The photochemistry of ozone

~tm,tspAwic Printed

Envirmunmr

in Great

Vol. 21. No. 8. pp. 1683-1694.

1987

Briuin.

ooo4-@81/67

53.00+0.00

Pn~mon

Journ&Ltd.

REVIEW ARTICLE THE PHOTOCHEMISTRY

OF OZONE

R. P. WAYNE Physical Chemistry Laboratory, South Parks Road, Oxford, OX I 3QZ. U.K. (First received 1 December 1986 and received for publication I I February

1987)

dstrrrt-lbii review swvqs laboratory data concerning the photochemistry of ozone, and indicates the rekvance of the laboratory studies to int~~tations of atmospheric chemistry. Emphasis is placed on the nature of the electronic states of the atomic and molecular oxygen fragments of photolysis, and the efficiencies with which the various speciesare formed. The primary quantum yield for O(‘D) formation is certainly less than unity for I < 274 nm, and it may thus also be lessthan unity in the atmospherically critical region around 1 = 300 nm. Similar considerations are likely to apply to the efficiencyof formation of excited singlet molecular oxygen, 02(‘As). On the other hand, 02(‘As) seems to be formed with high efficiency at

O(‘D) production. Calculations of atmospheric [O&J that depend on m~surement ofthe intensity of the O,(’ A, -* ‘xi) infrared atmospheric band may therefore

wavelengths longer than the i. ‘c 310 nm threshold for be in error both if they assume a

quantum yield of unity for O,(‘As) production at ?.< 310 nm and if they

assume that the quantum yield is zero at longer wavelengths. The wavelength dependencesof the quantum efficienciesare interpreted in terms of the spectroscopyof ozone; evidence for the breakdown of simple spin conservation arguments is presented,and some explanations for the bchaviour are suggested.Photofragment energy analysis, coherent Raman, and fluorescence techniques have been used to probe details of the dissociation dynamics. Resuits from some of these experiments are used to show how further information needed for atmospheric studies may eventually be won. Key word index: Ozone, photochemistry, electronically excited species, photodissociation spectroscopy, airglow.

Ozone plays a major role in the chemistry of Earth’s atmosphere, its importance in the stratosphere and in the troposphere being particularly well documented (see, for example, Becker and Cox, 1986; Angeletti and Restelli, 1987; Watson et al., 1986; Wayne, 1985a; World Meteorological Organization, 1986). 03’s absorption of U.V. light, especially in the stratosphere, not only prevents short wavelength U.V. radiation from reaching the Earth’s surface, but also modifies the spectral composition of the light that penetrates to the troposphere. At the same time, the absorption of radiation by OS leads to its photofragmen~tion and thus to the initiation of thermal atmospheric chemistry that could not otherwise arise. Particularly significant from this point of view is the possibility that the atomic oxygen fragment of dissociation can be electronically excited, and thus have enough energy itself to react with moiecuics. such as water or methane, for which the interactions with ground state atomic oxygen are thermochemically unfavourable. The products of these reactions include radicals such as OH (hydroxyl) which are central to tropospheric chemistry and for which it is dilXcult to postulate alternative low-altitude sources, It is thus clearly of the greatest importance to have a sound knowledge of the efficiencies with which OS is

dynamics,

photolysed and with which excited atoms, O(‘D), are produced, and to understand what factors influence the variation of those efficiencies with wavelength of the solar radiation absorbed. Further interest extends to the molecular oxygen fragment of the photodissociation, because it, too, can be formed in an excited electronic state. Although it is unlikely that the excited 0, greatly influences the course of atmospheric chemistry, the airglow emission from the species is used for remote monitoring of atmospheric O3 concentrations, for example by instrumentation on the Solar Mesosphere Explorer (Thomas er al., 1984). Once again, therefore, it is imperative that the quantum yields for excitation of molecular oxygen be known accurately. Quantitative information, of the kind described in the last paragraph. is likely only to be available from laboratory experiments. Motivation enough exists, therefore, for physical chemists to study the photochemistry of0,. ~rticu~ar~y with respect to the nature of the photolytic fragmentsand to the way in which the wavelength of the photolysing radiation-or energy of the absorbed photons-influences the pathways and efficiencies of decomposition. There is, in addition, a strong fundamen~i physico-chemicaf interest in the phot~hemistry of 0, that has prompted investi~tion of the details of dissociation at increasingly sophisti-

