J. Phys. Chem. A 2010, 114, 9709–9719
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Isotope Evidence for Ozone Formation on Surfaces† Christof Janssen*,‡,§,| and Be´la Tuzson|,⊥ Laboratoire de Physique Mole´culaire pour l’Atmosphe`re et l’Astrophysique, UniVersite´ Pierre et Marie Curie, case 76, 4 place Jussieu, 75252 Paris Cedex 05, France, Centre National de la Recherche Scientifique, LPMAA, France, and Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for ¨ berlandstrasse 129, 8600 Du¨bendorf, Switzerland Air Pollution and EnVironmental Technology, U ReceiVed: February 27, 2010; ReVised Manuscript ReceiVed: April 16, 2010
Ozone formation in the gas phase is associated with a large and unusual isotope effect of widespread use in geochemistry and climate research. Little is known whether similar nonstandard mass dependent fractionations also occur in other recombination reactions. Here we report on the pressure and temperature dependence of the isotopic composition of ozone formed by electric discharge in molecular oxygen. Isotope signatures at low pressures show a standard mass dependent depletion, their magnitudes strongly depending on temperature. Our analysis confirms the formation of ozone at Pyrex reactor walls with an atom recombination coefficient γ ) (0.4 ( 0.1)% at room temperature and slightly higher values at lower temperatures. Thus, although neglected so far, wall assisted ozone formation is an essential part of oxygen plasma chemistry and it could also provide a mechanism explaining the presence of ozone on icy satellites. Recombination reactions on the surface are not likely to show the isotope anomalies associated with ozone formation in the gas phase. 1. Introduction Surface-assisted formation of ozone (O3) has long been reported,1,2 but direct observations or quantitative treatments of the formation mechanism are still lacking. Ozone formation at the gas-solid interface could possibly explain abiotic ozone observed on solar system icy satellites.3 Nevertheless, the reaction is generally not considered in the literature and, in particular, it seems to not be accounted for in the studies investigating the chemistry of oxygen atoms at surfaces and in molecular oxygen discharges.4,5 Given the great interest in understanding these processes due to plasma surface treatments having a wide range of applications in semiconductor, medical, and biomedical technologies,6 this is somewhat astonishing. In the rare cases that ozone wall formation is mentioned, it is deemed unimportant for the heterogeneous chemistry of atomic oxygen.7 This is despite the fact that atomic oxygen appears to be efficiently converted into ozone at reactor walls. Nekrasov et al.1 claim a conversion efficiency of about 100% at liquid nitrogen temperatures, which seems to be in striking contrast to recent interpretations of actinometric studies where heterogeneous ozone formation is not taken into account and where surface recombination coefficients for atom loss of 0.5% or less on Pyrex walls are inferred for temperatures between 80 and 300 K.8 Plasma discharge studies published so far have almost exclusively focused on the measurement and modeling of lifetimes and concentrations of the species (radicals, atoms, ions) relevant for plasma chemistry and physics. The additional information contained in the isotopic composition of these species has not yet been explored, although the kinetic scheme of a discharge model may include over a hundred of reactions. †
Part of the “Reinhard Schinke Festschrift”. * To whom correspondence should be addressed: E-mail: janssen@ lpmaa.jussieu.fr. Phone: +33(0)144279672. Fax: +33(0)144277033. ‡ Universite´ Pierre et Marie Curie. § Centre National de la Recherche Scientifique. | Experiments were carried out when the authors were at the Max-PlanckInstitut fu¨r Kernphysik, Heidelberg, Germany. ⊥ Empa.
Remarkably, the gas-phase recombination or association reaction of ozone
O + O2 + M f O3 + M
(1)
has an unusually large (∼10%) isotope effect and shows almost equal enrichments of the ozone molecules containing two different oxygen isotopes. The phenomenon, often denoted as the mass independent isotope effect, has been the subject of several recent reviews9-12 and still lacks full understanding on the quantum chemical level.13-15 Isotope enrichments and depletions E(X) in ozone are defined as relative deviation between the relative content of a heavy ozone isotopomer16 X (expressed as [X]/[16O3]) in a sample and that in a standard:10,11
E(X) )
([X]/[16O3])sample ([X]/[16O3])stat
-1
(2)
The heavy isotopomer content of the standard (stat) is calculated from the atomic isotope abundance in molecular oxygen (O2) assuming that ozone is formed without preference for any molecular combination (i.e., statistically). When the heavy isotopes are rare in abundance and the distribution of heavy isotopes within the different ozone isotopomers (e.g., 16O18O16O and 16O16O18O) is not relevant, one may equally employ the δ-notation, which in this case becomes equivalent to the above definition of isotope enrichments, i.e., δ18O ) E(16O218O) ) E(16O16O18O + 16O18O16O) and δ17O ) E(16O217O) ) E(16O16O17O + 16O17O16O).11 In this paper both notations will be used interchangeably. Isotope effects in reaction 1 show a marked pressure dependence.17,18 Large enrichment values E(16O218O) J 10% were observed in reactor chambers with a typical length scale l J 16 cm for pressures between 3 and 100 Torr. At pressures > 50 Torr enrichment values start to decrease. At low pressures,
10.1021/jp1017899 2010 American Chemical Society Published on Web 05/06/2010
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Figure 1. Oxygen discharge setup.
