J. Phys. Chem. 1996, 100, 7849-7853
7849
Photoreactivity of Oxygen Dimers in the Ultraviolet Laura Brown and Veronica Vaida* Department of Chemistry and Biochemistry, UniVersity of Colorado, Campus Box 215, Boulder, Colorado 80309-0215 ReceiVed: September 11, 1995; In Final Form: NoVember 10, 1995X
The photochemistry of the oxygen dimer was investigated using the technique of resonance-enhanced multiphoton ionization. Ozone and oxygen atoms were produced following photolysis of oxygen dimers at wavelengths between 251 and 266 nm, which are below the energy needed to break the O2 bond at 242 nm. The production of ozone in the upper stratosphere following (O2)2 photolysis was evaluated, with e1% of the daily ozone at 40 km being produced from this mechanism.
Introduction Weak intermolecular interactions responsible for complexation have been shown to modify the photoreactivity and absorption spectra of molecular clusters.1-9 Changes in the absorption spectra of molecules as a result of complexation have been well documented. New vibronic features and enhancements of forbidden or weak transitions can be caused by the cluster-induced perturbations of the potential energy surfaces1-3 analogous to similar spectroscopic effects studied in matrices and crystals.10 Covalent bond chemistry as the result of complexation was found to occur in previous studies in our lab for (CS2)2, (OCS)2,11 (CH3I)2,2 and O3‚H2O.4,5 Specifically, the production of S2 from (CS2) 2 and (OCS) 2, and I2 from (CH3I) 2, was detected. OH production from the ozone water complex was observed at 266 and 355 nm, suggesting that such chemistry would occur in the atmosphere.4 We have studied the photochemistry of the oxygen dimer in an effort to find covalent bond chemistry in the cluster, leading to odd oxygen formation. The large amount of theoretical and experimental work on the oxygen dimer make it an attractive system to study.12-22 In addition, photochemical processes involving oxygen are of interest with regard to its reactivity in the atmosphere. The role of the oxygen cluster in modifying the absorptions of molecular oxygen has been researched extensively. For instance, the Herzberg continuum has a well-characterized pressure-sensitive absorption cross section described by the following:21,23-26
σp(λ) ) σ0(λ) + R(λ)P where σp(λ) is the total cross section. The O2 cross section extrapolated to zero pressure is σ0(λ), and the cross section involving two molecules of O2 is R(λ)P, where P is the oxygen pressure. This pressure-dependent absorption has been attributed to a short-lived oxygen dimer with a binding energy of ∆Hf ) -1.1 kcal/mol.19,27 Oxygen absorptions in the visible and infrared have the same type of pressure dependence as the Herzberg continuum. Excitations of one O2 within the cluster have been observed, as well as electronic excitations of both oxygen molecules. For example, the transition to the 1∆g(ν)1) state of a single O2 in * Author to whom correspondence should be addressed. For VV, telephone, (303) 492-8605; fax, (303) 492-5894; e-mail, vaida@ spot.colorado.edu. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)02671-2 CCC: $12.00
the cluster has been observed at 1065.2 nm.20,28 The double transition where both oxygens are excited to the 1∆g(ν)0) state is seen at 630 nm.20,28 These transitions are spin-forbidden and therefore very weak yet can be detected in the atmosphere.29 Our studies have probed the Herzberg III oxygen absorption from 251 to 266 nm which has been found to increase in intensity 3 orders of magnitude upon clustering.21 The large increase in cross section was first attributed to the oxygen dimer in 192830 and has been assigned to the (O2)2 A′ 3∆u+ X 3∑gr X 3∑g- + X 3∑g- transition.17,31-34 We have explored the photochemistry of (O2)2 following excitation of this transition and detected O3 and O as photoproducts at energies below the O2 bond dissociation energy of 242 nm. The implications of these results to oxygen chemistry in the mesosphere and upper stratosphere are discussed. Experimental Section The oxygen dimer is produced by expanding neat oxygen of 99.9% purity through a pulsed nozzle to produce a supersonic expansion. The ultrahigh-purity oxygen was received from LINDE and used without further purification. A General Valve 0.8-mm-diameter pulsed nozzle was jacketed with a copper block in thermal contact with a circulating cooling bath. The cooling bath allowed the nozzle to be cooled to ≈ -30 °C for maximum production of clusters. The concentration of (O2)2 in the expansion could be controlled by varying the stagnation pressure of oxygen behind the nozzle between 10 and 70 psi. For example, the (O2)2 concentration increased 520% when the pressure behind