Quantum Yield Of CO Research Articles

Effects of photoinhibitory treatment on CO2 assimilation, the quantum yield of CO2 assimilation, D1 protein, ascorbate, glutathione and xanthophyll contents and the electron transport rate in vine leaves

ABSTRACTThe responses of photosynthesis to high light and low temperature were studied in vines cultivated in the greenhouse in low light. Exposure to high light (1000 /umol m−2 s−1) or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, but the combination of high light and low temperature caused rapid loss of photosynthetic capacity and a decrease in the efficiency of photosynthetic energy conversion. After a 15 h exposure to 5°C at high light, the Fv/sb/Fmratio had decreased by 80% and the apparent quantum yield by 75%. Nevertheless, when the leaves were returned to low light at 22°C, these parameters recovered rapidly. The foliar pools of ascorbate and glutathione decreased in the first hours of photoinhibitory treatment while the zeaxanthin content increased from negligible levels to about 50% of the total foliar xanthophyll pool. There was a clear correlation between the zeaxanthin content of the leaves and their Fv/Fm ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in theFv Fm ratio and the measured decrease in the total foliar levels of the antioxidants ascorbate and glutathione. The amount of D, protein diminished over the same period as the zeaxanthin levels were increasing. This approach, involving simultaneous measurements of several parameters considered to influence photosystemy II activity, clearly demonstrates that measured decreases in Fv/Fm may not simply be related to zeaxanthin levels or to amounts of D1 protein alone but result from multifactoral influences.

Mechanism of 4-carboxybenzophenone-sensitized photooxidation of methionine-containing dipeptides and tripeptides in aqueous solution

The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) x 10{sup 9} M{sup -1} S{sup -1} for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly (S.{sup {center_dot}}.S)-bonded radical cations, and intramolecularly (S.{sup {center_dot}}.N)-bonded radical cations derived from peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of CO{sub 2} formation were measured by steady-state photolysis. 50 refs., 6 figs., 3 tabs.

Effect of Shading on Vine Morphology and Productivity and Leaf Gas Exchange Characteristics in Grapevines in the Field

The photosynthetic activity of grapevine leaves (Sangiovese/Kober 5BB) was evaluated under field conditions on mature vines grown under three different levels of photosynthetically active radiation (PAR, 100%, 60%, and 30% sunlight) by measuring the response of net photosynthesis (Pn) to PAR at flowering and veraison. The diurnal trends of PAR, Pn, leaf transpiration rate (E), stomatal conductance to HL₂O vapor (g_s), substomatal CO₂ concentration (C_i), and leaf water potential (ψ) were determined. Responses of Pn to PAR were analyzed using asymptotic exponential curves, which provided estimates of the radiation saturated rate of Pn (Pn_sat), dark respiration (R_d), light compensation, and saturation points (PAR_c and PAR_sat, respectively) and the apparent quantum yield of CO₂ assimilation (φ_i). At flowering and veraison, the leaves of shade-grown vines (60% and 30% sunlight) showed significantly lower Pn_sat, R_d, PAR_c, and PAR_sat values, whereas the φ_i was significantly higher. In comparison to unshaded vines, the Pn_sat in the vines grown at 60% and 30% sunlight measured between 0900 and 1100 hours was about 62% and 54% at flowering and 81% and 65% at veraison, respectively. At both phenological stages, the diurnal pattern of Pn, g_s, and ψ were positively correlated with PAR. Leaf transpiration rate was significantly reduced by shading in the early morning during flowering and in the early afternoon during veraison, whereas the calculated C_i was unaffected. All of these modifications that occurred under shading are associated with a decrease in specific leaf dry weight, leaf soluble carbohydrates and starch content, vine yield, total soluble solids in the berries, total leaf area per vine, number of axillary shoots per cane, and winter pruning weight. The increase of φ_i, leaf chlorophyll content, and the change in the growth habit to a more open canopy increased the PAR trapping efficiency, which indicates an adaptability of the grapevine to low light intensity. Vine yield and berry quality decreased linearly with increasing shade intensity. A profitable management strategy is, therefore, necessary in order to assure that most of the leaves receive approximately 700 to 900 µmol m^-2 S^-1 of PAR for the greater part of the day during the entire crop cycle. All the factors that could modify the light availability at the canopy level during the growing season, such as different vineyard exposure, cloudiness, windbreak and horizontal hail netting presence, tall trellis system, and excessive shoot vigor, must be correctly evaluated in order to reduce the risks of low photosynthetic activity, low vine yield, and poorer grape quality.