1684

R. P.

cated levels. Elementary chemical reactions must be interpreted in terms of the interactions as the constituent atoms of the reactants move to the positions they occupy in the products. The laws of physics must determine the motions of the particles under the influences of the forces described by the potential energy surface for the reaction. Detailed predictions can be made of the dynamics of reactions of small pofyatomic molecules such as 0,. Ultimately. one might hope to understand (and predict) inter- and intra-molecular dynamics in state-to-state detail (i.e. in terms of the internal excitation and velocity parameters as the reactants approach and the fragments separate). From the viewpoint of experimental studies aimed at this rather grand goal, photodissociation is a particularly fruitful process to study since it involves only a ‘half collision’ between a photon and the reactant, rather than a full collision between two molecules, OJ photochemistry has been studied in the laboratory for well over a century, and it is the results of these increasingly sophisticated investigations that aeronomers and atmospheric chemists have been able to apply to their own endeavours. The earliest experiments, performed with ‘classical’ (or steady-state) techniques, already provided indirect evidence for the formation, under some circumstances, of excited atomic and molecular fragments. The development of time-resolved methods, such as flash photolysis, together with direct identification of the fragments. provided the first steps towards probing the detailed dynamics ofdissociation. In general, subsequent effort has been directed towards examining the excitation in the fragments, and sometimes their translational energies and angular distributions of release as well, at times increasingly close to the moment of absorption of the photon, so that the measurements reveal more about the fragments as they are born and less about what happens to them in subsequent collision. Experiments have now been performed that examine the dynamical behaviour on the femtosecond timescale: that is, they examine the intermediate species, during and immediately after absorption. as it is in the process of falling apart (Imre et al.. 1982. 1984). The significance of the new experiments is two-fold. First, the more detailed is our understanding of the phenomenon of photodissociation, the more reliable will be the predictions of behaviour under conditions not accessible in the laboratory. Such predictions may have particular pertinence for atmospheric studies. Secondly, the results obtained. while not primarily directed towards the elucidation of atmospheric behaviour, nevertheless require that consideration be given to their relevance to atmospheric chemistry. The purposes of this review are to describe the development of laboratory experiments on 0, photochemistry. to point out what the potential implications for atmospheric chemistry might be. and to discuss briefly the progress being made in understanding the detailed dynamics.

WAVNF

2. SPECTROSCOPY AND DlSSOClATIOY

fHASXE1.S

Three photochemically active absorption regions are normally recognized for 0, at wavelengths longer than the vacuum U.V. The strongest of these is the Hartley band, centred on i. = 254 nm. and probably due (Hay et al., 1982) to the I’B, + I’A, transition. Merging at longer wavelengths with the Hartley continuum, but orders of magnitude less intense. are the Huggins bands. A locally bound region on the I ’ B2 surface may be responsible (Katayama. 1979) for these bands, but there is also speculation (Brand er al.. 1978) that the 2’A, state may be involved. Finally, in the red region of the spectrum are found the Chappuis bands, probably resulting from a I’B, + l’A, transition. Considerable theoretical effort has gone into calculating potential energy surfaces for 0s (Hay and Dunning, 1977; Thunemann et al., 1978; Wilson and Hopper, 1981; Jones, 1984, 1985; Morin et al., 1985) that assist in understanding the photochemistry and dissociative pathways (Way et al., 1982; Sheppard and Walker, 1983). The richness of 0,‘s photochemistry is in part a consequence of the rather weak energy of the 0,-O bond (just over 101 kJmol-’ or 1 eV). Since the peak of the 0, absorption spectrum in the U.V. region corresponds to about 470 kJ mol- ‘. it is apparent that much excess energy is available in the U.V. photolysis which can lead to the excitation of the fragment species. Both atomic and molecular oxygen have several accessible excited states. The table below shows some of the thermochemical thresholds, expressed as wavelengths in nm, for different dissociation channels. Molecule Atom