the nozzle was increased from 30 to 60 psi. Electron impact ionization with a quadrupole mass spectrometer was used to characterize the oxygen expansion, allowing the relative concentration of (O2)2 to be determined at the pressures and temperatures used in the experiment. The experiments were conducted in a differentially pumped vacuum system. The expansion chamber is separated from the detection chamber with a 2.01-mm-diameter skimmer. Alignment between the nozzle opening, the skimmer, and the ionization region allowed only the center of the oxygen expansion to be probed in the detection chamber. These conditions ensure a collision-free environment at the ionization region of the mass spectrometer. Typical pressures in the detection chamber were 3 × 10-6 Torr during experiments. Photoproduct studies employed the technique of resonanceenhanced multiphoton ionization, REMPI. The third harmonic of a Nd-Yag laser was used to pump a dye laser containing either Coumarin 500 or Coumarin 460. The output of the dye © 1996 American Chemical Society
7850 J. Phys. Chem., Vol. 100, No. 19, 1996
Brown and Vaida
laser was then doubled using an Inrad tracking system. This setup allowed generation of tunable ultraviolet light from 251 to 266 nm and 223 to 239 nm depending upon the dye. The fourth harmonic of the Nd-Yag laser provided 266-nm radiation, and an ArF excimer laser produced 193-nm light. The laser was brought into the chamber perpendicular to the molecular beam and focused into the ionization region of the mass spectrometer with a 25-cm focal length lens. Typical laser energies were 0.5-0.8 mJ/pulse. The laser energies were recorded during all of the experiments to correct the ion signals for fluctuations in laser power. At the wavelengths used in the experiment, one photon was needed to excite the (O2)2 molecules to the A′ 3∆u+ X 3∑gstate. Products from (O2)2 photolysis were probed using the same laser pulse. For the case of the ground-state oxygen atom, three additional photons are needed to ionize O, resulting in an overall (1 + 3) REMPI process. The REMPI spectra were obtained by collecting the desired mass signal as a function of photolysis wavelength. The laser was scanned with a typical step size of 0.5 Å and 500 laser shots per step. The ion signals were averaged over the 500 shots, to help reduce noise in the spectra. The laser was scanned through the (O2)2 band from 251 to 266 nm. Results Electron impact ionization was used to determine the relative concentration of (O2)2 at various clustering conditions. The highest yield of clusters was achieved with 70 psi of stagnation pressure and the nozzle cooled to ≈ -30 °C. The concentration of clusters showed a marked decrease as the stagnation pressure was lowered. This observation was used to track the ratio of the O+ signal to the O2+ signal following laser excitation as a function of cluster concentration. The observed ion signals in the experiment were O+, O2+, and O4+. At the expansion conditions used during these experiments, there was no mass spectral evidence for oxygen clusters larger than the dimer. Odd oxygen production from the photolysis of (O2)2 at energies below the O2 bond dissociation energy of 242 nm occurs according to the following reaction:
(O2)2 + hν f O3 + O
(1)
with MPI detection of the photoproducts. An O3+ signal was not expected because ozone efficiently dissociates at the onephoton wavelengths, precluding up-pumping and ionization of O3. The photochemistry of (O2)2 was studied following excitation to the A′ 3∆u+ X 3∑g- electronic state of the dimer. The two competing processes in this experiment are reaction 1 given above or direct ionization of (O2)2 followed by fragmentation to O+ and O2+. Several different experiments which are described below were conducted to determine the origin of the O+ photolysis product. The averaged O+ and O2+ signals were collected following excitation at 266 nm at six different stagnation pressures from 10 to 60 psi. The pressure inside the chamber was held constant at 3 × 10-6 Torr for all of the experiments. The ratio of O+ to O2+ vs stagnation pressure at 266 nm is shown as the upper line in Figure 1. As the concentration of clusters in the expansion increased, the O+/O2+ ratio also increased. Had O+ come from O2+ fragmentation, this ratio would be expected to remain constant. Due to the collision-free conditions of the supersonic expansion, the change in the O+/O2+ ratio showed that increased odd oxygen production correlates with the
Figure 1. stagnation photolysis photolysis
Ratio of the O+ and O2+ MPI signals as a function of pressure. The upper trace (O) is the signal ratio following at λ ) 266 nm, and the lower trace (b) is following at λ ) 228 nm.