heavy ozone isotopomers may be depleted instead of being enriched,18-20 leading to a clear transition between two regimes of strikingly different isotopic composition (e.g., see Figure 3 further below). While isotopic compositions at high pressures evidently reflect the effect of the gas-phase reaction (eq 1), the low pressure plateaus of depletions of 18O-containing O3 with values between -7%19 and -3%20,21 are not explicable in terms of reaction 1. Therefore, a heterogeneous wall-assisted reaction
O + O2 + wall f O3 + wall
(3)
has been proposed.18,19 Yet, the elementary steps of this wall reaction have been neither specified nor identified and efforts to interpret the low pressure depletion remain unsatisfactory. In the attempt to explain depletions of 16O218O and 16O217O by -7.0% and -4.0%, respectively, Bains-Sahota and Thiemens19 suggested, inter alia, the mechanism of O atom diffusion limited ozone wall formation. They argued that diffusional velocities of 18O and 16O differ by 6%, which is about what would be required to explain the observations. This idea, however, neglects that only one-third of the 18O-containing ozone is formed from reaction of 18O with 16O2. About two-thirds form in the competing 16O + 16O18O reaction. Thus, the suggested explanation leaves room for a depletion of 16O218O by -2% at most, which falls short of the observation. Moreover, the above cited variability of depletions between -7% and -3% observed so far in different experiments19,21 remains completely unexplained. Janssen et al.21 proposed a combination of isotope effects resulting from atom diffusion and a gas-phase isotope equilibrium through O + O2 isotope exchange reactions which possibly could explain these variations but provided little detail. Therefore, the mechanism of the isotope selection in the ozone forming process at low pressure must be regarded as an open question, which does currently not allow confirming whether and at what rate ozone is formed by the heterogeneous process in eq 3. Interest in understanding the isotopic details of wall-assisted ozone formation stems from the need to accurately characterize line strengths of heteronuclear ozone molecules relevant for atmospheric research.22,23 This requires generation of ozone samples where relative amounts of different isotopomers with the same molecular mass, such as 16O18O16O and 16O16O18O, for example, are known. Unlike ozone formation in the gas
phase, the standard mass dependent character of wall-assisted ozone formation, in which isotope effects in 18O-substituted ozone are almost twice as large as effects in 17O-containing molecules, appears to make this ozone formation process particularly suitable for the preparation of such calibration mixtures. Moreover, there is the general question whether nonstandard isotope fractionation, as observed in reaction 1, occurs exclusively in gas-phase reactions or also at the surface. It has recently been suggested that surface reactions analogous to reaction 3, such as O + XO f XO2 (X ) Si, Ca, Al, ...), could show almost equal isotope fractionation in the 17O- and 18O-containing dioxides,24 which would allow us to explain the remarkable oxygen isotope composition of early solar system material.25 Experiments that could provide an answer to this fundamental question are sparse and ambiguous and even the decomposition of ozone on quartz and Pyrex does lack an isotope anomaly.26 To the best of our knowledge, only the thermal decomposition of carbonate minerals has been reported as evidence for a nongas-phase reaction producing an anomalous isotope distribution.27 The observed effects were so small (∼0.02%), however, that it is questionable whether these indeed signify a nonstandard mass dependence.28 In particular, competition between thermodynamic equilibrium and kinetic evaporation processes as observed in the evaporation of Cd-melts29 could have led to fractionation constants smaller than the reference value, thus possibly explaining the observations. In this article we present temperature dependent measurements of the isotopic composition of ozone formed in an O2 discharge at low pressures for characterizing and constraining the mechanism of heterogeneous ozone formation. On the basis of our previous proposition in ref 21, a simple kinetic model is introduced that accounts quantitatively for the observed pressure and temperature dependencies, thus firmly establishing the process of wall-assisted ozone formation. 2. Experimental Methods Ozone is produced by electric discharge in natural oxygen (research grade with 99.9995% purity, Messer Griesheim). The oxygen gas is flown through an U-shaped Pyrex tube with inner diameter and total length of 10 mm and 110 cm, respectively (see Figure 1). Flow rates ranging between 5 and 95 sccm are monitored by a stainless steel flow meter (100 sccm range, MKS Instruments) and result in discharge pressures varying between
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0.3 and 110 Torr. The pressure is measured by a temperature stabilized capacitive gauge (Baratron 390HA, MKS Instruments). Two copper ring electrodes are attached to the outside of the tube and connected to a radio frequency high voltage generator (custom-made, ATPE). Under low pressure conditions a glow discharge is established with the visible glow being restricted to the lowermost part of the U-tube. For low temperature measurements (∼87 K), more than the lower half of the U-tube can be placed in a bath of liquid argon (LAr). The downstream arm of the U-tube is isolated by a foam wrap that can be purged with vapor from a liquid N2 reservoir. An all glass and Teflon sealed valve positioned 10 cm upstream of a cold trap reduces the pressure between the cooled discharge region and the cold trap containing LN2. The trap can be kept at temperatures between 64 and 78 K by adjusting the vapor pressure. Oxygen gas pressure in the trap is measured by a 10 Torr capacitive pressure head (Barocell 600AB, Edwards). With appropriate LN2 bath temperatures, ozone produced in the discharge condenses inside the coldfinger, while molecular oxygen remains in the gas phase and is pumped away into a sorption pump (MDC). A series of measurements has also been performed in a 28 mm diameter and 11 cm long Pyrex reactor (not shown in Figure 1) with two tungsten electrodes placed inside. After collection, trapped ozone is transferred to a small stainless steel tube (8 mm inner diameter, 15 cm length) immersed in liquid helium. Thereafter, the tube is brought to room temperature and surface decomposition reactions lead to a complete conversion of the ozone sample into molecular oxygen. In this way the overall isotopic composition of ozone can be determined with a small (1.5 in) Mattauch-Herzog type mass spectrometer (MS), originally designed for operation in space research.30 3. Results and Discussion Ozone isotope enrichments displayed in Figure 2 show a marked transition between two distinct plateaus. For both 17O and 18O, enrichments are consistently higher at high pressures. Ozone at low pressures is always depleted in the heavy isotopes (E < 0) and, unlike O3 formed at high pressure, it is fractionated in a standard mass dependent manner with δ17O ∼ 0.5 δ18O. The low and high pressure plateau values as well as transition pressures are summarized in Table 1. These values have been obtained from global fits on the δ17O and δ18O data in Figure 2, using the following model function
E(p) ) Eg +
Ew - Eg 1 + (p/pt)nt
(4)
This model function describes the pressure (p) dependence of the transition from an enrichment Ew to Eg, which is located at the half-height pressure pt and whose steepness is given by the exponent nt. Enrichments also show a temperature dependence with values being lower at low temperatures not only in the high but also in the low pressure regions. We note that our low temperature/ low pressure value Ew(16O218O) ) -11.8% falls below all other existing measurements of 16O2 18O. Nevertheless, the sizable amplitude Eg - Ew (12% for 18O) is smaller at low temperature (87 K) than at room temperature (18%). Transition pressures pt depend on both the wall temperature Tw and the reactor diameter 2R. The isotope enrichments Eg observed at high pressures agree well with those of previous
Figure 2. Pressure dependence of isotope enrichment in ozone in (a) a 28 mm diameter reactor at 300 K, (b) a 10 mm diameter reactor at 300 K, and (c) a 10 mm diameter reactor at 87 K: black symbols, δ18O; gray symbols, δ17O. One pair of points at 40 Torr and 87 K (designated by open symbols) has been obtained from a photochemical reactor experiment. Solid lines are global fits to the data using the model in eq 4.
