Quantum Yield Of CO Research Articles

Evidence for a biphotonic process in the mercury-photosensitized decomposition of ethylene oxide in the gas phase

The quantum yields of CO, H 2 and CH 4 at incident resonance radiation intensities of I 0 = 38.6 × 10 15 and 1.23 × 10 15 quanta s −1, used in the equations Φ = kI n−1 0, lead to values of n which are greater than unity. Considering the nature of the possible radical—radical and atom—radical reactions involved, this clearly indicates the presence of a biphotonic step or steps leading to the formation of these products in the mercury-photosensitized decomposition of ethylene oxide. The pressure dependence of the product yields at the higher intensity is thought to be the result of the deactivation of the intermediate (probably a triplet ethylene oxide molecule) before the second excitation can occur.

The photolysis of sulfur dioxide in the presence of foreign gases: IX. allene

SO 2 was photoexcited at 25 °C and 313.0 nm in the presence of allene. The quantum yields of the gas phase products, C 2H 4 and CO, were determined over a wide range of allene and SO 2 pressures as well as in the presence of CO 2, NO, and H 2O. The quantum yield of C 2H 4, Φ {C 2H 4}, increases with the ratio [allene]/[SO 2] to a maximum value of 13.1 × 10 −3. In the presence of excess CO 2, NO, or H 2O, or at higher allene pressures, Φ {C 2H 4} is reduced for any [allene]/[SO 2] ratio. At higher [allene]/[SO 2] ratios NO is a much less efficient quencher than at low ratios and thus both singlet and triplet states of SO 2 must participate in C 2H 4 production. The quantum yield of CO, Φ {CO}, also increases with the ratio of [allene]/[SO 2] maximum value of 20.9 × 10 −3. At low values of the ratio [allene]/[SO 2] as well as low total pressure, Φ {CO} is quenched initially by adding CO 2, NO, and H 2O in a manner similar to Φ {C 2H 4}. For continued increases in the pressures of CO 2, NO, and H 2O or at higher values of [allene]/[SO 2], Φ {CO} is enhanced. Both the singlet and triplet emitting states, as well as a previously proposed non-emitting triplet state are necessary to interpret the photochemistry of SO 2 in this study. A relatively complete mechanism is proposed, all of the pertinent rate coefficient ratios are derived and tabulated, and from these values Φ {C 2H 4} and Φ{CO} values are computed which agree well with the observed values.

The reaction of O( 3P) with CCl 2CH 2

The reaction of O( 3P), prepared from the Hg photosensitization of N 2O, with CCl 2CH 2 was studied at 25°C. The products of the reaction in the absence of O 2 were CO, CH 2ClCCl(O), polymer, and another unidentified compound (as well as N 2 from the N 2O). The quantum yields of CO and CH 2ClCCl(O) were 0.35 ± 0.02 and 0.06 ± 0.01, respectively, independent of reaction conditions. The reaction rate coefficient is identical to that for O( 3P) + C 2F 4, i.e. 6.0 × 10 8 M −1 s −1. In the presence of O 2 a long free radical chain process occurs in which CH 2ClCCl(O) is the major product and CCl 2O is produced about 4% of the time in the chain steps which are: ▪ The quantum yield of CO production also increases when O 2 is added, but CO is formed only in the initiating and terminating steps.

Photochemistry of anhydrides. Part 3.—Primary processes in the photolyses of (CF3CO)2O and (C2F5CO)2O and the use of (CF3CO)2O and (C2F5CO)2O as gas phase actinometers

The photolyses of the vapours of the anhydrides (CF3CO)2O and (C2F5CO)2O are quantitatively described by the overall equation (R = CF3 or C2F5), (RCO)2O +hv→ CO + CO2+ R2. The quantum yield of CO is independent of anhydride pressure in the range 0.5–16 Torr and increases slightly with temperature. For (CF3CO)2O, ϕCO(25°C)= 0.24 and ϕCO(200°C)= 0.29: for (C2F5CO)2O, ϕCO(25°C)= 0.26 and ϕCO(190°C)= 0.31. ϕCO is unaffected by the presence of mercury vapour and is virtually independent of whether photolysis is at 254 or 265 nm. Addition of CO2 or olefins indicates weak vibrational quenching. No evidence of involvement of excited triplet states was obtained, using the Cundall and biacetyl techniques. Weak emission of light was observed with ϕ(emission)⩽ 2 × 10–4 for (CF3CO)2O and ⩽ 2 × 10–3 for (C2F5CO)2O.