O(‘P) Of’@ O(‘Sj

W’L& 1180 411 237

%f’AJ

Od’ZJ

612 310 199

463 267 181

The wavelengths indicated in the table would be the absolute limits for the formation of any particular product pair. were it not for the possible contribution of internal energy in the molecule. Vibrational and rotational energy can, in fact, contribute towards the total energy necessary for dissociation. so that the onset of participation of a channel does not have to be a step-function of wavelength, and it may be a function of temperature, as we shall see later. Whiie the thermodynamics may impose a limiting constraint on a dissociation channel, other conditions may also need to be satisfied for the actual occurrence of that pathway. In particular, reactions are generally eflicient only if they proceed udiahatirul~~ that is. a single continuous potential energy surface connects products with reactants. Of the correlation or conservation rules that are based on this concept, the most important for the present purpose is that of Spin Conservation. Applj~tion of the Spin Conservation Rule to a photodissociative process starts by assuming

Review: the photochemistry of ozone that there is no change in electron spin on optical absorption (AS = 0). In the case of O,, the ground state is a singlet (S = CI),so that any excited state populated in a (strong) absorption is also likely to be a singlet. If the 0 and Oz fragments of the dissociation are to correlate with the excited state, the argument runs that both must be singlets or both must be triplets. since the rules of quantum vector addition allow that both S = Of S =OandS=l+S=lcangiveS=O,butS=O+S = I cannot. With this rule imposed, some of the pairs of products suggested by the table are excluded. The validity of the rule will be discussed in more detail later in this review. Experimental evidence shows that absorption in the visible region of the spectrum leads to the formation of two tripiq ground state, fragments, in accordance with the thermochemistry constrained by spin conservation (see table). In the strong part of the Hartley band, at i, c 300 nm, the productsare mainly the two singlets suggested by the table [t&(‘A ) + O(lD)]. At wavelengths shorter than 237 nm, Q( !S) formation becomes energetically possible, although experiments (Lee ef al.. 1980) in the wavelength range 170-240 nm show that the IS state atom is not produced to a significant extent. Absorption of a sufficiently energetic photon does not, therefore, guarantee that an energetically accessible route will in fact be followed. Indeed, at i. = 157.6 nm, about half the 0, molecules fragment to O(‘D) + OI(‘ZJ; the remainder of the 0, dissociates via a completely new route to 0 + 0 + 0 (Taherian and Sianger. f985). Static determinations (i.e. performed without provision for time resolution) of the overall quantum yield for ozone decomposition provided the first evidence for the formation of ground state products in the red region of the spectrum (Casteliano and Schumacher. 1969) and of excited products in the U.V. (Norrish and Wayne, 1965a, b; Lissi and Heicklen. 1972). Quantum yields of two are found in the red, but chain decomposition in the U.V. can lead to much larger values. A simplified reaction scheme can be used to interpret the resuhs: O3 + hv + O(‘D) i- Q#AJ d(‘D) -+ o(+)

(la)

#(“A,) t O1(‘;c,)

U(:’ D) + Cl3+ chain decomposition O>(*A& + 03-c 203 +Q(‘P) Of JP)+ 0, +20,.

(lb) (2) (3) (4)

This scheme predicts an overalf quantum yield & of the jorm @ = 2 + W’AJ

+f#CDW,J

(0

where f is a constant describing the extent of chain decomposition. Thus, in the red region of the spectrum. where only O(“P) is formed (i-e. #(ID) is zero), the quantum yield is two, because reaction (2) does not

1685

6

Fig. 1. Overall quantum yield for OJ photolysis as a function of pressure of OJ for photolysis at three wavelengths: (a) 1= 254 nm; lb) i = 3 13 nm; (e) i. = 334 nm. Data of Jones aed Wayne (1969, 1970). participate. On the other hand, if both &ID) and 4(‘A,) are unity, as the arguments presented so far suggest they might be in the U.V. region, then the quantum yield would be expected to be a linear function of [O,], with an extrapolated intercept .at zero fQ,] of four. Figure 1 shows some typical results obtained at three wavelengths in the U.V. (Jones and Wayne, 1969, 1970). Considering, for the moment, photoiysis at the shortest wavelength (254 nm) only, the chain nature of the decomposition is clearly indicted by the positive slope of the quantum yield-[OS] relationship, and the intercept of nearly four provides strong circumstantial evidence for. the production ofO,(‘A,) with a yield approaching unity.