studies relying on the discharge technique for the formation of ozone. Likewise, they agree with results on ozone generated by other means when temperature dependencies are taken into account. As observed previously,18 ozone generated in a discharge is slightly more enriched in the heavy isotopes than ozone produced in a photochemical reactor,31 supposedly due to local heating in the discharge that cannot be easily measured or controlled in the experiments. The temperature dependence of E (Figure 2b,c) is consistent with the strong negative temperature dependence of the rate coefficient for the ozone formation reaction in the gas phase. Lowering the temperature should correspondingly favor ozone formation in the gas phase at the expense of formation of ozone at the walls and therefore lower the pressure pt at which the transition between the two domains occurs. Seemingly contrary to the idea of wall assisted ozone formation dominating at low pressures, however, the transition occurs at a higher pressure in the reactor with the greater diameter (see Figure 2a,b or values reported for experiments a and b in Table 1). At a given pressure, wall formation should be less important in the reactor with larger diameter, because of the longer diffusional time scales and because of the smaller surface to volume ratio (S/V). To give a more quantitative discussion of the results and arguments why our observations are nevertheless consistent with a surface assisted formation of O3, a simple isotope kinetic model is introduced in the following section. 3.1. Model. For a better understanding of the pressure and temperature dependence of ozone isotopic composition in oxygen discharges, we developed a relatively simple isotope kinetic model that accounts for the processes of (i) heterogeneous ozone formation at the reactor walls, (ii) isotope exchange
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O+
16
of oxygen atoms with O2 molecules, and (iii) ozone formation in the gas phase. To that purpose, we postulate two different sources of ozone (wall (w) and gas phase (g)) having different isotopic signatures (Ew and Eg). Then we calculate their respective rates (rw and rg) to determine their relative importance xw ) rw/(rw + rg) and xg ) rg/(rw + rg), which allows us to obtain the actual ozone isotopic composition as
E ) xwEw + xgEg
16
16
O2 f
O3
(6)
k1
there are two for heteronuclear molecules containing two different oxygen isotopes. For example, we have for 16O218O
O+
18
16
O2 f
16
O218O
(7)
k2
(5) O+
16
While being rather simplistic, the approach has its merits in avoiding the problems of treating the complex chemical system of an oxygen discharge, but still being in accord with the general findings of current models. In the following sections 3.1.1-3.1.4 the fundamentals and premises of the model and the three different sources of isotope fractionation (i-iii above), which contribute to the isotopic composition of ozone, are presented in detail, followed by a short summary of the model predictions in section 3.1.5. 3.1.1. General Remarks. Complete modeling of the chemistry in an oxygen discharge is a challenging task, which is due to the number of different species involved, such as electrons, ions, electronically excited atoms and molecules. The large number of reactions between all these, as well as the requirement of a correct description of the boundary conditions (reactor geometry, electromagnetic field, temperature) contribute to the difficulty of the venture. A full treatment is thus beyond the scope of this paper, not only because discharge parameters are not all measured and controlled in our setup but especially because isotope dependent rate coefficients are not available. Interestingly though, the concentration of ozone at the reactor outlet is determined by only a few chemical reactions that take place in the afterglow of the discharge. This is due to the fact that ozone is formed and destroyed effectively by species, such as O and O2(a1∆g), that survive the chemical climate of a discharge on the time scale of milliseconds, unlike ionic and highly excited species that exist on the microsecond time scale only. Quasi equilibrium concentrations of O3 are thus almost independent of the discharge conditions, which may vary from a very low degree to complete oxygen dissociation.32 In agreement with these findings of Jacobs et al.,32 we will thus adopt the picture according to which the gas is first excited in the active zone of the discharge reactor and ozone and then finally forms in the afterglow region, when O atoms and O2(a1∆g) fade away (see Figure 12 of ref 32). Ozone produced in an electric discharge can thus be expected to acquire, at least to a large extent, its isotopic composition from the formation processes according to eq 1 or eq 3, which has led us to adopt the isotope approach depicted in eq 5. Independent of whether ozone is formed heterogeneously or in the gas phase, its isotopic composition depends on the individual pathways from which an isotopomer can be formed. While there is only one reaction channel for the isotopomer of mass 48 u
16
O18O f
O218O
16
(8)
k3
and equivalent reactions apply to 17O-containing ozone. Enrichments are calculated using the atom isotope ratio fm ) 18O/16O (or 17O/16O) in molecular oxygen. For atmospheric oxygen fm is small (3.80 × 10-4 and 2.01 × 10-3 for 17O and 18O, respectively),33 and similar values apply to the oxygen gases used in the measurements. Since fm , 1, we can make the assumption 2fm ) [16O18O]/[16O2]. In general, the isotopic composition of atomic oxygen fa ) [18O]/[16O] may or may not differ from the molecular value fm and, therefore, ozone produced at any instant will have (neglecting multisubstituted species) an isotope enrichment of
E(16O218O) )
(
)
k3 1 fa k2 +2 -1 3 fm k1 k1
(9)
which signifies that the enrichment may arise from both the differences in rate coefficients and the difference in the isotopic composition of atoms and diatomic molecules. Two cases are of special interest here. In the first case, we assume that atoms and molecules differ in their isotopic composition fa * fm, but their isotopic composition is kept fixed during the formation of ozone. Then, the isotope enrichment will be given by eq 9, which reduces to E ) (fa/fm - 1)/3, if all rate coefficients are equal. In the second case, we consider that all atoms, once formed, react to produce ozone. Then mass balance likewise requires E ) (fa/fm - 1)/3, but with fa being here the initial ratio of atoms. Interestingly, this latter result is entirely independent of the rate coefficients, implying that it holds whatever isotope dependence the rate coefficients have. 3.1.2. Heterogeneous Ozone Formation. We propose that wall assisted ozone formation in eq 3 proceeds analogously to the gas-phase process in eq 1, with the wall replacing the molecule M as an efficient quencher. At this point we do not impose any particular reaction mechanism for the surface reaction and the wall assisted reaction may or may not require adsorption and desorption of O, O2, and O3. By comparison with the gas-phase reaction, any isotope effect in the wall reaction should be small, because vibrational energy transfer of adsorbate molecules can be fast (∼10-100 ps)34 and may thus be only about a factor of 10-100 slower than the time scale for oxygen isotope exchange