Photoinduced Electron Transfer between Sulfur-Containing Carboxylic Acids and the 4-Carboxybenzophenone Triplet State in Aqueous Solution

The mechanism of photoinduced electron transfer was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of COO[sup [minus]] groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) x 10[sup 9]M[sup [minus]1] s[sup [minus]1] depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional absorption band at approximately 410 nm in the transient absorption spectra for some of the acids was assigned to intramolecularly (S O)-bonded species (for acids for which a five-member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of CO[sub 2] formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is the main photochemical pathway. 29 refs., 4 figs., 3 tabs.

Photochemistry of formic acid monomer at 222 nm

AbstractThe yields of CO2 and CO formed from the gas‐phase photolysis at 222 nm of very low pressures of formic acid where the monomer predominates have been determined using FTIR spectroscopy. The observed ratio of CO2/CO approaches unity as the formic acid pressure approaches zero. Previous studies have shown that the quantum yield for HCOOH + hv → OH + HCO (or H + CO) is 0.70 at 298 K. The present experimental results indicate that the ratio of the quantum yields of the possible molecular photolysis channels forming H2 + CO2 (ϕ1b) and H2O + CO (ϕ1c) is ca. 1. Including this result in an analysis of previously reported quantum yields of CO and CO2 determined at higher pressures of formic acid, where both monomer and dimer contribute significantly to the products, indicates that ϕ1b = ϕ1c = 0. © 1994 John Wiley & Sons, Inc.

Photochemical Behaviour of [Co(NH 3 ) 5 HC 2 O 4 ] 2+ in Microheterogeneous Media

In the present work a comparative study of the photochemical behaviour (steady-state photolysis) of oxalatopentaamminecobalt (III) perchlorate, [Co(NH 3 ) 5 HC 2 O 4 ](ClO 4 ) 2 , in pure aqueous as well as in premicellar and micellar solutions of three different (anionic, cationic and non-ionic) surfactants has been undertaken. The photochemical reactivity of the complex has been found to charge in the following respects. (1) The photoaquation of ammonia does not take place in micellar solutions of all the three surfactants used. (2) The quantum yield of Co 2+ (Φ Co 2+) changes as the concentration of surfactant changes with a concomitant change in the structure of the solution. The manner in which Φ Co 2+ varies depends on the nature of the surfactant. The CMC of a surfactant can be determined from the plot of Φ Co 2+ vs. -log [Surfactant]. (3) The effect of [CH 3 COO - ] variation on Φ Co 2+ is more prominent in micellar solutions.

Photosynthetic productivity of an immature maize crop: changes in quantum yield of CO2 assimilation, conversion efficiency and thylakoid proteins

Abstract. The effect of growth temperatures on quantum yield (φ) was examined for leaves at different stages of development within the immature canopies of two crops of field grown maize (Zea mays cv. LG11) sown on 3 May and 20 June 1990. During the period of 23 to 49d after sowing, the crop sown on the 3 May experienced temperatures below 10°C on 19 occasions compared with only two for the crop sown on 20 June. A period of severe chilling at the end of May and the beginning of June was associated with a marked reduction in φ for all leaves in the early‐sown crop. This chill‐induced depression in φ was greater in recently emerged than more mature leaves in the canopy and was found to be accompanied by modifications in the polypeptide profiles of thylakoids isolated from the leaves. During the chilling period, decreases in some polypeptides, notably in the range of 41–42 and 20kDa apparent molecular size, and increases of polypeptides of c. 15–16kDa were observed compared with leaves developing at warmer temperatures in July. The efficiency of converting intercepted radiation into dry matter (conversion efficiency) was 42% lower in the early‐ than late‐sown crop, but no significant relationship between conversion efficiency and quantum yield was found in either treatment.