3. WAvELENGTHDEPENDENCEOF UI’Dj MELD So far as the stratosphere and troposphere are concerned, the most important photochemical region is the long wave tail of the Hartley-Huggins bands. In this region, solar intensity rises rapidly, but is accompanied by a sharp decrease in absorption cross section. The variation with wa~~ength of quantum yield for O(tDf production thus has a profound effect on the predicted rate of excited atom formation. Static experiments, such as those just described, can be used to obtain the information needed. According to relation (If. slopes of the overaii quantum yield-[Cl,] piots (e.g. Fig. I )shouid be pro~ortjonai to the primary quantum yieid for Of’D) furmation. #(‘Dj. It is apparent from Fig. 1 that there is a marked decrease in &‘D) as i, increases from 254 nm to 334nm, the yield being essentially zero at the longer wavekngth. At i. = 310nm. #I’D) is around 0.1 of its value at 2 = 254 nm. Such behaviour is. at least qualitatively. that expected if photoiysis is spin conserved, since, as shown in the table, d = 310 nm is approximately the thermochemical threshold for formation of two singtet products. Subsequent experiments, both static and those using laser flash photolysis, have greatly refined

R.P.

1686

6 ’ .<

Q

0.6 2s % L 06 e r 9

z

E 3

O4

E

9

= 0.2? 2 ii

280

I 290

I Jo0

310

Wavelength

320

330

340

(nrn1

Fig. 2. Primary quantum yield for O(‘D) production in 0, photolysir as a function of wavelmpth. Curve derived from NASA recommended values (DeMon ef ol., 1985).

the early results (Arnold et al., 1977; Brock and Watson, 1980; Cobos et al., 1983; Fairchild and Lee, 1978; Kajimoto and Cvetanovic, 1979; Moortgat and Wameck. 1975; Philen et al.. 1977). Figure 2 shows the form of the quantum yield-wavelength curve currently recommended by NASA (DeMore et al.. 1985) for use in stratospheric modelling. The value of the absolute limiting yield will be discussed in section 4, but the figure makes it p&in that there is, indeed,a sharp fall in the relative &‘D)at a wavelength of roughly 310 nm in accordance with the predictions. The exact form of the fall-off curve is temperature dependent (Kajimoto and Cvetanovic, 1976; Kuis et al., 1975; Moortgat et ol., 1977). The absorption spectrum is also temperature dependent, probably as a result of vibrational excitation (Adler-Golden, 1983; Adler-Golden et al., 1982; Astholz et al., 1982). While the chemistry of vibrationally excited ozone is probably of immediate importance only in the mesosphere and above (Rawlins, 1985).changes in absorption cross section and in quantum yield for O(‘D) formation might be important at lower altitudes. especially for photolysis in the wavelength threshold region. Direct infrared laser excitation can leadto changes in the U.V. absorption spectrum (McDade and McGrath, 1980); absorption of solar infrared radiation by atmospheric OS could similarly affect the U.V. absorption. OS in the atmosphere is produced by the 0 + O2 recombination reaction. There is evidence from the laboratory that the newly formed O3 is vibrationally excited (Jocns et al., 1982) and that it displays a modified U.V. spectrum (Kleindienst et 01.. 1980; von Rosenberg and Trainor. 1975a. b). Infrared fluorescence in the v3 band ofozone has been observed in the atmospheric nightglow at altitudes of about 100 km (Rawlins et al., 1985), and is likely to be excited by the recombination &action. Solomon et ol. (1986)discuss further evidence for nonequilibrium excitation of the v3 mode of mesospheric OS, and some of the likely consequences. The contribution of excited O3 to photolysis will obviously depend on vibrational relaxation rates. There is, however, direct laboratory evidence that shows vibrational