concentration of oxygen dimers. Collisions between excitedand ground-state O2, which have been postulated to lead to the formation of odd oxygen in cell experiments,35 are not possible under our experimental conditions. These experiments were also conducted at two other wavelengths. At 228 nm, the O+/O2+ ratio increased 40% when the backing pressure was changed from 20 to 70 psi as shown by the lower line in Figure 1. Excitation at this wavelength is in the Herzberg continuum of oxygen. Previous investigations have also shown an increased production of ozone in the Herzberg continuum at 214 nm when conditions favored the presence of the oxygen dimer.36 The O+/O2+ ratio was observed to increase with clustering following excitation at 193 nm, although the effect was not as pronounced. In addition, a photon-dependence study of the ion signals was performed at 193 nm as a function of backing pressure. The O4+ signal was not consistently reproducible until 60 psi. At 70 psi of backing pressure, the O+ signal required one more photon to produce compared to the O4+ signal. This is consistent with the oxygen atom being produced via reaction 1 as compared to fragmentation of O4+ which would have the same photon dependence for both signals. To further confirm that the oxygen atom was a product of (O2)2 photolysis, the wavelength dependences of the O+, O2+, and O4+ signals were determined as a function of clustering. The laser was scanned over a portion of the A′ 3∆u + X -∑gr X 3∑g- + X 3∑g- transition from 251 to 266 nm. With a low stagnation pressure of 10 psi, the O4+ was not observed. A spectrum of the O+ signal at this pressure showed no dependence on wavelength. Additionally, the O2+ signal showed no dependence on wavelength or concentration of (O2)2 in the expansion. When the stagnation pressure was increased to 70 psi, the O4+ signal returned. A REMPI spectrum of O4+ is shown in Figure 2 along with the (O2)2 absorption spectrum taken by Shardanand.21 The O4+ action spectrum shown is the average of 23 wavelength scans. A comparison shows good agreement between the (O2)2 REMPI and absorption spectra. Four bands appear in both spectra centered at 263.3, 259.2, 255.4, and 251.9 nm corresponding to the ν′ ) 5, 6, 7, 8 r ν′′ ) 0 transitions in (O2)2, respectively.17 Previous high-resolution absorption work showed that each of these diffuse bands was a triplet due to transitions to the Ω ) 1, 2, and 3 sublevels of each vibration.17 This structure is not seen in the REMPI spectra, and the averaging of additional spectra did not appear to resolve any more features. The O+ signal was also collected as a function of wavelength at 70 psi. A comparison of the O+ and O4+ REMPI spectra is shown in Figure 3. The O+ spectra are an average of 50 wavelength scans. As can be seen from Figure 3, the O+ signal maps out the (O2)2 spectra in this wavelength range. As the
Photoreactivity of O2 Dimers in the UV
J. Phys. Chem., Vol. 100, No. 19, 1996 7851
Figure 2. Comparison between the (O2)2 absorption spectra taken by Shardanand21 (A) and the O4+ REMPI spectra (B).