TABLE 1: Characteristics of Ozone Isotopic Composition at the Transition as a Function of Reactor Dimension (Radius R) and Wall Temperature Twa exp
R (mm)
Tw (K)
pt (Torr)
nt
a b c
14 5 5
300 300 87
12.3(4) 4.7(1) 1.75(9)
2.6(2) 3.3(2) 2.0(2)
49
Ew (%)
-2.0(2) -2.5(2) -6.2(1)
50
Ew (%)
-3.4(2) -4.2(2) -11.8(2)
49
Ew/ 50Ew
0.61(7) 0.61(5) 0.53(3)
49
Eg (%)
11.3(2) 10.3(2) 1.0(2)
50
Eg (%)
14.4(2) 14.1(2) -0.3(2)
Values in parentheses denote 1σ uncertainties in the last digit determined in a global fit of δ17O and δ18O, while MEw,g denotes δ17O (M ) 49) and δ18O (M ) 50) of ozone formed at the wall (w) or in the gas phase (g), respectively. a
Isotope Evidence for Ozone Formation on Surface
O+
16
O2 h
16
O+
16
O2 h
16
18
17
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O18O +
16
k4, k-4
(10)
O17O +
16
k5, k-5
(11)
O O
The typical time scale for the oxygen isotope exchange reaction, whose rate coefficient and its temperature dependence is firmly established,35,36 is of the order of 1 ps or longer.37,38 The dynamics of the exchange, in turn, determines the relevant time scale for the formation of excited ozone, which may become collisionally relaxed to form the stable molecule by either the gas phase (eq 1) or the wall-assisted (eq 3) reaction. The disappearance of the ozone isotope effect in the gas phase occurs at pressures (ph in Figure 3) of about 2000-4000 Torr,17,18,39 which is at least a factor of 20-40 smaller than the high pressure falloff of the rate coefficient for reaction 1 that is situated at above 100 atm.40 The isotope anomaly that arises from the O + O2 interaction41 is thus not expected to prevail in eq 3 due to the effectiveness of the surface assisted vibrational energy transfer. Because details of the adsorption and desorption processes for the O + O2 reaction on the surface are not known, we use a common phenomenological formulation for the rate of heterogeneous loss42 of O atoms to derive the rate rw of heterogeneous ozone formation for a cylindrical reactor
rw ) τw-1 )
(
R2 R 2-γ + 2 V γ 2.405 DO th
)
-1
(12)
where Vth ) [8kBT/(πmO)]1/2 denotes the average gas-phase thermal velocity of the oxygen atoms (kB being the Boltzmann constant, mO the oxygen atom mass, and T the temperature), DO is the diffusion constant, R is the reactor radius, and γ is the reaction coefficient. The diffusion constant DO for diffusion of O in O2 has been measured to be 222 cm2 s-1 at 1 Torr and 280 K.43 Equation 12 is an approximation for an infinite cylinder of radius R that covers the whole range of γ (0 < γ e 1) with an accuracy of better than 11%.42 The first term on the righthand side indicates the time of diffusion to the wall (τd), whereas the second term indicates the time it takes to react there (τr). The limits γ f 0 and 1 represent collision or diffusion controlled wall reactions, implying the limitation of the rate either by the second or by the first term in eq 12. Because the diffusional
Figure 3. Model predicted pressure dependence of oxygen isotope enrichment in ozone E(p). Pressure regimes dominated by formation of ozone either in the gas phase (“gas phase”) or at the reactor walls (“wall”) are shaded differently. The model predicts a transition in the low pressure (“wall”) dominated region due to the competition of wall formation with isotope exchange at p ) pl. The transition at p ) ph is an intrinsic effect of the gas-phase reaction (eq 1). Key: black arrows at the wall, gas-phase transition indicating the pressure range investigated in this study.
constant DO is the only pressure dependent quantity (DO ∝ p-1),44 the partitioning between diffusional and wall loss term R ) 1/(τd/τr) determines the pressure dependence of the overall rate. For R , 1 we have a p-1 dependence, and for R . 1 a p0 dependence. For intermediate cases (R ∼ 1), rw can be shown to approximately follow a power law pc (see Appendix B), with the exponent c being
c ) -(1 + R)-1
(13)
which is consistent with the above-mentioned limiting values of -1 and 0. Assuming a wall reaction coefficient that is independent of the isotopes, isotope effects enter through the inverse square root dependence of the constant for diffusion of O in O2 on the reduced mass µO-O2
DO ∝ (µO-O2)-1/2 )
mO + mO2 mOmO2
and that of the atom thermal velocity (Vth ∝ 1/mO1/2) on the atom mass. Here, we use the symbols mO and mO2 to indicate masses of oxygen atoms and molecules, respectively. Using eq 9, we can calculate the enrichment values due to the kinetic isotope effects predicted for the wall reaction. Assuming that atomic oxygen has the same isotopic composition as O2 (fa ) fm), enrichments of 18O-containing ozone due to heterogeneous reaction of oxygen atoms should therefore be between -1.3 and -1.9% (-0.7 to -1.0% for 17O), depending whether the reaction is controlled by the diffusion process (τd > τr) or by the reaction at the wall (τd < τr). Nevertheless, the difference between the two limiting cases is not very large and in anticipation of the measurement results we present values that are determined by the wall reaction step, if not stated otherwise. As already pointed out in section 1, both effects lead to a depletion of the heavy isotopes, but they are quite small despite the large relative difference in atomic masses. This is due to the fact that the isotope effect only affects the reaction pathway (eq 7) that contributes one-third to 16O218O, while the other one (eq 8) that contributes two-thirds remains unaffected. It should be noted that this isotope fractionation mechanism does not predict any dependence on temperature. 3.1.3. Isotope Exchange. The double role of the isotope exchange reactions 10 and 11 for the isotopic composition of ozone has been discussed repeatedly.10,11,45,46 First, these reactions proceed on the same potential surface as the ozone formation reaction, where they share the association and dissociation dynamics with the latter. Second, when the reactions proceed at a much faster rate than ozone formation, such as in the low pressure regime of eq 1, they establish a dynamic isotope equilibrium between atoms and diatomic molecules, which is dependent on temperature. This isotope equilibrium is nonstatistical and leads thus to a depletion of heavy atoms with respect to molecular oxygen (fa < fm) at all finite temperatures. As a quantum mechanical isotope effect related to differences in zero-point energies,47 it becomes more pronounced at low temperatures and its variability has well been experimentally confirmed in the temperature range between 130 and 320 K.48 In equilibrium fa ) 2fm/K(T), where K(T) is the equilibrium constant of one of the reactions in eqs 10 and 11, and using eqs 20 from Appendix B, we calculate for 16O218O depletions of -2.5, -5.6, and -9.2% at 300, 150, and 90 K, respectively.
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TABLE 2: Kinetic and Diffusional Constants rate coefficient/diffusion constanta
reaction
ref
O + O2 + M f O3 + M
k1
6.0 × 10
O + O2 f O O + O O + O2 + wall f O3 + wall
k4
3.4 × 10-12 cm3 s-1(300 K/T)1.1
18
16
16
18
16
DO
2
-34
222 cm s
-1
6
-1
2.6
50 36
cm s (300 K/T) Torr/p(280 K/T)
-1.5
43
a The rate of the wall recombination reaction (eq 12) depends on the diffusion constant DO and the wall recombination coefficient γ, which is determined from the measurements.
