Photolytic Source

3 Photolytic Source In case of photochemically active molecules, supersonic beams of radicals can be generated via photodissociation of helium-seeded precursors (Fig. 11.4). 

Typically, these are bromide or iodide precursors such as bromoacetylene (CaHBr), vinyl bromide (C2H3Br), ° and propargyl bromide (C3H3Br) ° to yield supersonic beams of helium-seeded ethynyl, vinyl, and propargyl radicals, respectively. Briefly, the radical precursor is entrained in helium at a ratio of typically 1%. The gas mixture is released by a pulsed valve at 700 Torr backing pressure. A teflon extension with a slit located parallel to the expansion direction of the pulsed beam was 

---

Free radical oxidation of nitrite by OH, for example, can also occur. This reaction (which will occur during daylight hours when photolytic sources of OH are present) and the chemistry of associated nitrogen oxides in solution have been studied by Lpgager and Sehested (1993) ... 

Chemical, as opposed to photolytic, sources of S-MIF are also possible in the Archean atmosphere. By analogy with oxygen MIF produced during O3 formation [2], which is attributed to a symmetry-dependent non-statistical redistribution of internal energy in the vibrationally excited O3 [7], it is likely that reactions such... 

Further photolytic sources are dimethylnitrosamine (J), for which the initial reaction is probably... 

In addition to the value of 1.0 for the CO quantum yield obtained by Warneck near 150 nm. Quick and Cvetanovic (66) identified 0( D) as a primary product of the photolysis at the emission wavelengths of a bromine lamp, 157.5 and 163 nm. It was previously (67) established that insertion of 0( D) into neopentane leads to a 65 percent production of neopentanol. Thus the yield of neopentanol in combination with the effect of various additive gases serves as a qualitative test for the presence of 0( D). The quantum yield of neopentanol can then be used to measure the quantum yield for 0( D). These authors find the 0(1d) quantum yield to be near unity at these two wavelengths. This result is in conflict with that of Inn and Heimerl (50), who measured a quantum yield of 0.5 for CO production in the interval from 150 to 167 nm. These latter measurements were made by Fourth Positive fluorescence with a hydrogen continuum serving as the photolytic source. They have... 

This reaction is of interest because current models underestimate the production of HONO, which is an important photolytic source of hydroxyl radical (Ammann et al. 1998 Finlayson-Pitts and Pitts 2000). This reaction could also be important as a reductive pathway for nitrogen species in the atmosphere, which is in contrast to most atmospheric reactions which tend to oxidize trace gases (e g., the conversion of SO2 to H2SO4). Molar yields of HONO via Equation (31) (relative to the moles of NO2 lost) typically range from 50 to 100%, while yields of NO are 4 to 30% (Gerecke et al. 1998 Stadler and Rossi 2000). It appears that the formation of NO results from the secondary reaction of HONO at the surface of the soot (Stadler and Rossi 2000). The mechanism of... 

A number of other p-stannylalkyl radicals have been used as sources of stannyl radicals. Thus isobutyltrimethyltin can be used as a photolytic source of trimethylstannyl radicals (equation 20-13),44 and bis(O-trialkylstannyl) derivatives of benzpinacol can be used as a thermal source (equation 20-14).42 45 47... 

Diaryl oxalates afford a convenient photolytic source of phenoxyl radicals (ref. 188) The oxido grouping has been employed for imparting conformational immobility to a complex multi-ring structure in ligand binding studies (ref. 189). 

Figure 5.10 shows the OH mixing ratio as a function of altitude at three different latitudes and the OH concentration as a function of solar zenith angle. The strong solar zenith angle dependence reflects the photolytic source of OH. Figure 5.11 shows OH, H02, and total HO versus altitude. 

Dissolved CO exhibits diurnal variability in the surface ocean in response to its photolytic source, although this is also indicative of a strong sink term. The decline in the surface mixed-layer CO concentration in the dark results from a combination of CO oxidation, vertical mixing, and air-sea exchange. As the equilibration time between atmosphere and oceanic surface mixed layer is on the order of a month, this suggests that the former two processes dominate. Superimposed upon the diurnal cycle of CO in the surface ocean are spatial and seasonal gradients that result from the interaction of photoproduction and the sink processes. Below the euphotic zone CO is uniformly low throughout the intermediate water column. 

The reaction of ozone with alkenes produces OH and other radicals. Work in LACTOZ has provided quantitative data for the yield of OH for a variety of alkenes, including biogenic hydrocarbons. Yields of around 50 % were obtained for the reaction of ozone with typical alkenes, making this a source of atmospheric free radicals, which is especially significant at night-time when photolytic sources are absent. 

The interaction of NO with organic peroxy radicals a non-photolytic source of OH radicals,... 

Of similar photochemical importance as H2O2 is nitrate, which absorbs light above 290 nm. The ubiquitous presence of nitrate in environmental ices is well documented for cirrus ice particles [296, 297] as well as permanent and perennial snow packs [298-301]. hi aqueous solution, photolysis of nitrate ion leads to either OH and NO2 or 0( P) and nitrite ion, with typically significantly higher quantum yields for the first pathway [197, 200]. In the upper troposphere, it is currently thought that uptake of HNO3 to ice makes it ineffective as a photolytic source of... 

Photodissociation quantum yields for hexafluoroazomethane have been considered briefly, following earlier studies by Wu and Rice and photolysis of the azo-compound, both alone and in the presence of helium and carbon monoxide, in the range 30—200 °C has been studied. The azocompound has been employed as a photolytic source of trifluoromethyl radicals in a study of diffusion-controlled reactions in a variety of solvents." ... 

Holloway, A.L., J. Treacy, H. Sidebottom, A. Mellouki, V. Daele, G. Le Bras, and I. Barnes (2005), Rate coefficients for the reactions of OH radicals with the keto/enol tautomers of 2,4-pentanedione and 3-methyl-2,4-pentanedione, allyl alcohol and methyl vinyl ketone using the enols and methyl nitrite as photolytic sources of OH, J. Photochem. Photobiol A, Ghem., 176, 183-190. 

---