WAYNE

excitation to be effective in shifting the 6(’ D)-l curve to longer wavelengths. A two-laser experiment was used (Zittel and Little, 1980) to demonstrate that, beyond the 1 = 3 10 nm threshold, the cross section for O(‘D) production increases by nearly two orders of magnitude on excitation OJ(v,). The effect of vibrational excitation drops with increasing photon energy, a result expected because internal energy needs to make a smaller contribution to the total dissociation energy. The increased photodissociation cross section is not solely attributable to the increased absorption cross section: there seems to be a real enhancement of the quantum yield for O(‘D) production by a factor of about six. Electronic excitation of O3 might also play some part in modifying the &ID)-i relationship. It has been invoked (von Rosenberg and Trainor. 1975a, b) to explain part of the infrared emission observed on the recombination of0 + 0,. In addition, a transient enhanced U.V. absorption in 0, obtained on flash pumping OJ with visible radiation from a dye laser has been attributed to electronic excitation (McGrath et 01.. 1983). One possible explanation of this observation is that states such as l’B, or 11A2, populated in 0, by the absorption of visible light, could themselves absorb the U.V. radiation (e.g. to excite the 2’BI or 1‘B, states). If the states first populated dissociateat 1> 310 nm to yield O(‘D), or if collisional quenching of them produces ‘active’ vibrationally excited OJ, then electronic excitation by visible radiation might enhance the overall rate of O(lD) production in the atmosphere. 4. ABSOLUTE YIELD OF O(‘D)

Two experimental facts upset the neat picture presented in section 2 of thermodynamically controlled spin-conserved photodissociation of OJ. The first concerns the absolute values of &O’D) at wavelengths shorter than the i. = 310 nm threshold. Evidence about to be presented will show that the quantum yield may be less than unity even at wavelengths much shorter than the thermodynamic limit. Discussion of the second unexpected result will be deferred until section 5. Two time-of-flight photofragment spectroscopy studies have indicated that the quantum yield for ground state, 0(3P), atom formation is about 0.1 at i. = 274 nm (Fairchild ef al., 1978) and I = 266 nm (Sparks et al., 1980). Photofragment spectroscopy is carried out by crossing a beam of radiation (photons) with a beam of reactant under collision-free conditions, and analysing the energy (and angular distribution) of release of the fragments. The two studies cited examined the atomic and the molecular fragments, respectively. Figure 3 shows some results of‘ Sparks et al. (1980) obtained by accumulating the molecular oxygen signal for 150,000 laser shots; the figure represents the centre-of-mass translational energy obtained from the photofragment time-offlight spectrum. Most of the photofragments can be

1687

Review: the photochemistry of ozone

Lab angle = 30’

C.M.energy.E’(KCal

mole“)

Fig. 3. Ccntrc-of-mass photofragmentation spectrum of 0, for photolysis at A = 266 nm. Reproduced from Sparks

er al. (1980).

identified with formation of02(r A,) + O(t D), the four peaks corresponding to u = f&l,2 and 3 in the 0,; the broad high energy feature, however, can only result from dissociation in the 0z(3Sg) + 0(3P) channel, which must contribute roughly 10% to the total

dissociation. Fairchild et al. (1978) reached similar conclusions from their study of the atomic oxygen fragment. Flash photolysis experiments tend to confirm that some of the dissociation proceeds via the triplet channel. Photolysis at A =i 248 nm (Amimoto et ai., 1980; Greenblatt and Wiesenfeld, 1983) and at 3. = 256 nm (Brock and Watson, 1980) shows that some O(lP) is formed promptly after the photolytic flash (and thus presumably in the primary step), while additional O(jP) builds up subsequently as a result of the secondary reactions anticipated. Figure 4 displays the results of Greenblatt and Wiesenfeld (1983) for photolysis at 1 = 308 nm (Fig. 4a) and at I = 248 nm (Fig. 4b). At the shorter wavelength, the O(‘P) formed promptly after photolysis again accounts for about 10 % of the photolysis, in agreement with the time-ofRight experiments. The longer wavelength (308 nm) is already in the ‘fail-off region (cf. Fig. 2), so that the production of some O(“P) is expected; the experimentaf result suggests that $fO’D) is 0.79. However, this result indicates another curious facet of the investigations, because combining it with the accepted &* D)-J relationship gives &O’D)=0.96 at 1 = 300 nm. That

Time

-60

-30

0

30

( ps 1

60

so

I20

I50

Time I~z.1

Fig. 4. Transient [O”P]-time detection

profile obtained by resonance fluorescence on photolysis of ozone: (a) I = 308 nm; (b) A= 248 nm. Reproduced from Gmnblatt and Wiesenfeld (1983).