Depletions of 16O217O are about 0.54-0.57 this size over the temperature range from 300 to 90 K and the effect of isotope exchange thus has a standard mass dependence. Compared to the above predicted isotope effects for ozone formation at the walls (section 3.1.2), the fractionation due to the isotope exchange reaction is more important, especially at low temperatures. But it must once more be emphasized that kinetic isotope effects in ozone formation (by eq 1 or by eq 3) can only play a role when the equilibrium of isotope exchange is established. Only then is the atom isotopic composition tightly bound to the molecules with fixed fa < fm. Otherwise, all atoms once created form ozone, implying that the original atom isotope composition is fully transferred to the product ozone. Provided there are no other important sinks for ozone and O atoms, the ozone isotopic composition thus reflects that of the bath O2 and that of the oxygen atoms (see section 3.1.1), with corresponding weights of 2 and 1. Therefore, when at very low pressures O3 wall formation (eq 3) becomes faster than the rate of isotope exchange in the gas phase (τwk4, τwk5 < 1, at pl in Figure 3), we expect that the depletions predicted in sections 3.1.2 and 3.1.3 disappear, presuming that the atoms have an initial composition that resembles that of the molecular oxygen (under these conditions, the notion of an atom thermal velocity starts to break down too). This situation is analogous to the isotope composition of gas-phase ozone formed at very high pressure. 3.1.4. Gas-Phase Formation. Homogeneous formation of O3 in the gas phase occurs in the recombination reaction 1. The associated isotope effect is well documented, but open questions remain despite the many efforts made during the past 30 years.9-11,14 Like the isotope anomaly itself, its pressure and temperature dependencies are not entirely understood. Models that are capable of reproducing the measurements49 still require adjustable ad-hoc parameters. Nevertheless, pressure and temperature dependencies in the gas phase are well characterized by experiments, both for total enrichments17,18,31,48 and individual rate coefficients,39,48 such as k2 and k3. To summarize the experimental evidence on the enrichment, plateau values Eg of 9.5 and 11.5% below pressures of about 50 Torr and at 300 K have been found for 16O217O and 16O218O, respectively. Above 50 Torr a decrease in enrichment sets in (see Figure 3 for a schematic representation). The enrichment then disappears at a typical pressure ph of 2000-4000 Torr. The aforementioned plateau value Eg shows a strong temperature dependence and, due to the effect of the isotope exchange reactions (eqs 10 and 11), enrichments eventually become depletions at low enough temperatures (∼100-150 K).48 3.1.5. Summary of the Model. Due to the very different pressure dependencies of the two ozone formation processes (eqs 1 and 3), which are rg ∝ p2 and rw ∝ p-1 to 0 (see eq 13), respectively, the two channels, which for a given temperature are characterized by the respective isotope enrichments Eg and Ew, compete for O atoms with a rg/rw ∝ p2 to 3 dependence. We thus expect a pressure dependence according to eq 4, with nt varying between 2 and 3, depending whether the reaction at
the wall is controlled by collision or by diffusion (see Figure 3 for a schematic illustration). The transition pressure pt is defined by the equality of rates for the gas phase (k1[O][O2]2) and wall assisted formation (eq 12). It can be determined from the corresponding oxygen number density
[O2]t )
(
rw R2 1 R 2-γ ) + 2 k1 V γ 2.405 D th √k1 O
)
-1/2
(14)
and the limiting enrichments follow from experimental observation (Eg) or from calculations (Ew, see sections 3.1.2 and 3.1.3). Ew is given by
Ew(16O218O) )
(
)
1 2 k2,w -1 3 K18 k1,w
(15)
where K18 is the equilibrium constant for isotope exchange (see Appendix A) and the ratio of rate coefficients k2,w/k1,w for ozone formation at the wall is between (µ16O-16O16O/µ18O-16O16O)1/2 and (m16O/m18O)1/2. An equivalent relation holds for 17O. Figure 3 also indicates the low pressure transition pl, at which isotope effects disappear due to the lack of isotope exchange. It is defined by the oxygen number density where time scales of wall assisted formation (τw) and of exchange (1/(k4[O2])) are of equal importance. Relevant rates and diffusion constants are given in Table 2. 3.2. Comparison with Model. 3.2.1. High Pressure Plateau (Eg). Table 3 summarizes the results on the plateau values derived from fits to the measurements (see Table 1) together with the modeling results. Because our model completely relies on experimental observation at high pressure, model values for Eg have no real predictive power. Nevertheless, the low temperature measurements presented in Figure 2c once again confirm the agreement between ozone generated in a discharge and ozone produced in a photochemical reactor from ground state constituents (reaction 1). Generally, ozone formed in a discharge is more enriched in the heavy isotopes due to the additional heating and the positive temperature dependence of the enrichments.18,31,48 We accounted for this effect by allowing for elevated gas-phase temperatures as we modeled the enrichments for measurements at room temperature, which were performed without coolant that would have efficiently removed the additional heating caused by the discharge. For measurements at low temperature, the gas-phase temperatures are thought to be relatively close (within 3 K) to temperatures at the wall due to the LAr bath used. We thus compare our results with those obtained in the earlier temperature dependent studies,18,31,48 assuming gas-phase temperatures Tg of 355, 340, and 90 K for experiments a-c, respectively. For 17O the
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TABLE 3: Comparison of Measured (msd) Enrichments with Model (mod) Results for an Assumed Temperature Tg of the Gas Phase a
b
c
exp
moda (355 K)
msd
moda (340 K)
msd
moda (90 K)
msd
Eg (%) Eg (%) 50 Ew (%) 49 Ew (%)
14.0 11.2 -3.8 -2.0
14.4 11.3 -3.4 -2.0
13.3 10.8 -3.9 -2.1
14.1 10.3 -4.2 -2.5
-1.7 0.4 -10.6 -6.1
-0.3 1.0 -11.8 -6.2
50 49
a Gas-phase enrichments Eg based on ref 31. Model temperatures (Tg, in parentheses) have been chosen in agreement with experimental conditions to obtain consistent results for low and high pressure regions.
TABLE 4: Analysis of Transition Parameters and Determination of the Wall Recombination Coefficient γ exp
set
Tg (K)
pt (Torr)
R (mm)
nta
τd (ms)
τg (ms)
τv/τg
R
γR (%)b
γk (%)c
a b c c
1 1 1 2
355 340 90 100
12.3 4.7 1.75 1.75
14 5 5 5
2.6 2.9 2.4 2.4
12.8 0.67 1.82 1.55
23.1 130 2.07 3.35
20 1.2 175 97
0.67 0.11 1.5 1.5
0.48 18 1.1 1.2
0.40 0.012 11 1.5
a Value taken from within the (2σ confidence limits such as to be compatible with γk; see text. b Based on comparison with diffusion time (R). c Determined by comparison with time constant/rate for ozone formation (eq 14).
