Trace gases photodissociation

Several authors have been able to show experimentally that natural air containing trace amounts of SO2, exposed to sunlight, is photochemical-ly oxidized and that the rate of SO2 oxidation was a complex function of the trace gas composition of the air mixture (NO, NO2 - hydrocarbons level and composition). The oxidation of SO2 within the natural troposphere is therefore expected to occur largely through reactions 7,8 and 9 of table I. The main mechanism is through radicals which might be formed in various ways such as from ozone photodissociation and from the reaction ... 

Evidence for a PDR in NGC 7027 comes from comparing the H2 and PAH emission. H2 and PAH emission are coextensive outside the H H region, and the H2 loops form the outer boundary of the PAH emission. Since the PAH feature traces the penetration of FUV light into the surrounding gas this reveals the interface between ionized and molecular gas, and therefore traces the photodissociation region in NGC 7027. The same stratification of H n, PAH, H2, and CO is seen in the Orion Bar PDR (SeUgren et d. 1990). 

The T dependence of (ib) was unexpected as it implies that there are strong attractive forces acting between 02(A ) and I at relatively large internuclear separations. To explore this phenomena in more detail, and to test the reliability of Eqs. (20) and (21), we examined reaction (lb) at a temperature of 150 K. ° These measurements were carried out using a Laval nozzle to provide low temperature gas flows. Traces of I2 were entrained in He/02 mixtures, and pulsed laser photolysis was used to generate I by exciting just above the 12(B) dissociation limit. Near threshold photodissociation was used to ensure that I was produced with a translational temperature in equilibrium with the local conditions. I decay kinetics were... 

Photochemistry of gas-phase molecules, including species of atmospheric importance the molecular photodissociation dynamics are traced by photofragment ion (velocity map) imaging. 

Figure 27.13 Translational energy distributions of the methyl (CH3) photofragment from the 222 nm photodissociation of CHaBr. (a) Gas-phase results, derived from data by van Veen et al, Chem. Phys., 1985, 92 59, with permission of Elsevier, (b) Adsorbate-mediated results, derived from data by Harrison etat, J. Chem. Phys., 1988, 89 1475, with permission of the American Institute of Physics solid-line trace CHaBr on an annealed LiF(OOl) substrate long-dash trace CHaBr ice short-dash trace CHaBr on a non-annealed (i.e. rough) LiF substrate. The arrows in the lower panel indicate the positions of the gas-phase peaks...

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