Effect of the Long-Term Elevation of CO2 Concentration in the Field on the Quantum Yield of Photosynthesis of the C3 Sedge, Scirpus olneyi

CO(2) concentration was elevated throughout 3 years around stands of the C(3) sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO(2) atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO(2) uptake (ø(abs) and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO(2) concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. An Ulbricht sphere cuvette incorporated into an open gas exchange system was used to determine ø(abs) and a portable chlorophyll fluorimeter was used to estimate the photochemical efficiency of photosystem II. When measured in an atmosphere with 10 millimoles per mole O(2) to suppress photorespiration, shoots showed a ø(abs) of 0.093 +/- 0.003, with no statistically significant difference between shoots grown in elevated or control CO(2) concentrations. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO(2) atmosphere. Shoots grown and measured in 680 micromoles per mole of CO(2) in air showed a ø(abs) of 0.078 +/- 0.004 compared with 0.065 +/- 0.003 for leaves grown and measured in 351 micromoles per mole CO(2) in air; a highly significant increase. In accordance with the change in ø(abs), the light compensation point of photosynthesis decreased from 51 +/- 3 to 31 +/- 3 micro-moles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO(2) in air, respectively. The results suggest that even after 3 years of growth in elevated CO(2), there is no evidence of acclimation in capacity for photosynthesis under light-limited conditions which would counteract the stimulation of photosynthetic CO(2) uptake otherwise expected through decreased photorespiration.

The Sequence of Change within the Photosynthetic Apparatus of Wheat following Short-Term Exposure to Ozone

The basis of inhibition of photosynthesis by single acute O(3) exposures was investigated in vivo using analyses based on leaf gas exchange measurements. The fully expanded second leaves of wheat plants (Triticum aestivum L. cv Avalon) were fumigated with either 200 or 400 nanomoles per mole O(3) for between 4 and 16 hours. This reduced significantly the light-saturated rate of CO(2) uptake and was accompanied by a parallel decrease in stomatal conductance. However, the stomatal limitation, estimated from the relationship between CO(2) uptake and the internal CO(2) concentration, only increased significantly during the first 8 hours of exposure to 400 nanomoles per mole O(3); no significant increase occurred for any of the other treatments. Analysis of the response of CO(2) uptake to the internal CO(2) concentration implied that the predominant factor responsible for the reduction in light-saturated CO(2) uptake was a decrease in the efficiency of carboxylation. This was 58 and 21% of the control value after 16 hours at 200 and 400 nanomoles per mole O(3), respectively. At saturating concentrations of CO(2), photosynthesis was inhibited by no more than 22% after 16 hours, indicating that the capacity for regeneration of ribulose bisphosphate was less susceptible to O(3). Ozone fumigations also had a less pronounced effect on light-limited photosynthesis. The maximum quantum yield of CO(2) uptake and the quantum yield of oxygen evolution showed no significant decline after 16 hours with 200 nanomoles per mole O(3), requiring 8 hours at 400 nanomoles per mole O(3) before a significant reduction occurred. The photochemical efficiency of photosystem II estimated from the ratio of variable to maximum chlorophyll fluorescence and the atrazine-binding capacity of isolated thylakoids demonstrated that photochemical reactions were not responsible for the initial inhibition of CO(2) uptake. The results suggest that the apparent carboxylation efficiency appears to be the initial cause of decline in photosynthesis in vivo following acute O(3) fumigation.