agreement is 0.6% or better, which is compatible with errors in this study and in the experiments serving as a reference here. Agreement for 18O is slightly less convincing but, given the difficulty and sparsity of low temperature data as well as the need to extrapolate on the temperature scale, it can still be regarded sufficient. 3.2.2. Low Pressure Plateau (Ew). Measurements performed at room temperature (entries a and b in Table 3) are very close to model predictions, when we allow for the same temperature elevation as before. At low temperatures, agreement is particularly good for 17O, but given the very large fractionation, agreement for 18O is also tolerable. Modeled values have been calculated on the basis of heterogeneous recombination being limited by the reaction step at the wall. Both absolute values and variability with temperature of the low pressure depletions are thus well explained by a heterogeneous ozone formation channel. As discussed in section 3.1.3, the variability is ultimately due to the strong temperature dependence of the isotope exchange equilibria, which despite the weighting factor of 1/3, already lead to a strong 18O depletion of -9.2%, to which also contribute the effects in the wall reaction itself (section 3.1.2). Moreover, and contrary to previous interpretations, ozone forms not only at low temperature walls but also at walls that are at room temperature or above. If it were formed exclusively in the cold trap, as discussed by Sabadil et al.2 and Nekrasov et al.,1 there would be no observable variation of the low pressure enrichment Ew with wall temperatures upstream of the trap. 3.2.3. Transition Region (pt). As discussed in section 3.1, the transition region is characterized by the competition between ozone formation in the gas phase and at the wall. Both the pressure pt at half-height and the steepness coefficient nt (eq 4) provide information about the relative importance of the wall process, thus allowing us to determine the recombination coefficient γ from the comparison either with kinetic data on the gas phase (eq 14) or with the diffusion time scale (eq 13). Due to the quadratic pressure dependence of the rate of reaction 1 we have
R ) τr /τd ) (nt - 2)-1 - 1
(16)
In general, results for pt and nt in Table 1 seem to comply with model predictions. In particular, nt values determined from
the fits are compatible with the expected range between 2 and 3, when error limits (2σ) are taken into account. However, the fact that the transition pressure pt is smaller for the smaller reactor indicates certain limitations, either on the experimental or on the modeling side. For a more detailed analysis, we summarize parameters that are compatible with experimental conditions and γ values derived therefrom in Table 4. In addition, the ratio of the time it takes to reach the cold trap after the discharge (τv, determined from the flow conditions) to the characteristic time (τg) for the gas-phase reaction at transition pressure pt is tabulated. This ratio indicates whether the reaction could go to completion before the cold trap was reached. Both methods lead to compatible results for the conditions of experiment a. The value of nt ) 2.6 ( 0.4 (2σ) corresponds to a large range of γ between 0.08 and 100%, indicating that this method to determine γ is not very sensitive. From pt (eq 14) on the other hand, the more concise result γ ) (0.4 ( 0.1)% (1σ) is obtained. We need to assert, however, that the different methods give incompatible results for experiment b. From the lower limit value nt ) 2.9, we conclude that γ should exceed 18% under measurement conditions, whereas pt ) 4.7 Torr implies that γ should be at least 3 orders of magnitude smaller. This incompatibility is not recoverable by invoking other measurement uncertainties but hints to a systematic drawback of this particular experiment. Flow and pressure conditions at pt are such that a significant fraction of O atoms enters the cold trap (τv/τg ) 1.2) before reacting. The much colder environment there implies that our assumption of roughly constant temperature conditions over the whole pressure range is not fulfilled and that a non-negligible portion of ozone could have formed there. Because the gasphase reaction 1 becomes quite efficient at lower temperatures, it shifts weights in disfavor of ozone wall formation. This explains qualitatively why in the smaller diameter reactor the transition is observed at a lower pressures than predicted. We note that this interpretation does not necessarily imply a conflict with the temperatures derived for the reactor upstream of the cold trap, because low pressure ozone formed in the gas phase has a slight depletion at the very low temperatures in the trap, probably not too different from the values given in Table 3. In the high pressure plateau, on the other hand, pressures are about a factor of 10 higher than in the transition region, implying a reaction time that is reduced accordingly by a factor of 100.
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The formation of high pressure ozone had thus been completed well before it was frozen out and isotope enrichments on either end of the covered pressure range should thus well reflect temperatures in the zone upstream of the cold trap. Finally, a more consistent result is obtained for the low temperature experiment c. From R we obtain an upper limit of about 1% for the recombination coefficient (see Table 4). Determining γ from eq 14 gives a lower limit, strongly depending on the temperature in the gas phase. Because temperatures higher than 100 K are certainly not compatible with the enrichment data, a value of 1.5% is thought to be a reasonable lower limit estimate. Refraining from a full error analysis (including an evaluation of the rate coefficients at 90 K) that would certainly widen the ranges of tolerance given here, we regard the agreement between the two methods, though not complete, nevertheless sufficient to conclude that γ is on the order of 1% or higher. Inspection of τv/τg in Table 4 indeed confirms that ozone has well been formed before the gas entered the cold trap and that the model predicted temperature conditions thus prevailed. 3.3. Comparison with Earlier Results. Our study gives the first quantitative analysis of earlier hypotheses18-21 that ozone can be formed in a surface assisted process. Our model covers all the pressure range (Figure 3) with the only drawback that it has no predictive power for the high pressure region, where the yet not fully explained isotope effects inherent to the O + O2 reaction system require that the model is based on experimental observations in that pressure range. The assertion of Janssen et al.21 that ozone wall formation in combination with isotope exchange could explain the results of earlier studies has been confirmed. Interestingly, Morton et al.18 obtained their results in stainless steel reactors, demonstrating that the reaction also proceeds on this particular surface material. Bains-Sahota and Thiemens19 have conducted their experiments in a quartz reactor at even lower pressures and their data further show the model predicted turn up to zero (when the pressure is reduced) at pressures below pl ) 52 mTorr (see Figure 3), which at the time of publication went unnoticed. In any case, a quantitative analysis of this experiment is difficult, because the discharge has been created just a few centimeters upstream of the cold trap, leading to largely undefined temperature conditions. Remarkably though, the authors stated a temperature of 150 K for which our model predicts depletions of -7.2 and -4.0% for 16O218O and 16O217O, respectively. This prediction agrees very well with the measured averages in the low pressure plateau, which are -6.9 and -4.0%. The available isotope data thus provide a very consistent picture, which requires the presence of a wall assisted ozone formation reaction (eq 3) that seems to be relatively independent from the wall material. Its temperature dependence and its efficiency have been determined for the first time. 3.4. Implications. So far (in eq 12), we have not made any assumption on a particular mechanism for the surface reaction. Nevertheless, the transition pressures observed indicate that adsorbed species (O(ads), O2(ads), or both) are involved, implying a reaction following the Eley-Rideal or the LangmuirHinshelwood scheme. On the contrary, the proposed51 mechanism of simple “wall quenching” of excited ozone formed in the gas phase must likely be ruled out on the basis of our and other results. Under steady state conditions, the number density of vibrationally excited ozone (O*3 ) is determined by the equilibrium between its association and its removal rate. Maximizing the rate coefficient of association by twice the rate coefficient for isotope exchange 2k4, we can estimate an upper