Although the similarity between the wavelength dependence of O+ and the (O2)2 absorption is consistent with both reaction 1 and ionization of (O2)2 followed by fragmentation to produce O+, there is strong evidence that reaction 1 is significantly contributing to the O+ signal. The photon dependence of the O+ and O4+ signals combined with the small laser fluxes used in the experiment is indicative of reaction 1 occurring. The appearance of both (O2)2 and O3 absorption bands in the O+ spectrum further confirms that odd oxygen was produced by photodissociation of (O2)2 according to reaction 1. Neutral O3 needed to give rise to the O3 absorptions in the O+ spectrum can only be produced in the experiment through reaction 1. On the basis of the stagnation pressure, wavelength, and photon dependence of odd oxygen signals discussed above, we conclude that photolysis of (O2)2 leads to O and O3 in the wavelength range 251-266 nm. Discussion
Figure 3. O4+ and O+ REMPI spectra. The O4+ spectra (A) are the average of 23 wavelength scans. The O+ spectra (B) are the average of 50 wavelength scans. The diamonds ([) indicate positions of the O3 absorption resonances in the Hartley band.
laser wavelength came into resonance with the (O2)2 absorption band, more oxygen atoms were produced according to reaction 1, leading to the observed wavelength dependence of the O+ signal intensity. The presence of (O2)2 resonances confirms that oxygen atoms were produced from (O2)2 photolysis. The oxygen atom could also be coming from dissociation of ozone according to the following:
O3 + hν f O + O2
(2)
At the wavelengths used in the experiment, ozone produced from (O2)2 photolysis would absorb light in the Hartley band and quickly dissociate. The increased width of the band centered at 259.2 nm in the O+ spectrum compared to the O4+ spectra could be due to the presence of O3 absorptions. The structure on the top of the Hartley band at 257.2, 258.7, 260.7, and 262.4 nm37 may be enhancing the production of oxygen atoms in this range and accounting for the differences. Other ozone absorption features in the Hartley band that would lead to an increased O+ signal fall within the (O2)2 absorption bands and could thus be obscured. The increased noise in the O+spectra compared to the O4+ spectra can be attributed to several factors including reaction dynamics. The appearance of the O4+ signal indicates that the A′ 3∆u+ X -∑g- state is relatively long lived, and therefore, there is a competition between dissociation of the cluster and up-pumping to the ionization continuum. The quantum yield for dissociation to odd oxygen is therefore less than one. The ionization process for (O2)2 also has a lower photon dependence, (1 + 2) REMPI process, which serves to increase the ion signal compared to the (1 + 3) photons needed for formation of O+.
The structure of (O2)2 has been investigated by a number of studies.12-15,19,20,27 The infrared and visible spectrum of (O2)2 in a neon matrix is consistent with a ground-state, X 3∑g- + X 3∑g-, structure of a rectangle with D2h symmetry and an intermolecule bond distance of 3.41 Å.15,20 In addition, these experiments showed that the 1∆g + 1∆g excited state of (O2)2 had the same D2h geometry. Several theoretical studies have examined the gas-phase structure of the oxygen dimer. Ab initio studies of Bussery, Wormer, and van der Avoird all predict that the absolute minimum potential surface of (O2)2 is either a singlet or a triplet state with a rectangular D2h geometry.12-14 Previous experimental results of the magnetic deflection of (O2)2 prepared in a molecular beam showed that the ground state is paramagnetic, suggesting the triplet.22 In addition, recent ab initio work on the 1∆g + 1∆g excited state agrees with experiments in predicting a similar geometry as the ground state.12 However, ab initio studies by Slanina et al. found that a quintet linear state was lowest