Photodissociation quantum yield

Figure A3.6.12. Photolytic cage effect of iodme in snpercritical ethane. Points represent measured photodissociation quantum yields [37] and the solid curve is the result of a numerical simnlation [111].

The simple difhision model of the cage effect again can be improved by taking effects of the local solvent structure, i.e. hydrodynamic repulsion, into account in the same way as discussed above for bimolecular reactions. The consequence is that the potential of mean force tends to favour escape at larger distances > 1,5R) more than it enliances caging at small distances, leading to larger overall photodissociation quantum yields [H6, 117]. 

Dardi P S and Dahler J S 1990 Microscopic models for iodine photodissociation quantum yields in dense fluids J. Chem. Phys. 93 242-56... 

The early work on the photolysis of water was in the gas phase employing one photon. The branching ratio of the photodissociation into H + OH and H2 + O was reported by McNesby et al. [28] as 3 1 at a photon energy of 10.03 eV. Ever since, that ratio has been consistently revised in favor of the H + OH reaction with the final result of Stief et al. [29] giving 0.99 0.01 for 6.70-8.54 eV photon energy and 0.89 0.11 for the interval 8.54-11.80 eV. In the absence of direct determination these ratios often are assumed valid in the liquid phase. In the early work of Sokolev and Stein [30], mainly the photodissociation quantum yield in liquid water was measured, but a small photoionization yield of -0.05 was attributed to the process... 

Minton et al. (1992) ratios converted to absolute values assuming a photodissociation quantum yield of 1 and only two channels. 

Figure 4.34 compares the absorption spectra of the diatomic halogens, F2, Cl2, Br2, and I2. Cl2 is of particular recent interest in the troposphere in that levels up to 150 ppt have been observed in marine areas (Keene et al., 1993 Pszenny et al., 1993 Spicer et al., 1998). Table 4.30 summarizes the absorption cross sections of Cl2, Br2, and BrCl (DeMore et al., 1997 Marie et al., 1994 Hubinger and Nee, 1995). These diatomics all dissociated with a photodissociation quantum yield of 1 (Calvert and Pitts, 1966). 

Nevertheless, even from steady-state measurements of iodine photodissociation quantum yields, Noyes [270] found that the quantum yield increased as the wavelength of photoexciting light decreased, presumably due to the greater separation of the iodine atoms when formed with greater... 

The little information available (see above) suggests that photolysis of gas-phase PCBs in the troposphere will be negligible for those PCBs with <4 chlorine atoms, and this may also be the case for the more chlorinated PCBs. However, it appears that the PCDDs and PCDFs will absorb radiation in the actinic region in the troposphere, and hence the importance of photolysis will depend on the photodissociation quantum yields in the gas phase, which are, however, not presently known. 

In this Section, we present a brief overview of the measured absorption cross sections and photodissociation quantum yields which are used for the calculation of the photolysis rates for the key molecules of the middle atmosphere. More details can be found in the evaluation of chemical and photochemical data periodically published by the Jet Propulsion Laboratory. 

Magnotta, F., and H.S. Johnston, Photodissociation quantum yields for the NO3 free radical. Geophys Res Lett 7, 769, 1980. 

A) more than it enhances caging at small distances, leading to larger overall photodissociation quantum yields [U6, 117]. 

Absorption cross sections and quantum yields for acetone have been summarized by Atkinson et al. (1992). An average photodissociation quantum yield for CH3CO formation is about 0.33 over the wavelength region 280-330 nm. 

A recent study of iodine atom recombination in solution by Luther et al. [294] used a dye laser (wavelength 590nm, pulse duration 1.5ps) to photodissociate iodine molecules in n-heptane, -octane or methyl cyclohexane at pressures from 0.1 to 300 MPa. Over this pressure range, the viscosity increases four-fold. The rate of free-radical recombination was monitored and the second-order rate coefficient was found to be linearly dependent on inverse viscosity. This provides good reason to believe that the recombination of free iodine atoms is diffusion-limited, especially as the rate coefficient is typically 10 °dm mol s . The recombination of primary and secondary pairs is too rapid to be monitored by such equipment as was used by Luther etal. [294] (see below). Instead, the depletion of molecular iodine absorption just after the laser pulse was used to estimate the yield of (free) photodissociated iodine atoms in solution. They found that the photodissociation quantum yields (survival probability) were about 2.3 times smaller than had been measured by Noyes and co-workers [291, 292] and also by Strong and Willard [295]. This observation raises doubts as to the accuracy of the iodine atom scavenging method used by Noyes et al. or perhaps points to the inherent difficulties of doing steady-state measurements. In addition, Luther et al. 