Photodissociation quantum yields of CO2 between 1200 and 1500 Å

Measurements of the photodissociation quantum yields of CO2 have been carried out using various atomic line sources between 1200 and 1500 Å. The production of CO was monitored by resonance fluorescence, and the results indicate that where the CO2 absorption spectrum is continuous, the CO quantum yield is high, but where there is structure, the yield is low. At the commonly employed 1236 Å resonance line, the CO quantum yield is 0.85±0.17, relative to the absolute value at 1470 Å of 0.75 obtained by Inn. On the same scale, the other values are (a) 1216 Å, 0.57 ±0.11, (b) 1302–1306 Å, 0.21±0.07, (c) ∼1390 Å, 0.46±0.05, (d) 1492–1495 Å, 0.58±0.06. The CO2 pressures employed were in the range 0.4–4.0 Torr. If these same quantum yields are valid at the pressures of the Mars and Venus atmospheres, the CO and O atom production rates are less than presently assumed.

The reaction of O(3P) with C2HCl3

AbstractThe reaction of O(3P), prepared from the Hg photosensitization of N2O, with C2HCl3 was studied at 25°C. The products of the reaction in the absence of O2 were CO, CHCl3, and polymer (as well as N2 from the N2O). The quantum yields of CO and CHCl3 were 0.23 ± 0.01 and 0.14 ± 0.05, is respectively independent of reaction conditions. The reaction mechanism is equation image with k14a/k14 = 0.23, where k14a + k14b. Most of the HCl and CCl2 combine to form CHCl3, but some other products must also be formed to account for the difference in the CO and CHCl3 quantum yields. The C2HCl3O* adduct polymerizes without involving additional C2HCl3 molecules, since the quantum yield of C2HCl3 disappearance, Φ{C2HCl3}, was about 1.0 at high values of [N2O]/[C2HCl3]. The rate coefficient for the reaction of O(3P) with C2HCl3 is 0.10 that for the reaction of O(3P) with C2F4.In the presence of O2 the free radical chain oxidation occurs because of the reaction equation image The main product is CHCl2CCl(O) with smaller amounts of CO and CCl2O, and some CO2. The chain lengths were long and values of  Φ {C2HCl3} up to 90 were observed.

Stoichiometry of reduction and phosphorylation during illumination of intact chloroplasts

1. 1. Intact chloroplasts which had retained their envelopes and were capable of photoreducing CO 2 at rates ranging from 70 to 240 μmoles·mg −1 chlorophyll·h −1 showed a large increase in their endogenous levels of ATP and NADPH on illumination in the absence of electron acceptors. At low light intensities, addition of 3-phosphoglycerate, whose reduction consumes ATP and NADPH in an equimolar ratio, drastically decreased the levels both of chloroplast ATP and NADPH. Only at light intensities not far from light saturation for 3-phosphoglycerate reduction did ATP become available in excess of NADPH, as revealed by the different response of the endogenous levels of ATP and NADPH to addition of 3-phosphoglycerate. 2. 2. Addition of bicarbonate to preilluminated chloroplasts or reduction of the light intensity during photosynthetic CO 2 reduction decreased only the ATP, not the NADPH levels, indicating a rate limitation of CO 2 reduction by ATP. 3. 3. Quantum requirements for the reduction of 3-phosphoglycerate or oxaloacetate at low light intensities were consistently close to 4. For the reduction of CO 2 they were never significantly below 12 and usually much higher. As the reduction of CO 2 requires twice as much reduced pyridine nucleotide as that of 3-phosphoglycerate or oxaloacetate, this indicates that the ATP generated during pyridine nucleotide reduction is insufficient to drive CO 2 reduction. 4. 4. Dithiothreitol improved significantly quantum yields of CO 2 reduction, but not of 3-phosphoglycerate reduction. It also stimulated the reduction of oxygen (Mehler reaction) by intact or broken chloroplasts. 5. 5. The light-dependent increase of both chloroplast NADPH and ATP was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) at concentrations which are ineffective in suppressing Photosystem I-dependent cyclic photophosphorylation of broken chloroplasts. 6. 6. From the data it is concluded that not much more than one molecule of ATP is formed in intact chloroplasts during the transport of two electrons from water to NADP +. Coupling of phosphorylation to electron transport appeared to be flexible, not tight. The extra ATP needed for the reduction of CO 2 was produced by an independent photochemical reaction presumably reducing oxygen.

CO quantum yield in the photolysis of CO2at λ1470 and λλ1500-1670

CO quantum yield in the photolysis of CO₂at λ1470 and λλ1500-1670

The action spectrum, absorptance and quantum yield of photosynthesis in crop plants

The measurements were made to provide a basis for discussion of the definition of “photosynthetically active radiation”. The action spectrum, absorptance and spectral quantum yield of CO 2 uptake were measured, for leaves of 22 species of crop plant, over the wavelength range 350 to 750 nm. The following factors were varied: species, variety, age of leaf, growth conditions (field or growth chamber), test conditions such as temperature, CO 2 concentration, flux of monochromatic radiation, flux of supplementary white radiation, orientation of leaf (adaxial or abaxial surface exposed). For all species and conditions the quantum yield curve had 2 broad maxima, centered at 620 and 440 nm, with a shoulder at 670 nm. The average height of the blue peak was 70% of that of the red peak. The shortwave cutoff wavelength and the height of the blue peak varied slightly with the growth conditions and with the direction of illumination, but for the practical purpose of defining “photosynthetically active radiation” the differences are probably insignificant. The action spectrum for photosynthesis in wheat, obtained by Hoover in 1937, could be duplicated only with abnormally pale leaves.