in energy.38 In their calculations, the D2h geometry exhibited imaginary vibrational frequencies and therefore was excluded from further consideration. Temperature-dependent studies of the dimer absorption at 6 µm have found a bond energy for (O2)2 of ∆Hf ) -1.1 ( 0.5 kcal/mol.27 This is consistent with the infrared work of Long and Ewing that gave ∆Ef ) -530 cal/mol.19,20 Using these values, a calculation of ∆Hrxn shows that to break an O-O bond according to reaction 1 is thermodynamically possible for light λ e 308 nm. In contrast, only 385 cm-1 (1.1 kcal/mol) is needed to break the weak intermolecular bond of the dimer. Since the present studies probe the photoreactivity from 251 to 266 nm, reaction 1 is thermodynamically feasible at these wavelengths. We performed additional experiments that attempted to photolyze (O2)2 at 532 nm, corresponding to the 1∆g + 1∆g (ν ) 2) state28 of (O2)2, and did not see any photolysis products. We have investigated such covalent bond reactions for several molecular dimers which offer a precedent for reaction 1.2,4,5,11 The (O2)2 absorption bands in the wavelength region studied in our experiments were described as a weak set of diffuse triplets by Finkelnburg31,32 and have since been assigned to the A′ 3∆u+ X 3∑g- r X 3∑g- + X 3∑g- transition of the oxygen dimer.34 The wavelength region of this absorption band extends from 280 to 200 nm where it becomes buried under the more intense Herzberg continuum.39 The intensity of the transition depends on the square of the oxygen pressure, indicative of a dimer.33 In comparing the transition in free O2 and (O2)2, Coquart and Ramsey17 found that the frequencies of the (O2)2 triplets had systematic differences from the subband origins of
7852 J. Phys. Chem., Vol. 100, No. 19, 1996 O2. The small shifts between the O2 spectrum and the diffuse triplets further confirm the absorption as that of (O2)2. The role of the oxygen dimer in enhancing the absorption cross section of oxygen has been extensively studied.23-26,40,41 In the case of the transition probed in the above experiments, the cross section of the corresponding transition in the free O2 molecule, A′ 3∆u r X 3∑g- called the Herzberg III band, is 10-24 cm2.21,39 This cross section is 3 orders of magnitude smaller than the value of 10-21 cm2 measured for the dimer by Shardanand.21 The presence of nitrogen and argon also increases the absorption cross section of O2 in this band.42,43 The increased intensity is the result of a relaxation of the angular momentum selection rule, ∆Λ ) 0, ( 1, from free O2 due to the perturbing presence of an adjoining oxygen.44 The effect of the adjoining oxygen has been seen in several other forbidden electronic states of oxygen.18,23-28,42 Most notably, the oxygen dimer has a large effect on the Herzberg continuum.18,23-26,42 Absorption of O2 from 195 to 240 nm is the primary source of oxygen atoms in the stratosphere and lower mesosphere. As a result, the absorption cross section of O2 in the Herzberg continuum has been studied in an effort to remove the contribution from (O2)2. Our results are consistent with earlier experiments of the photolysis of oxygen at 248 nm which was found to generate ozone.45 Slanger et al. reported ozone production via two different mechanisms following oxygen photolysis at 248 nm. The better understood autocatalytic mechanism involves dissociation of O3, from photolysis in the Hartley band, to form a highly vibrationally excited O2 (3Σg-, ν ) 26) and O(3P).46 The excited oxygen is able to react with ground-state O2, producing O3 and O(3P), eventually leading to a net formation of two ozone molecules. The initial formation of ozone is not as well understood and has been attributed to several sources. The photolysis of vibrationally excited ground state O2 (3Σg-, ν ) 1) at 248 nm would have enough energy to break the oxygen bond although the population in ν ) 1 would be small