The lifetime of NO3 is very short and is much shorter in daytime because of effective photodissociation (quantum yield 1.0 for A < 587 nm) at wavelengths less than 587 nm, NO3 radical dissociation is exclusively to NO2 + O ( P) ... 

From the relationship between the photo-excitation and absorption spectrum, photodissociation quantum yields are in general unity when a molecule is excited to the repulsive potential curve as shown in Fig. 2.1. On the other hand, when an absorption spectrum has a band stmcture and a molecule is excited to a bound state as shown in Fig. 2.2, photodissociation quantum yields are in general 0 < cp < 1 and the value has to be determined experimentally. 

Photodissociation Quantum Yields The wavelength thresholds below which photodissociation is energetically possible in the photolysis of O3 are shown in Table 4.3 (Okabe 1978). From the table, energetically possible photodissociation processes by the solar flux reaching to the troposphere are the following five processes,... 

Talukdar, R.K., Burkholder, J.B., Hunter, M., Gilles, M.K., Roberts, J.M., Ravishankara, A.R. Atmospheric fate of several alkyl nitrates Part 2 UV absorption cross-sections and photodissociation quantum yields. J. Chem. Soc. Faraday Trans. 93, 2797-2805 (1997a)... 

Prom this data, as well as the previous data on the photodissociative quantum yield, it is evident that short wavelength UV light is extremely detriment to the... 

Protonation of a heterocyclic nucleus has been found to decrease slightly the photodissociation quantum yield (cp) of 4-azidopyridine and 4-azidoquinoline, do not influence on the (p value for 9-azidoacridine, and reduce by two orders of magnitude the (p value for 9-(4 -azidophenyl)acridine. 

The photochemistry of aromatic azides reeeives eontinuous attention because of their useful applications in heterocyclic syntheses, photoresist techniques and photoaflfinity labeling [1,2,3,4,5]. The key reaction in all cases is the photoinduced N-N2 bond dissociation with formation of highly reactive intermediate, nitrene. The main quantitative parameter of this reaction is the photodissociation quantum yield (cp), which determines the azide photoactivity. 

The second parameter characterizing the tendency of azide towards photodissociation is the range of spectral sensitivity, which is determined by the absorption spectrum of azide. Interrelation between azide photoactivity (photodissociation quantum yield) and spectral sensitivity will be discussed below. 

Systematic investigation of this series and some other heterocyclic azides allows revealing general relationship between azide structure and photoactivity. The dependence of photodissociation quantum yield on the size of the azide n-system and its charge - the size and charge effects and interrelation between them - have been foimd experimentally and justified quantum-chemically. 

Systematic quantitative investigations of the spectral properties and photodissociation quantum yields of azidopyridine and azidoquinoline in dependence on charge state (neutral or cationic) were performed in [27,28],... 

The photodissociation quantum yields (cp) of the heteroaromatic azides were calculated from the kinetic curves of the azide disappearance and are shown in Table 1. It is seen that both neutral and eationie azidopyridine and azidoquinoline are photoaetive azides with quantum yields rp > 0.1 [28,35], Nevertheless, one can note some effects. Insertion of positive eharge into azide moleeule results in decrease of the (p value for example, in MeCN, quantum yield deereases from 0.83 to 0.22 on going from 4-azidopyridine Al to its hydroehloride and in somewhat less extent, from 0.49 to 0.37, on going from 4-azidoquinolinev42 to its hydrochloride. For neutral 4-azidopyridine quantum yield deereases slightly also on going from acetonitrile to acetonitrile-ethanol mixture and further to pure ethanol (Table 1). 

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