1688

R. P, KAYNE

is, the quantum yield is apparently higher at the threshold than it is at shorter wavelengths. Such behaviour is not excluded on theoretical grounds, although it is not necessarily predicted, either. The formation of any triplet products in the U.V. region is presumably a result of a predissociation from the electronic states that more frequently yield the singlet fragments by direct dissociation Figure 5 represents potential energy curves suggested (Hay PI nf., 1982) for Q,. States A or 3 for a mixture) are those first populated by abs5rption from the ground, X, state, and they correlate with the usual singlet fragments. However, it is speculated that these states are crossed by a repuhive state, tabefied R, that correlates with ground state products. Radiationiess transition to this state wiif then permit the formation of 0(3P). Efficiency of crossing will be determined by the mixing of the electronic states and the velocity ofapproach to the intersection, and the variation of this efficiency with wavelength will thus depend also on the exact energy and geometry of the surfaces. In particufar, if crossing occurred at a much smaller value of R, than that indicated in Fig. 5, so that the energy needed to reach it exceeded the binding energy of the A or B curves, then the triplet channel might be accessible only at wavelengths appreciably shorter than the threshofd for the singlets. Without a better knowledge of the identity and characteristics of the repulsive R surface, further speculation is probably pointless. However, the real values for the quantum yields at wavelengths near 300 nm are of vital importance for quantitative atmospheric modelhng. The current NASA recommendation @More er ol., 1985) is explicit in scaling the quantum yields to a limiting value of #(‘D) of 0.9. Some

modehers continue, nevertheless, to assume a hmit of unity. There is clearly an urgent need to find out if there reatly is an increase in &‘Df from 0.9 to almost 1.0 as the threshold wavelength is approached from the shorr wavelength side. 5. THE

MOLECULAR

0, FRAGMENT OF 0, PHOTOLYSIS

Static photolysis fCastellano and Schumacher, 1969), time of Aight photofragment spectroscopy {Fairchild et al., 1978) and Coherent Anti-Stokes Raman Spectroscopy (CARS: see section 6) (Moore er al., 1983) studies ail indicate that 02(‘As) is not a product of ozone photofysis in the Ionger wavelength Chappuis absorption region, even though there is enough energy for the moiecufe to be excited. This result again accords with the demands of spin conservation as outlined in section 2. Considerable evidence from atmospheric studies had accumulated by the late 1960s to indicate that excited singlet molecular oxygen, O,(‘As), is a product of the U.V. photolysis of 0,. Airglow of the O,(’ As) -+ 02(3Xs) system had been recognized for many years, although early observations were confined to the (0,I) band at 1, = 1.58 pm because of atmospheric reabsorption of the (0,O)band. Aircraft (Noxon, 1982; Noxon and VaI~an~-Jon~, 1962; Wraight and Gadsden, 1975), baboon (Pick et al., 1971), and rocket (Wood et af., 1970) borne instruments allow measurement of the more intense (0,O)emission. Observation of the decay ofintensity at twilight (Noxon and VallanceJones, 1962; Pick et ol.. 1971) or in eclipses (Bantle er al., 1984; Noxon, 1982; Wraight and Gadsden, 19781 can help in the e~u~jdation of excitation and deexcitation mechanisms. Good reviews of the observ-

Fig. 5. Ab inirio potential energy curves for 0,. One O-O bond length (I?,) is varied, with the other (R,) beidfixed812.5lB5hrand with a bond angle of 1I@‘. Repro&reed from Hay et al. (19g2). Copyri&t 1982, American chemical Society.