Adaptation of the Photosynthetic Apparatus in Maize Leaves as a Result of Nitrogen Limitation

In maize (Zea mays L., cv Contessa), nitrogen (NO(3) (-)) limitation resulted in a reduction in shoot growth and photosynthetic capacity and in an increase in the leaf zeaxanthin contents. Nitrogen deficiency had only a small effect on the quantum yield of CO(2) assimilation but a large effect on the light-saturated rate of photosynthesis. Linear relationships persisted between the quantum yield of CO(2) assimilation and that of photosystem II photochemistry in all circumstances. At high irradiances, large differences in photochemical quenching and nonphotochemical quenching of Chl a fluorescence as well as the ratio of variable to maximal fluorescence (Fv/Fm) were apparent between nitrogen-deficient plants and nitrogen-replete controls, whereas at low irradiances these parameters were comparable in all plants. Light intensity-dependent increases in nonphotochemical quenching were greatest in nitrogen-deficient plants as were the decreases in Fv/Fm ratio. In nitrogen-deficient plants, photochemical quenching decreased with increasing irradiance but remained higher than in controls at high irradiances. Thermal dissipative processes were enhanced as a result of nitrogen deficiency (nonphotochemical quenching was elevated and Fv/Fm was lowered) allowing PSII to remain relatively oxidised even when carbon metabolism was limited via nitrogen limitation.

The photolysis of CO at 193 nm in the liquid and gas phases

The photolysis of CO at 193 nm, which excites the v′ = 2 level of the ã( 3Π) state, has been studied in the liquid phase at 77 K and the gas phase at room temperature. Products observed were CO 2 and C 3O 2, with quantum yields of CO 2 of about 10 −2 and 10 −3 in the gas and the liquid respectively. Quantum yields of C 3O 2 were about 1% of the CO 2 in both phases. Ozone was also a product and was shown to have been derived from traces (20 – 30 ppm) of O 2 present in the CO. Mechanisms of product formation are discussed.

The photo-oxidation of i-C 3H 7CHO vapour

The steady state photolysis of iso-butyraldehyde ( i-C 3H 7CHO) was studied in the presence of O 2 at 263 and 294 K at several incident wavelengths. The quantum yields of CO and C 3H 8 were measured. From them the primary quantum yield Φ(C 3H 8) of the molecular process to produce CO + C 3H 8 and the primary quantum yield Φ(rad) of the free-radical process to produce i-C 3H 7 + HCO were deduced. Likewise the flash photolysis of i-C 3H 7CHO was studied in the presence of air at 298 K. The transient absorption of RO 2 radicals was monitored, and relative quantum yields were obtained with 284.0, 302.5, 311.7, 325.0 and 330.5 nm incident radiation. The quantum yields were not pressure quenched, except for very slightly at 330.5 nm. They followed the same trends with incident wavelength as seen in the steady state photolysis. The mechanism describing the primary process is ▪ where R represents i-C 3H 7, A represents the aldehyde, the superscripts 1 and 3 represent excited singlet and triplet states respectively, the subscripts n and 0 represent excited and ground vibrational levels respectively and M represents O 2 and N 2. k 2a/ k 2 = 0.43 and k 2a/ k 2 = 0.31 at 253.7 nm and 280.3 nm respectively and k 2a/ k 2 = 0 at 302.2 nm and longer wavelengths. The quenching of 3A n by O 2 or N 2 is quite inefficient, the half-pressure for quenching increasing with temperature and incident energy. Under all conditions studied it is 1200 Torr or greater. However, quenching of 3A n by i-C 3H 7CHO is quite efficient with k 5/ k 8a = 32.1 ± 10.0 Torr at 312.8 nm and 294 K. The quenching of A n is insensitive to variations in temperature or incident energy. An approximate value for k 3/ k 4 was estimated to be about 108 Torr. From the mechanism, radical quantum yields could be estimated at atmospheric pressure at several wavelengths. These lead to atmospheric photodissociation coefficients for radical formation at 298 K of 7.6 × 10 −5 s −1 and 5.9 × 10 −5 s −1 for solar zenith angles of 30° and 58° respectively.