at room temperature.47 This mechanism would not explain our data as the population of oxygen molecules in ν ) 1 would be extremely small in our cooled molecular beam. Additionally, the reaction between electronically excited oxygen and ground state oxygen in a cell experiment has been studied by Shi and Barker.35 Oxygen excited to one of the Herzberg bands, A 3Σu+, A′ 3∆u or c 1Σu-, at λ ) 248 nm was found to react with ground-state O2, producing O3 and O, even though there was not enough energy present to break the O2 bond. Due to the fast chemical reaction, a low activation barrier for this reaction was estimated. Since wavelength resolution of this process was not available, the actual excited state of O2 involved could not be determined. As this reaction is analogous to eq 1 studied above, our results can shed some light on the process. From the wavelength dependence of the oxygen atom photodissociation product in our experiment, the excited state is clearly identified as the A′ 3∆u state. Shi and Barker ruled out contribution from the cluster since the reaction cross section increased with N2 and Ar as well as O2. However, since the A′ 3∆u is the excited state involved, all three of these gases have been seen to dramatically increase the absorption into this state.21,33 In the case of our experiments , the adjoining O2 is responsible both for an increase in the absorption cross section and for providing the reactive partner. The molecular beam ensures a collision-free environment at the ionization region which would prevent reactions between two separated oxygen molecules. The cluster is therefore necessary not only for absorption of light at these wavelengths but also to bring both participants of the reaction together. From the results presented
Brown and Vaida above, we suggest that the initial formation of ozone in the Slanger experiments could also be the result of (O2)2 photolysis. Previous investigations have shown that the presence of (O2)2 enhances the production of ozone in the Herzberg continuum.18,36 The 214-nm photolysis of oxygen at room temperature was studied in a cell at pressures from 380 to 1300 Torr. The formation of ozone was found to scale with the square of the oxygen pressure, indicating that (O2)2 played an important role. The increased ozone is believed to be the result of the larger absorption cross section of oxygen23-26 at high pressures due to contributions from (O2)2. Our results at 228 nm agree that increased odd oxygen is produced, specifically that photolysis of (O2)2 directly produces odd oxygen in competition with breaking the weak cluster bond. The oxygen dimer is found in the atmosphere, and in the troposphere, (O2)2 absorption bands have been detected spectroscopically.29,48 The fundamental vibration-rotation band of (O2)2 at 6 µm is very weak;27 however, the long path lengths available in the atmosphere can lead to appreciable absorption.29 These bands have been studied in the laboratory in an attempt to remove the atmospheric (O2)2 absorption from measured field spectra. We have evaluated the contribution that (O2)2 may have to the production of oxygen atoms in the upper atmosphere, as the results shown here suggest that photolysis of (O2)2 could provide a source not previously considered. The major advantage of odd oxygen production from the dimer over free oxygen is that the wavelength range available for photolysis in the atmosphere is now extended from 242 nm to the end of the A′ 3∆u+ X -∑g- transition at 280 nm.21 In addition, the dimer has a larger absorption cross section in the Herzberg continuum compared to O2. The concentration of (O2)2 in the atmosphere was calculated previously using second virial coefficients and varies from a number density of 105 molecules/cm3 at 80 km to 109 molecules/cm3 at 50 km.49 The following steady-state equations were used to evaluate the production of odd oxygen from 80 to 40 km:
POx ) 2∑Kp[(O2)2]
(3)
Kp ) ∑σ(λ)φ(λ)J(λ)
(4)
alt
λ
In the above equations, POx is the total production of ozone, Kp is the photolysis rate, [(O2)2] is the concentration of the dimer as a function of altitude, and J(λ) is the incident solar flux. The absorption cross section of (O2)2 , σ(λ), measured by Shardanand21 was used in the calculation. A quantum yield, φ(λ), of unity for reaction 1 was used to calculate the best case scenario. The solar flux was corrected for attenuation by absorbers in the atmosphere at altitudes below 50 km.50,51 The factor of 2 in the expression for POx comes from the assumption that the oxygen atom produced will react with an oxygen molecule resulting in the net formation of two ozone molecules per (O2)2 photolyzed. Using the above equations, the rate of odd oxygen production peaks at 12 × 104 molecules/(cm3 s) at 40 km and quickly drops to 0.5 × 104 molecules/(cm3s) at 55 km. These values are overestimates, as the quantum yield for this mechanism is most likely much less than one. The ozone produced from the above mechanism could become incorporated in the autocatalytic ozone cycle mentioned previously,46 which would serve to amplify the amount of odd oxygen produced. Comparing this proposed mechanism with the conventional mechanism for ozone production, photolysis of oxygen in the Herzberg continuum, shows that the dimer could account for
Photoreactivity of O2 Dimers in the UV at most ≈1% of the ozone production at 40 km.52 At higher altitudes, the contribution would drop dramatically to only 0.01% at 55 km. The very small contribution to ozone production is the result of the combination of a small (O2)2 concentration at high altitudes and the altitude dependence of the available light. The concentration of (O2)2 is a function of pressure and temperature. The lower temperature in the mesosphere and upper stratosphere, ≈270-220 K, is not enough to compensate for the large decrease in pressure. Lower altitudes, where the concentration of (O2)2 is much higher, is below the ozone layer, and light of λ ) 251-280 nm is no longer available for reaction 1 to occur. Conclusions Resonance-enhanced multiphoton ionization was used to study the photochemistry of (O2)2 following excitation of the A′ 3∆u+ X 3∑g- transition. The action spectrum of (O2)2 from 251 to 266 nm agreed very well with previous absorption measurements.21 The action spectrum of the oxygen atom product also showed very good agreement with the (O2)2 spectrum with the exception of an increased width of the band centered at 259.2 nm. The difference between the two spectra is the result of oxygen atoms produced in the photodissociation of ozone, which exhibit the absorption features of the Hartley band of ozone. Pressure-dependent studies at 266, 228, and 193 nm confirm that more oxygen atoms are produced during photolysis when there is an increased concentration of (O2)2 in the expansion. A photon-dependence study of the O+ and O4+ signals indicates that the two ions have different photon dependencies, as expected for reaction 1. From these results, we conclude that new covalent bond chemistry is occurring with the production of O and O3 following (O2) 2 photolysis at energies below the O2 bond dissociation energy. The atmospheric relevance of this mechanism was investigated with a simple steady-state model. Assuming a quantum yield of 1 for odd oxygen production, photolysis of (O2)2 could account for less then 1% of the ozone production at 40 km. The effect would decrease drastically with altitude in correspondence with the altitude dependence of the (O2)2 concentration. Acknowledgment. This work was funded in part by the NSF. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. L.A.B. thanks the NSF for an Atmospheric Training Grant Fellowship. References and Notes (1) Vaida, V.; Donaldson, D. J.; Sapers, S. P.; Naamam, R.; Child, M. S. J. Phys. Chem. 1989, 93, 513. (2) Donaldson, D. J.; Vaida, V.; Naaman, R. J. Phys. Chem. 1988, 92, 1204. (3) Donaldson, D. J.; Gaines, G. A.; Vaida, V. J. Phys. Chem. 1988, 92, 2766. (4) Frost, G. J.; Vaida, V. J. Geophys. Res. 1995, 100, 18803. (5) Vaida, V.; Frost, G. J.; Brown, L. A.; Naaman, R.; Hurwitz, Y. Ber. Gunsen-Ges. Phys. Chem. 1995, 99, 371.