ational data are given by Llewehyn and McDade (1984) and Llcwellyn et al. (1973). Feak concentrations of O,(lAs), determined by rocket photometry, are typically (Llewellyn PI al., 1973) 2 x lOlo molecule cm-s at SO-69 km altitude, and the concentrations drop rapidly after the Sun fails below the horizon or is eclipsed. A photochemical source of O,(‘A& is therefore indicated, and the only sensible (Wayne, 1967, 1985b) reaction that can give the concentrations observed is the primary step of 0, photodissociation in the U.V. region. Laboratory evidence, described in detail by Wayne (1972,1984,1985b),showsexp~i~tiy that O,(‘As) is the molecular fragment of Oa photolysis in the U.V. Izod and Wayne (1968) first detected emission at ii = 1.27 pm from the products of 0, photolysis, although they believed that 0, was needed as well to generate the singlet molecular oxygen. H&man ef at. (1969) observed vacuum U.V. absorption bands, assigned to O,( ‘4), in 0s irradiated at i = 253.7 nm. Gauthier and Snelling (1970, 1971) attributed optical emission at d = 1.27 pm seen on photolysis of 0, at li = 253.7 nm to O,(*As) formed in the primary step. Al~ou~ 0,(3YZs) could be produced at the photolytic wavelengths used (see table), a limit of < 5 % is placed on its excitation eficiency (Gauthier and Snefling, 1971). Absolute calibration for O,(‘As) emission intensities allowed Jones and Wayne (1971) to show that the species was formed with an efficiency of 0.83 & 0.1 i at I = 253.7 nm. The photofragment spectroscopy experiments (Fairchild er al., 1978; Sparks et al., 1980) confirm that 0,(‘4) is a product of O3 photolysis by U.V. radiation. However, they further suggest, of course, not only that 4(O’D) is less than unity, but also that the quantum yield for 0~(i4~~oduction will be fess than one in the photolysis of 0,. Jones and Wayne (1971) took their measurement of 0.83 + 0.11 to indicate that $(‘As) approached unity, but it now seems that the result should be taken at face value. The significance of this conclusion for atmospheric studies is that the production rates ofOs(‘Asf in thea~ospheremust be scald by the primary quantum yield. One application, for example, where a value of tp rr Q8-0.9 might have an impact is in the derivation of 0, concentrations from airglow intensities in the infrared atmospheric band, O,(’ As -)I “Es), as applied to data from the SME infrared instrument (Thomas et al., 1983, 1984). At the beginning of section 4, it was pointed out that two experimental results conflict with the Simple picture of spin-conserved photolysis oS0,. The second of these results concerns the production of O,(lAs) at wavelengths longer than the j, = 310 nm threshold (Jones and Wayne, 1969, 197Q; Castehano and Schumacher, 1972). The evidence suggests that the quantum yield for O,(‘Asf formation remains unjty at A = 334 nm, even though that for O(‘D) has dropped to essentially zero. That is, the dissociation appears to be spin-forbidden in this weak absorption region. Spin’forbidden’ processes are a consequence af

spin-orbit coupling that makes San imperfect descrip tion of the quantum state. Optical transitions cart thus occur with AS # 0, even though they may be much weaker than fully allowed transitions. Spin-forbidden radiationless transitions are also possible, so that the rigour with which a (pre-)dissociation has to retain its spin state is rdaxed. Hay and Dunning (1977) show that the fragments 0(3P) $0,(14) correlate with the 13B, and 2”B2 states of OS. Wayne (1985b) shows how some electron energy-loss spectroscopic results of Swanson and Celotta (19751 might be used to fix the energy of the 2sB2 state at N 3.6 eV (r 337 nm). Theoretical calculations also predict ~une~nn et al., 1978) that a 3B1 state of GJ lies at about 3.27 eV, corresponding to an onset of absorption at 1 = 379 nm. Direct, but forbidden, population of the 23B, state might therefore be achieved by the weak opticat absorption at 1= 334 nm. An alternative route to the photoiytic fragments that correlate with triplet states of G3 would, ofcourse, be optical absorption in the singlet system, and (forbidden) crossing to the triplet. The production of O,(‘As) at ii > 310 run needs to be conshiered if LB. Atmospheric band intensities are to be used to estimate O3 concentrations in the lower stratosphere and below (Crutu?n et al., 1971). Long wavelength photolysis is likely to show the largest influence for large solar zenith angles and at low altitudes. Figure 6 gives one way of displaying the data: here, the fraction of 0,f’As) produced in the ‘forbidden’ region is shown as a function of solar xenith angle for three altitudes under irradiance typical of spring. Even at an altitude of 30 km, for zenith angles greater than about 70” half the O,(‘As) is derived from the spectral region not usually included. It is evident that disregard of the contribution made by long wavelength photolysis is unjustifiable, even though the scaling of the O,(* As)yield in the ‘ahowed’ region, needed for the

Fig. 6. ThefraclionFofO,(‘Al)produccd by radiation at wavelengths longer than 310 nm plotted 8s a function of the solar zenith angle for three aititudes during spring. From Crutzen ef al. (1971).