Hg(63P1)-sensitized decomposition of HNCO vapor and HNCO–H2 mixtures; the reaction of hydrogen atoms with HNCO

The Hg(63P1)-sensitized decomposition of HNCO vapor has been briefly studied at 26 °C with HNCO pressures from about 3 to 30 Torr. The products detected were the same as in the direct photolysis, CO, N2, and H2. The quantum yield of CO was appreciably less than unity, compared with a value of 1.5 in the direct photolysis under similar conditions. From this and other observations it is tentatively concluded that a single primary process occurs:[Formula: see text]From a study of the mercury-photosensitized reactions in mixtures of HNCO with H2, it was concluded that hydrogen atoms react with HNCO to form CO but not N2. The initial step is probably addition to form NH2CO. From the competition between reaction [1] and the corresponding quenching by H2, the cross section for reaction [1] was estimated to be 2.3 times that of hydrogen.

Quantum yields and primary processes in the photolysis of HNCO vapor at 2062 Å

Quantum yields of CO, N2, and H2 have been measured in the photolysis of HNCO vapor (2–23 Torr, 25 °C) at 2062 Å. Extrapolated values at zero pressure of HNCO were about 1.0, 0.40, and 0.13 respectively. Φ(CO) and Φ(N2) increased with increasing HNCO pressure and became approximately constant at values of 1.5 and 0.6 respectively between 10 and 20 Torr. From these and other data it was concluded that two primary processes, each with a quantum yield of about 0.5, occur.[Formula: see text]The abstraction of H from HNCO by methyl radicals was found to be slow, even at 250 °C, and it was concluded that abstraction of H from HNCO plays no part in the photolysis mechanism, contrary to previous conclusions. A mechanism is suggested, based on radical–radical reactions at low HNCO pressure or high intensity, with radical reactions with HNCO becoming important at higher pressures or low intensity. It is concluded that H2 was formed in the system by reactions of H atoms.

Primary Photochemical Processes in Glyoxal at 4358 Å

Quantum yields of CO and of H2 have been measured in the gas-phase photolysis with 4358-Å radiation at temperatures between 37° and 207°C. φCO is generally about 0.6; φH2 is about 0.01. The data suggest that about 70% of the excited molecules cross to the triplet state by a first-order process, and the remaining excited singlet molecules return to the ground state with fluorescence. The principal reaction of triplet molecules is a dissociation into CO and HCHO, while a small fraction of triplet molecules are destroyed by collision with ground-state glyoxal molecules. About 3% of the triplet molecules undergo a first-order intersystem crossing to the ground state, forming molecules with about 55 kcal of vibrational energy. These either dissociate into H2 and CO or relax to vibrational equilibrium by collision with heat-bath molecules. At pressures near 100 Torr, where higher concentrations of triplet molecules are formed because of increased absorption, reactions of free radicals produced in collisions between triplet molecules also contribute significantly to φH2. The data suggest that temperature variation has little effect on the reactions destroying the triplet state, but that the rate of vibrational relaxation in the ground state increases markedly at higher temperatures.

Photochemical Studies in Flash Photolysis IV. Intensity and Wavelength Effects

AbstractWavelength and intensity effects which we have observed in the flash photolysis of acetone are reviewed. Marked nonmonotonic wavelength effects occurred in the different regions investigated (260, 280 and 300 mμ) and serve to emphasize the problem of interpreting any data obtained with unfiltered light. The data at 300 mμ were compared with those obtained with continuous light sources 106 fold less intense. A considerable difference occurred in the C2H6/CO ratio. To investigate this intensity effect, flash photolyses were made over a wide range of reduced intensities, as low as 104 fold less than the maximum one, and the transition intensity region for change in C2H6/CO ratio and CO quantum yield was located. This phenomenon illustrates the more general fact that the apparent primary processes observed at typical flash intensities need not be the same as the primary processes found at low intensities.

THE LIQUID PHASE PHOTOLYSIS OF DIETHYL KETONE AND METHYL ETHYL KETONE

The liquid phase photolysis of diethyl ketone has been studied in the temperature range from −35° to 95 °C. The CO quantum yield at 95 °C. was found to be close to unity. At 28 °C. decrease in intensity and addition of heptane led to a substantial increase of the CO and the ethane yields.The methyl ethyl ketone liquid phase photolysis at temperatures between 5° and 75 °C. led to the same observations. Arrhenius plots of RE/RB1/2[K] gave for both compounds a value of 5 kcal./mole.Gas phase studies in the temperature range of 0° to 60 °C. confirmed the low CO quantum yield reported previously and showed evidence for disproportionation and recombination reactions between ethyl and propionyl radicals.