Nitrous oxide quantum yield

Fig. 24. Adsorption of nitrous oxide on a platinum surface. Ordinate photoelectric yield I in electrons per light quantum. = 265.5 mg T = 83°K. (a) smashing of the N2O capsule (6) complete removal of the liquid air from N2O [according to (58)].

The relative rate of reaction of Of1/)) with N20 and C3H8 exceeds 0 2,352a so that under these conditions less than 20% of any Of1/)) will react with propane rather than with nitrous oxide. Our preliminary results110 indicate that the 02 quantum yield is less than 0.02%. 

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. 

Not being aware of the earlier work, the present author first noticed the phenomenon in 1981. Geiger and Huber10 had photolyzed dimethylnitrosamine in the gas phase at 1 Torr and under 100 Torr N2 buffer. This compound fragments from the first excited singlet state into dimethylaminyl radicals and nitrous oxide NO with unity quantum yield, but neither photoproducts nor a decrease of the initial compound pressure were observed. Even after 20 h photolysis the back-reaction was complete to more than 99.9% (Scheme 6). This seemed quite puzzling because sterically unhindered aminyl radicals are transient and readily self-terminate by coupling and disproportionation. 

Figure I. Quantum yield of nitrogen in the 1470-A. photolysis of alkane-nitrous oxide solutions at 13°C. Yields are relative to 4>(H2) = 1 for liquid cyclohexane photolysis at 1470 A. Ordinate is concentration of N20 in moles/liter...

The main photodissociation process for nitrous oxide is N20— N2 + O( D). It occurs with a quantum yield of nearly unity (Cvetanovic, 1965 Greiner, 1967a Paraskepopoulos and Cvetanovic, 1969). The photoproducts N2 + 0(5P) and NO + N(4S), although energetically allowed, would violate the spin conservation rule and are expected to be generated with low probability. For the second of these processes Preston and Barr (1971) have established a quantum yield of less than 2%. 

Obenauf et al. [271] have measured the quantum yield of fluorescence per molecule of target gas consumed in the reaction of barium vapour with nitrogen dioxide and nitrous oxide, obtaining values of 0.20—0.27 for the stronger fluorescence from the nitrous oxide reaction, and 0.015 0.003 for the nitrogen dioxide reaction. Using the values for total reaction cross-section reported by Jonah et al. [270], Obenauf et al. [271] estimate the cross-sections for the chemiluminescent reactions to be < 6—7 for the nitrous oxide reaction and about 2—3 for the nitrogen dioxide reaction. 

Natural water samples and humic substance solutions were probed for their phototransient behavior. Laser flash kinetic spectroscopy was used to study two transients common to most samples. One transient with a maximum around 720 nm was quenched by decreasing pH and nitrous oxide. It was present In all waters with DOC and had a spectrum which resembled chat of a solvated electron. The signal was linear with laser power. The quantum yield for this transient was measured. Samples with higher ground state absorbance yielded a transient with a maximum at 475 nm Chat was quenched by oxygen. This transient seemed to be a photophysical hybrid with triplet and radical cation character. Additional work done to characterize these transients and predict their environmental fates Is discussed. 

After T720 was tentatively characterized as an aquated electron with nitrous oxide and electron transfer studies, the quantum yield for the production of the aquated electron In dilute natural waters containing dissolved organic matter and In humic substance extract was measured by the comparlslon method. In this method the literature value for the extinction coefficient of the solvated electron at 720 nm was used (21). The excited state absorption of T720 was measured. These were used In algebraic ratio to an actlnomlter for which a quantum yield had already been thoroughly determined. 

Carbajo, P.G., Smith, S.C., Holloway, A.-L., Smith, C.A., Pope, F.D., Shallcross, D.E., Orr-Ewing, A.J. Ultraviolet photolysis of HCHO absolute HCO quantum yields by direct detection of the HCO radical photoproduct. J. Phys. Chem. A 112, 12437-12448 (2008) Carlon, N.R., Papanastasiou, D.K., Fleming, E.L., Jackman, C.H., Newman, P.A., Burkholder, J. B. UV absorption cross sections of nitrous oxide (N2O) and carbon tetrachloride (CCLi) between 210 and 350K and the atmospheric implications. Atmos. Chem. Phys. 10, 6137-6149 (2010)... 

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