Comparison of Photosynthetic Performance in Triazine-Resistant and Susceptible Biotypes of Amaranthus hybridus

The rate of CO(2) reduction in the S-triazine-resistant biotype of smooth pigweed (Amaranthus hybridus L.) was lower at all levels of irradiance than the rate of CO(2) reduction in the susceptible biotype. The intent of this study was to determine whether or not the lower rates of CO(2) reduction are a direct consequence of the same factors which confer triazine resistance. The quantum yield of CO(2) reduction was 23 +/- 2% lower in the resistant biotype of pigweed and the resistant biotype of pigweed had about 25% fewer active photosystem II centers on both a chlorophyll and leaf area basis. This quantum inefficiency of the resistant biotype can be accounted for by a decrease in the equilibrium constant between the primary and secondary quinone acceptors of the photosystem II reaction centers which in turn would lead to a higher average level of reduced primary quinone acceptor in the resistant biotype. Thus, the photosystem II quantum inefficiency of the resistant biotype appears to be a direct consequence of those factors responsible for triazine resistance but a caveat to this conclusion is discussed. The effects of the quantum inefficiency of photosystem II on CO(2) reduction should be overcome at high light and therefore cannot account for the lower light-saturated rate of CO(2) reduction in the resistant biotype. Chloroplast lamellar membranes isolated from both triazine-resistant and triazine-susceptible pigweed support equivalent rates of whole chain electron transfer and these rates are sufficient to account for the rate of light-saturated CO(2) reduction. This observation shows that the slower transfer of electrons from the primary to the secondary quinone acceptor of photosystem II, a trait which is characteristic of the resistant biotype, is nevertheless still more rapid than subsequent reactions of photosynthetic CO(2) reduction. Thus, it appears that the lower rate of light-saturated CO(2) reduction of the resistant biotype is not limited by electron transfer capacity and therefore is not a direct consequence of those factors which confer triazine resistance.

Photodissociation of HCHO in air: CO and H2 quantum yields at 220 and 300 K

New data are presented for the quantum yields of CO and H2 produced in the photodecomposition of formaldehyde. The data were obtained at the two temperatures 220 and 300 K, as a function of wavelength in the spectral range 253–353 nm, and as a function of pressure at the wavelengths 339.3 and 353.1 nm. The influence of temperature occurs mainly on the quenching constants describing the pressure dependence. The results are compared with previous data and improved quantum yield curves are presented for applications in atmospheric chemistry.

Differential efficiency of photosynthetic oxygen evolution in flashing light in triazine-resistant and triazine-susceptible biotypes of Senecio vulgaris L.

Photosynthetic oxygen evolution in response to flashing light was studied in triazine-susceptible and triazine-resistant biotypes of Senecio vulgaris L. Studies were conducted to determine if the modification of the herbicide-binding site which confers s-triazine resistance also affects the oxygen-evolving system. Oxygen evolution was measured using a Joliot-type oxygen-specific electrode on broken, stroma-free chloroplasts of both biotypes. We observed abnormal patterns of oxygen evolution in resistant chloroplasts. The S′ 1 → S 2 transition is slower while the S 2 decay is faster. The S′ 2 → S 3 transition, in contrast, is slightly faster in resistant chloroplasts, while the decay of the S 3 state is the same as in susceptible chloroplasts. These altered kinetics may be due to altered Q → B (B −) electron flow in resistant chloroplasts. These results are also consistent with the hypothesis that back-reactions from the reducing (acceptor) side of Photosystem II to the oxidizing (donor) side occur with greater frequency in resistant than susceptible chloroplasts. These events are responsible for lower oxygen yield and increased ‘misses’ and ‘double hits,’ resulting in abnormal yield patterns and lower quantum yield of CO 2 fixation in resistant chloroplasts compared to the susceptible ones.

Insensitivity of Water-Oxidation and Photosystem II Activity in Tomato to Chilling Temperatures

Chilling tomato plants (Lycopersicon esculentum Mill. cv. Rutgers and cv. Floramerica) in the dark resulted in a sizable inhibition in the rate of light- and CO(2)-saturated photosynthesis. However, at low light intensity, the inhibition disappeared and the absolute quantum yield of CO(2) reduction was diminished only slightly. The quantum yield of photosystem II (PSII) electron flow was 18% lower when measured in chloroplasts isolated from chilled leaves than in chloroplasts isolated from unchilled leaves. Even though the maximum rate of PSII turnover in these chloroplasts was 12% lower subsequent to chilling, it was in all cases two or more times that required to support the light- and CO(2)-saturated rate of photosynthesis measured in the attached leaf. The concentration of active PSII centers in chloroplasts isolated from leaves either before or after chilling was determined by measurement of the products of water oxidation from a series of saturating flashes short enough to turnover the electron transport carriers only a single time. There was no significant change in the concentration of active PSII centers due to dark chilling.It was concluded that PSII activity and water oxidation capacity are not significantly impaired in tomato by chilling in the dark and therefore are not primary aspects of the inhibition of CO(2) reduction observed in attached leaves.

