Photolysis quantum yield

While there have been a number of studies concerning the absorption cross-sections, photolysis rates and photolysis quantum yields of PCBs,107-109 PCDDs107 109-117... 

These results were interpreted with reference to an intramolecular heavy atom effect the authors concluded the carbene is formed from the excited singlet state of the molecule, in competition to intersystem crossing. A direct proof of this statement could, however not be obtained [24], It should be noted here that alternative explanations for the effect of halogen substitution on photolysis quantum yields have been put forward for the 4-haloanilines (see below). 

Nitrite is usually present in the environment at a lower concentration than nitrate, but its higher molar absorptivity and photolysis quantum yield can make it a competitive photoreactant under environmental conditions [13]. Usual concentration values of nitrite are below 2 jiM in seawater [13], below 0.1 mM in surface waters [7] and around 0.1-0.5 xM in the atmospheric aqueous phase in unpolluted areas [9]. Nitrite concentration was, however, found to reach up to 75 xM in fog water from California s Central Valley, and nitrite photochemistry was shown to account for 50-100% ofhydroxyl formation upon irradiation of the collected water samples [14],... 

One of the environmental sources of nitrite is represented by the irradiation of nitrate itself. The fact that nitrite, too, is a photoactive compound implies that the photochemical reactivity of nitrate and nitrite cannot often be dissociated, although the relative contributions to hydroxyl generation can be derived from the concentration values, photolysis quantum yields and radiation absorption calculations [6,8,12,14]. 

The transformation of various substituted phenols has been studied in the presence of nitrite under irradiation dihydroxybenzenes [78,79,113], nitrophenols [109], phenylphenols [114], Obviously, the observed transformation intermediates vary according to the reaction rate of each substrate and intermediate with the various reactive species formed during irradiation and according to the absorption spectrum and direct photolysis quantum yield of each compound. 

Photons are emitted by the sun. When a molecule absorbs a photon it may dissociate breaking one or more chemical bonds. The rate of a reaction in the atmosphere, e.g. ABC + hv A + BC, depends on the absorption cross section of the molecule ABC, the photolysis quantum yield and the actinic solar flux, shown in Figure 7.1, all of which are wavelength dependent. 

About half of formaldehyde reacts through each of these processes under atmospheric conditions. The absorption spectrum of formaldehyde shows extensive vibrational structure as shown in Figure 7.14, which also shows the photolysis quantum yields into the radical and molecular channels [136]. 

FIGURE 7.14 The cross section and photolysis quantum yields for formaldehyde [136]. The cross sections where divided by 10 ... 

CO, NO and O2. It was shown that the most probable coordinate of photo-dissociation is the e coordinate of the ligand bending mode, which is a superposition of two symmetrized distortions corresponding to the rotation of the ligand and to its displacement parallel to the porphyrin ring. Different APES s are obtained for ligands in linear or bent coordination the different photolysis quantum yields (0l 1 for linear and 10 10 for... 

Podoll and coworkers O) have redetermined the vapor pressure of 2,3,7,8-TCDD. Using this information together with the aqueous solubility, octanol/ water partition coefficient and photolysis quantum yields, investigators have estimated the half-lives for movement and transformation of 2,3,7,8-TCDD in water and air. Even though the vapor pressure (P) of 2,3,7,8-TCDD is low, the water solubility (S) is also very low and the Henry s law constant is therefore significant, and allows vaporization from water. 

The distribution of the photolysis process between Ionic and free-radical mechanisms Is mainly determined by the nature of the reaction medium. Free-radical processes are known with a photolysis quantum yield of two. Up to now an efficient and commercially workable sensitization has not yet been found. 

The primary quantum yield for electron formation, determined on a very short time-scale in laser experiments, is a measure of the electrons initially formed on interaction with light (Eq. 9). The primary quantum yield for electron formation has been measured at 355 nm by laser flash photolysis. Quantum yields were 4.6 x 10 to 7.6 X 10 for purified humic substances from several different natural waters and 1.7 x 10 to 4 x 10 for two commercial humic acids (normalized for carbon concentration) [85]. The caged pair generated in Eq. (9) can either collapse back or eject an electron and form the hydrated electron, e q, free in solution (Eq. 10). The steady-state yield, measured with electron scavengers under continuous irradiation, is a measure of the electrons which escape the DOM matrix and are free in solution. The electron thus occurs trapped within the DOM matrix and/or free in solution. 

Photon absorption provides another route for obtaining specific rate constants from highly excited molecules. Making some assumptions about collisional energy transfer, one may use the collision frequency as a clock in the measurement of the competition between photoreaction and collisional stabilization. Alternatively, one may make some model calculations for the specific rate constants for photoreaction and for collisional energy transfer. From these one may compute experimental quantities like photolysis quantum yields as a function of pressure and test them for consistency with experiments. This procedure has been used in a detailed study of NO2 photolysis at wavelengths between 313 and 416 nm (predissociation threshold at 397.9nm) and N2 pressures between 0 and 1000atm. The... 

One possible reason for this rate enhancement, at least in some instances, is a change in absorption cross sections and/or photolysis quantum yields due to selfassociation at the interface. This effect has been documented for aromatic compounds both spectroscopically and by simulations [278, 282, 283], and is a consequence of the different hydrogen bonding environment present at the air-ice interface compared to the liquid surface. In the case of benzene in particular, the self association gives rise to a significant red-shift in the absorption spectrum [279], such that benzene present at the air-ice interface may absorb available solar radiation in the lower atmosphere. This opens the possibility of a previously unconsidered fate for several aromatic pollutants present in snow- and ice-covered regions. 

By this definition, the maximum and minimum photolysis quantum yield is 1 and 0, respectively. 

Photolysis Quantum Yields. Table 4.6 shows the wavelength threshold below which the production of the pairs of either the ground or excited states of NO(X Il, A E ) and 0( P, D, S) in the photolysis of NO2 (Okabe 1978). From Table 4.6 it... 

Photolysis Quantum Yields The broad bands without rotational structure in the spectrum shown in Fig. 4.12 implies that the dissociation lifetime of the excited states is short. Indeed, the HONO molecules absorbing the radiation in this wavelength region is known to dissociate in the pathway,... 

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