Janssen and Tuzson limit for the concentration ratio κ ) [O*3 ]/[O] in the gas phase by 2k4τdec[O2], where τdec is the lifetime with respect to decay into O + O2. Because isotope exchange is so fast, this dynamic equilibrium is readily established under experimental conditions. Moreover, owing to the fact that vibrational energy transfer in single surface collisions is quite efficient even for small polyatomic molecules,52 we assume a wall reaction coefficient of almost 1 for the O*3 + wall f O3 + wall reaction. If this reaction was the dominant mechanism of heterogeneous ozone formation, then loss of O atoms would be at least κ times less efficient than the ozone wall stabilization process, implying an apparent O atom recombination coefficient γ < κ. Using reasonable average lifetimes of τdec e 200 ps,37 we find γ < 4 × 10-4 for our measurements (experiment a) and γ < 8 × 10-5 for the measurements of Morton et al.18 in a 9 L metal reactor. These values are at least 10-20 times smaller than what is observed experimentally, clearly showing that “wall quenching” plays a minor role at most. They likewise imply that either O2, or perhaps O (Eley-Rideal), or O and O2 (Langmuir-Hinshelwood) are adsorbed on the wall, before recombination takes place to form O3 at the surface. Heterogeneous ozone formation is likely to have a strong impact on the determination of the O atom recombination coefficient γ′ for
1 O + wall f O2 + wall 2
(17)
Usually, formation of ozone is completely neglected in the analysis of postdischarges used for this kind of measurements. This omission is justified normally by the small efficiency of the gas-phase process (eq 1) at low pressure. Ignoring ozone sources, such as the wall process (eq 3), however, leads to a systematic overestimation of the importance of eq 17. Assuming significant amounts of O and O2(a1∆g) in the postdischarge,32 ozone once formed may react with atomic oxygen
O + O3 f 2O2
(18)
leading to the catalytic conversion of odd (O, O3) to even (O2) oxygen. Alternatively, O atoms that have been consumed in the formation of ozone are just recycled when ozone reacts with excited molecular oxygen
O3 + O2(a1∆g) f O + 2O2
(19)
In terms of O to O2 conversion, reaction 3 followed by reaction 18 thus has an efficiency of 2, while reaction 3 followed by reaction 19 is a null cycle. At a temperature of 300 K rate coefficients for reactions 18 and 19 compete in a 2.1: 1 ratio.50 If [O] g [O2(a1∆g)], as modeled by Jacobs et al.,32 for example, wall formation of O3 implies an ozone catalyzed O f 1/2O2 conversion with an apparent wall reaction coefficient γ′ > 4γ/ 3. Provided that the chemical climate is in favor of O over O2(a1∆g), we thus predict that, to a large extent, the observed decay of O in an afterglow discharge is due to reaction 3, followed by 18. This also holds for the study of Braginskiy et al.,7 where the reaction in eq 3 was discussed but has been deemed unimportant. For example, [O]/[O2(a1∆g)] ∼ 1/2 has been measured at 2 Torr. Under these conditions O atoms are removed via the above scheme corresponding to an apparent wall recombination coefficient of γ′. If, as we have measured,
Isotope Evidence for Ozone Formation on Surface γ ) 4 × 10-3, this mechanism is much more efficient than eq 17, which in ref 7 has been assumed to have a coefficient γ′ ) 2 × 10-3. But even if O atoms are not the dominant sink for O3, our study still questions the validity of determining the wall recombination coefficient for eq 17 solely on the basis of the study of the O atom decay without analyzing the kinetic situation and, in particular, without considering the heterogeneous formation of ozone (eq 3). On the basis of the arguments presented before, we estimate that literature values on the recombination coefficient for eq 17 are likely overrated between 50 and 100%. It should be noted that actinometric measurements of O atoms in a discharge actually are complementary to our isotope experiment. While we perform a product study that probes the appearance of ozone formed at the wall, actinometer measurements study the decay of atomic oxygen without being able to detect reaction products. Because we do a relative measurement that relates to the well-known ozone formation in the gas phase, our method appears to be relatively robust with respect to possible interferences through the complex reaction network in a plasma discharge. Another significant consequence of the wall recombination coefficient of eq 3 being more important than that of eq 17 is the high degree of conversion of O into O3 when reactors are kept at LN2 temperatures. At these temperatures ozone is efficiently frozen out due to its low vapor pressure53 and ozone secondary chemistry (eqs 18 and 19) thus cannot further take place. Because O does not react with O2(a1∆g), conversion into ozone is the only atom sink. This analysis thus fully supports the findings of earlier reports1,2 of a very high O to O3 conversion efficiency at these low temperatures. Ozone formation at the gas-solid interface may also be important in other low pressure/low temperature environments. The observation of ozone on the icy satellites of the giant planets3 has triggered intense research on the conditions and mechanisms of ozone formation other than by the recombination reaction in the gas phase (eq 1). Commonly, radiolysis or photolysis of solid oxygen in water ices is thought to be at the origin of ozone formation, but laboratory simulations have not yet identified individual reaction steps and the search for the correct mechanism is still ongoing.3,54 The heterogeneous formation of ozone is also an interesting prototype for isotope effects in O + XO f XO2 reaction types. While the reaction in the gas phase is the prime example of a yet not fully explained isotope anomaly with large and almost equal enrichments of the heavy isotopes 17O and 18O, its equivalent on a surface, as shown here, follows a standard mass depencence δ17O ∼ 0.5 δ18O. Our result thus casts doubts on the proposed explanation24 for the δ17O ∼ δ18O records in some meteoritic material by O + SiO or O + AlO reactions on grain surfaces, even though the proposed mechanism for O + SiO and that underlying our O + O2 measurements may differ and even though we have certainly not probed the relevant temperature range of about 1500 K and above, where time scales for vibrational energy transfer and desorption rates are different. 4. Summary and Conclusion New measurements of the isotopic composition of ozone formed at low pressure have been presented. A simple model could successfully explain the observations, from which we conclude: (i) Ozone is formed at reactor walls. (ii) The reaction requires adsorption of O, O2, or both. (iii) The isotopic composition of ozone is determined by three processes: O +
J. Phys. Chem. A, Vol. 114, No. 36, 2010 9717 O2 isotope exchange, isotope fractionation in ozone wall formation, and the ozone isotope effect in the gas phase. (iv) Isotope effects in wall formation are small but likely lead to a depletion of the heavy isotopes. (v) The temperature dependence seen at low pressure is essentially due to isotope exchange reactions in the gas phase. (vi) Contrary to the gas-phase process, ozone wall formation has no isotope anomaly. (vii) The recombination coefficient for the wall reaction is γ ) (0.4 ( 0.1)% or higher at lower temperature, and (viii) it is sufficiently important to question the validity of published values for the O f 1/2 O2 wall recombination coefficient. Point vi in particular casts some doubts on the validity of the hypothesis24 that surface reactions analogous to ozone formation could have caused the isotope signature observed in early solar system refractory material.25 However, it makes ozone formed at low pressures an interesting candidate for preparing calibration mixtures for infrared absorption line intensity measurements. The reaction not only may occur in oxygen discharges but could possibly play a role in extraterrestrial environments, such as solar system icy satellites. In conclusion, the present study is ample illustration of how isotope measurements provide significant insight into complex reaction systems. Future experiments might benefit from improvements on the dynamics of the flow system, which might allow for better decoupling of the gas flow from variations of pressure. Appendix A. Equilibrium Constants The equilibrium constants for the isotope exchange reactions
O+
16
16
O+
16
16
18
17
O2 h O2 h
O18O +
16
O17O +
16
O O
K18 and K17, respectively, have been calculated by Kaye and Strobel.55 Their calculation depends, besides on the BornOppenheimer approximation, on the assumption of a rigid rotor-harmonic oscillator model for the energy levels of the diatomic molecules and further assumes the classical rotational partition functions to be sufficiently accurate. For a desired accuracy of better than 0.5%, however, higher order approximations must be incorporated into the calculation of the partition functions.56 We have thus redone these calculations following Mayer and Goeppert-Mayer57 to derive analytical expressions for the partition function of the diatomic molecules, from which the equilibrium constants could be calculated. In particular, higher order effects, such as rotational stretching, rotationvibration interaction, and the first anharmonic correction to the vibrational levels, have been taken into account. Using the following parametrization
K18 ) 1.9457 exp(31.779 K/T) × (1 - 9.733 × 10-6(T/K) + 2.031 × 10-8(T/K)2)
(20)
K17 ) 1.9714 exp(16.744 K/T) × (1 - 5.140 × 10-6(T/K) + 1.059 × 10-8(T/K)2)
(21)
an accuracy of 10-4 in the temperature range between 80 and 500 K is obtained.
