fe species. Studies of the picosecond decay of the excited state of (TPPFe)20 in benzene following a 532- or 355-nm 25-ps pulse suggest that the intermediate state is a photodissociated pair, (TPP -)Fe -l-TPPFe — (0 ), and a small amount of disproportionation reaction products, TPPFe -f TPPFe = O. ... 

The two most probable loss processes of phosgene in the stratosphere are photolysis, and transport into the troposphere. Phosgene can be formed in situ in the stratosphere [36a], and indeed it is one of the main photo-oxidation products in the upper troposphere and lower stratosphere from the breakdown of chlorinated hydrocarbons (of both natural and anthropogenic origin). For example, the relatively inert tetrachloromethane accumulates in the air, but can photodissociate in the stratosphere according to ... 

Studies of the photodissociation dynamics of chlorinated benzene derivatives have been reviewed. Photodissociation of chlorobenzene at 266 nm has been investigated by the crossed laser-molecular beam technique, and a hot molecule mechanism is considered probable. Similar studies have been carried out for bromobenzene and p-bromotoluene, which show that for each of these molecules the dissociation is fast and the transition dipole moment is almost perpendicular to the C-Br bond. In deoxygenated aqueous solutions, 254 nm photolysis of chlorobenzene yields phenol and chloride ions as the main products, along with benzene, phenylphenols and biphenyl.lodo-benzene adsorbed on sapphire(OOOl) at 110 K undergoes C-I bond cleavage when irradiated at 193 nm. ... 

Neutral molecule plus radical reactions in the outer CSE are probably the major production channel of longer C-chain molecules such as C2 H, and the cyanopolyynes (HCan+iN). For example, the reaction channel C2nH2 + C2H — C2n+2H2 + H and subsequent photodissociation C4H2 + H C4H +H can build up C4H and CeH from the parent molecule C2H2 62). 

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