J. Phys. Chem., Vol. 100, No. 19, 1996 7853 (6) Van Orden, A.; Provencal, R. A.; Giesen, T. F.; Saykally, R. J. Chem. Phys. Lett. 1995, 237, 77. (7) Buck, U.; Schmidt, B. J. Chem. Phys. 1995, 101, 6365. (8) Fan, Y. B.; Randall, K. L.; Donaldson, D. J. J. Chem. Phys. 1994, 98, 4700. (9) Young, M. A. J. Phys. Chem. 1995, 98, 7790. (10) Robinson, G. W. J. Chem. Phys. 1967, 46, 572. (11) Prinslow, D. A.; Vaida, V. J. Phys. Chem. 1989, 93, 1836. (12) Bussery, B. Chem. Phys. 1993, 184, 29. (13) Bussery, B.; Wormer, P. E. S. J. Chem. Phys. 1993, 99, 1230. (14) Bussery, B.; Umanskii, S. Y.; Aubert-Frecon, M.; Bouty, O. J. Chem. Phys. 1994, 101, 416. (15) Goodman, J.; Brus, L. E. J. Chem. Phys. 1977, 67, 4398. (16) Goodman, J.; Brus, L. E. J. Chem. Phys. 1977, 67, 4408. (17) Coquart, B.; Ramsey, D. A. Can. J. Phys. 1985, 64, 726. (18) Horowitz, A.; Schneider, W.; Moortgat, G. K. J. Phys. Chem. 1989, 93, 7859. (19) Long, C. A.; Ewing, G. E. Chem. Phys. Lett. 1971, 9, 225. (20) Long, C. A.; Ewing, G. E. J. Chem. Phys. 1973, 58, 4824. (21) Shardanand, Phys. ReV. 1969, 186. (22) Van Deursen, A.; Reuss, J. Int. J. Mass. Spectrom. Ion Phys. 1977, 23, 109. (23) Johnston, H. S.; Paige, M.; Yao, F. J. Geophys. Res. 1984, 89, 11661. (24) Cheung, A. S. C.; Yoshino, K.; Parkinson, W. H.; Guberman, S. L.; Freeman, D. E. Plan. Space Sci. 1986, 34, 1007. (25) Hasson, V.; Nicholls, R. W. J. Phys. B: Atom Molec. Phys. 1971, 4, 1778. (26) Shardanand; Prasad Rao, A. D. J. Quant. Spectrosc. Radiat. Transfer 1977, 17, 433. (27) Orlando, J. J.; Tyndall, G. S.; Nickerson, K. E.; Calvert, J. G. J. Geophys. Res. 1991, 96, 20755. (28) Greenblatt, G. D.; Orlando, J. J.; Burkholder, J. B.; Ravishankara, A. R. J. Geophys. Res. 1990, 95, 18577. (29) Rinsland, C. P.; Smith, M. A. H.; Seals, R. K., Jr.; Goldman, A.; Murcray, F. J.; Murcray, D. G.; Larsen, J. C.; Rarig, P. L. J. Geophys. Res. 1982, 87, 3119. (30) Wulf, O. R. Proc. Natl. Acad. Sci. U.S.A. 1928, 14, 609. (31) Finkelnburg, V. W. Z. Phys. 1934, 90, 1. (32) Finkelnburg, W.; Steiner, W. Z. Phys. 1932, 79, 69. (33) Dianov-Klokov, V. I. Opt. Spectrosc. 1965, 21, 233. (34) Herzberg, G. Can. J. Phys. 1953, 31, 657. (35) Shi, J.; Barker, J. R. J. Geophys. Res. 1992, 97, 13039. (36) Horowitz, A.; von Heldon, G.; Schneider, W.; Simon, F. G.; Crutzen, P. J.; Moortgat, G. K. J. Phys. Chem. 1988, 92, 4956. (37) Joens, J. A. J. Chem. Phys. 1994, 5, 3407. (38) Uhlik, F.; Slanina, Z.; Hinchliffe, A. THEOCHEM 1993, 285, 273. (39) Slanger, T. G.; Cosby, P. C. J. Phys. Chem. 1988, 92, 267. (40) McKellar, A. R. W.; Rich, N. H.; Welsh, H. L. Can. J. Phys. 1972, 50, 1. (41) Saxon, R. P.; Slanger, T. G. J. Geophys. Res. 1986, 91, 9877. (42) Shardanand, A. J. Quant. Spectrosc. Radiat. Transfer 1977, 18, 525. (43) Shardanand, A. J. Quant. Spectrosc. Radiat. Transfer 1978, 20, 265. (44) Blake, A. J.; McCoy, D. G. J. Quant. Spectrosc. Radiat. Transfer 1987, 38, 113. (45) Slanger, T. G.; Jusinski, L. E.; Black, G.; Gadd, G. E. Science 1988, 241, 945. (46) Miller, R. L.; Suits, A. G.; Houston, P. L.; Toumi, R.; Mack, J. A.; Wodtke, A. M. Science 1994, 265, 1831. (47) Freeman, D. E.; Yoshino, K.; Parkinson, W. H. Science 1990, 250, 1432. (48) Perner, D.; Platt, U. Geophys. Res. Lett. 1980, 7, 1053. (49) Calo, J. M.; Narcisi, R. S. Geophys. Res. Lett. 1980, 7, 289. (50) Brasseur, G.; Simon, P. C. J. Geophys. Res. 1981, 86, 7343. (51) Meier, R. R.; Anderson, D. E., Jr.; Nicolet, M. Plan. Space Sci. 1982, 30, 923. (52) Brasseur, G.; Solomon, S. Aeronomy of the Middle Atmosphere; D. Reidel: Dordrecht, 1986.
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