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Appendix B. Pressure Dependence of the Wall Reaction Here we show that the pressure dependence of the rate of the wall reaction approximately follows a power law ∝pc. We start from the pressure dependence for the wall loss rate implied in eq 12
r(p) ) (a + bp)-1
(22)
where r(p) is the rate, p is the pressure, a ) τr is the time constant for wall recombination, and bp ) τd is the diffusional time scale. The asymptotic limits ∝p0 and ∝p-1 are immediately apparent for p f 0 and p f ∞, respectively. To understand the pressure dependence in the intermediate ranges, we develop ln(r) ) -ln(a + bep˜) in powers of p˜ ) ln(p) around p˜ ) p˜0, and set r0 ) r(p˜0). This expansion and the choice of variables is motivated by the fact that we are concerned with relative rather than absolute changes. Keeping the two lowest order terms, we find
ln(r/r0) )
( ∂ln(r) ∂p˜ )
p˜)p˜0
(p˜ - p˜0) ) c ln(p/p0)
with c ) -br0p0 ) -τd/(τd + τr) ) -1/(1 + R) when we set R ) τr/τd, as before. This is the pressure dependence claimed in eq 13. Acknowledgment. We are grateful to Joachim Janicke for his invaluable technical support during the experiments. CJ thanks Konrad Mauersberger for many fruitful disussions on the subject and the directors of the Max-Planck-Institut fu¨r Kernphysik for having continuously supported this research. References and Notes (1) Nekrasov, L. I.; Skorochodov, I. L.; Kobozev, N. I. Zh. Fiz. Khim. 1966, 40, 2361–2365. (2) (a) Sabadil, H.; Biborosch, L.; Koebe, K. Beitr. Plasmaphys. 1975, 15, 319–332. (b) Wojtowicz, J. A.; Urbach, H. B.; Zaslowksy, J. A. J. Phys. Chem. 1963, 67, 713–714. (3) (a) Noll, K. S.; Johnson, R. E.; Lane, A. L.; Domingue, D. L.; Weaver, H. A. Science 1996, 273, 341–343. (b) Noll, K. S.; Roush, T. L.; Cruikshank, D. P.; Johnson, R. E.; Pendleton, Y. J. Nature 1997, 388, 45– 47. (4) Eliasson, B.; Kogelschatz, U. Basic data for modeling of electrical discharges in gases: Oxygen; Technical Report, 1986. (5) Diamy, A.-M.; Legrand, J.-C.; Al Andari, J. New J. Chem. 1997, 21, 177–185. (6) (a) Dylla, H. F. J. Vac. Sci. Technol. A 1988, 6, 1276–1287. (b) Poulsen, R. G. J. Vac. Sci. Technol. 1977, 14, 266–274. (c) Moisan, M.; Barbeau, J.; Crevier, M. C.; Pelletier, J.; Philip, N.; Saoudi, B. Pure Appl. Chem. 2002, 74, 349–358. (d) Chu, P. K.; Chen, J. Y.; Wang, L. P.; Huang, N. Mater. Sci. Eng. R 2002, 36, 143–206. (7) Braginskiy, O. V.; Vasilieva, A. N.; Klopovskiy, K. S.; Kovalev, A. S.; Lopaev, D. V.; Proshina, O. V.; Rakhimova, T. V.; Rakhimov, A. T. J. Appl. Phys. D 2005, 38, 3609–3625. (8) Macko, P.; Veis, P.; Cernogora, G. Plasma Source Sci. Technol. 2004, 13, 251–262. (9) (a) Thiemens, M. H. Ann. ReV. Earth Planet. Sci. 2006, 34, 217– 262. (b) Schinke, R.; Grebenshchikov, S. Y.; Ivanov, M. V.; Fleurat-Lessard, P. Annu. ReV. Phys. Chem. 2006, 57, 625–661. (c) Mauersberger, K.; Krankowsky, D.; Janssen, C. Space Sci. ReV. 2003, 106, 265–279. (10) Mauersberger, K.; Krankowsky, D.; Janssen, C.; Schinke, R. Assessment of the Ozone Isotope Effect. In AdVances in Atomic, Molecular, and Optical Physics; Bederson, B., Walther, H., Eds.; Elsevier: Amsterdam, 2005; Vol. 50, pp 1-54. (11) Brenninkmeijer, C. A. M.; Janssen, C.; Kaiser, J.; Ro¨ckmann, T.; Rhee, T. S.; Assonov, S. S. Chem. ReV. 2003, 103, 5125–5161. (12) Weston, R. E. Chem. ReV. 1999, 99, 2115–2136. (13) Grebenshchikov, S. Y.; Schinke, R. J. Chem. Phys. 2009, 131, 181103. (14) Sun, Z.; Liu, L.; Lin, S. Y.; Schinke, R.; Guo, H.; Zhang, D. H. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 555–558.
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