The photo-oxidation of propionaldehyde

C 2H 5CHO vapor was photolyzed with 313 nm radiation at 22 °C in the presence of O 2 and O 2He, O 2N 2, O 2NO and O 2 cis-2-C 4H 8 mixtures. The contents of the reaction mixture were allowed to bleed through a pinhole into a quadrupole mass spectrometer providing for continous monitoring of the C 2H 5OH and C 2H 5C(O)O 2H produced. CO, CO 2, C 2H 4 and CH 3CHO concentrations were determined by gas chromatography. From CO quantum yield determinations we found that the only photolytic process of importance is the following: ▪ The CO quantum yields also indicate that the reactive state of C 2H 5CHO is pressure quenched, and the half-quenching pressures for the various reactant gases were measured. The primary quantum yield in 1 atm of air is 0.30 ± 0.05. The data suggest that the reacting state is a vibrationally excited triplet state which consists of levels which are quenched with different efficiencies by any quenching gas. The more rapidly quenched leves account for about 40% of the total, while the less rapidly quenched levels account for about 60% of the total. Large quantum yields were measured for CO 2, C 2H 5OH and C 2H 5C(O)O 2H indicating a chain reaction initiated by abstraction of the aldehydic hydrogen of C 2H 5CHO by C 2H 5O radicals. The C 2H 4 quantum yields are dependent on the total pressure, and we believe that C 2H 4 is produced from the decomposition of vibrationally excited C 2H 5CO 3 radicals: ▪ The deactivated C 2H 5CO 3 radicals will either react with HO 2 or C 2H 5CHO or at the walls to produce C 2H 5CO 3H, or ultimately decompose to CO 2 + C 2H 5. We found that the following reactions are unimportant at 22 °C: ▪

Photolysis of a Cobalt(III) complex with a tetradentate macrocyclic (N 4 ligand. Photoredox reaction

The photolysis of CO(Me 4[14]tetraeneN 4)(OH 2) 3+ 2 in an aqueous acidic medium has been carried out with incident wavelengths at 254, 265, 313 and 365 nm. Spectrophotometric measurements in the uv, visible, ir and nmr region show that reduction of the metal centre takes place, but the macrocyclic ligand remains intact. The quantum yield of Co II (N 4) formation is small and decreases with decreasing photon energy. Addition of alcohol leads to an appreciable increase in the quantum yield, and the effect of glycerol is comparable to that shown by 2-propanol or t-butanol. The results are interpreted within the context of the radical-pair model with formation of hydroxyl radicals.

CO2 photolysis revisited

Measurements have been made of the absolute oxygen atom yields from CO2 photolysis at 1470 and 1302–1306 Å; in both cases the yield is unity. The CO generation rates have also been measured in these two wavelength regions, and they are proportional to the oxygen atom generation rates; by implication the CO quantum yields are thus also unity. The CO deficiencies observed in earlier work on this subject are probably caused by heterogeneous processes.

Selected-state photodissociation of glyoxal: Vibronic effects in the quantum yields of CO

The absolute quantum yield for the dissociation of glyoxal into carbon monoxide following excitation to seven (00, 71, 51, 81, 81 72, 21, 81 41) single vibronic levels (SVL) in the 1Au state are presented. Samples in the 1–10 torr pressure regime were studied. Significant vibronic and pressure dependencies were observed for all seven SVL examined. Dependencies arising solely from the characteristics of specific vibronic modes were found following excitation to two levels (81 and 81 41). The results are interpreted in terms of an excited-state mechanism which includes dissociation from vibrationally excited 3Au levels.