R. P. WAYNE

1690

reasons presented at the beginning of this section, may partially compensate for the shortfall. 6. TOWARDS

AN INTERPRETATJON

OF DJSSOCJATJON

DYNAMJCS

Increasingly sophisticated experiments are being performed to probe the detailed dynamics of 0, dissociation. Although the thrust of these investigations is towards an understanding of the underlying physico~he~~ nature ofdissoc~tion, the new information will also benefit the studies of atmospheric scientists. Trends in the experiments discussed so far have been to study the fragments of photolysis at ever shorter times after the absorption of the dissociating photon, and with the products as closely as possible reflecting the internal and external energies with which they were born. For a start, the ability to determine the electronic states of the fragments-a matter of supreme importance for atmospheric chemists-argues a capability of finding out about electronic excitation before relaxation processes have begun to force the system towards thermal equilibrium. Even relatively modest experiments have been used to look at the products on a time-scale short enough that vibrational relaxation has not wiped out more detailed information. Klais et ai. (1980) were able to use Bash photolysis, followed by absorption in the vacuum U.V. region, to study vibrationally excited O,(’ As). The experiments not only provide further confirmation that O,(‘As) is a primary product of photolysis, but also give an idea of the partitioning into the accessible vibrational levels. Time-of-flight photofragment spectroscopy provides even more detailed information. For example, Sparks er al. (1980) use the product energy information in injunction with polarization dependence data to show that the relative yields of photolysis at I

1

I

1390

I 1410

I

= 266 nm into v’ = 0,1,2 and 3 of O,(‘A,) are 0.57, 0.24, 0.12 and 0.07. The exact position of the I;’ = 0 peak indicates that about 17 % of the energy remaining after production of this level is deposited in rotational excitation, a result in accord with a model in which energy is released impulsively from a geometry near that of the ground state of 0,. Fairchild er al. (1978) investigated, in addition to the total energy release, the angular distribution of the fragment species, for dissociation both in the U.V. and the red regions of the spectrum. They show how the experimental data are consistent with the orientation of the transition dipoles in the dissociating molecule with respect to the electric vector of the photolytic laser radiation. The data are also used to explain the width of the time-of-flight spectra according to the same model of dissociation. Coherent Anti-Stokes Raman Spectroscopy (CARS) is a multiphoton spectroscopy dependent on nonlinear effects resulting from the use of high intensity lasers. Very high time resolution is inherent in the CARS technique, because the time scale is that of the Raman process itself, and the gathering of data under collision free conditions is much simpler than with absorption or infrared emission spectroscopy (Moore er al., 1983). Furthermore, the technique is of quite general applicability, since optically accessible electronic states are not necessary in Raman spectroscopy. That means that ground state molecular oxygen, 0,(32s), can be investigated in CARS studies of 0, photolysis as well as O,(‘As). CARS studies in the U.V. (Valentini, 1983) show that all four accessible vibrational states of O,(‘A,) are populated, as expected on the basis of the flash photolysis and photofragment results described earlier. Unexpectedly, however, the rotational structure shows an anomalous propensity for even J, as indicated in Fig. 7. Intensity alternations of this kind cannot be due to spectroscopic eflects or

I

Aomon

I

I

1430

l4!m shlil

I

I 1470

I

I

1490

(cm-‘)

Fig. 7. Vibrational Q-branch (ALI = 0, AJ = 0) CARS spectrum of the 02(’ As) fragment from the photolysis of ozone at i. = 266 nm. The grid above the spectrum identifies the transitions in c, J format. The trace at the very top is that of the dye laser etalon fringes, while that at the bottom is the spectrum of the empty cell. Reproduced from Vaientini (1983).

Review:the photochemistry of nuclear degeneracy differences. Rather, symmetry rcstrictions or a dynamical bias in the photodissociation must be behind the alternations. Intuitive interpretations that associate the population differences with simple symmetry restrictions turn out to be invalid. The effect is not yet properly understood, although it is thought that the explanation will involve symmetry considerations, possibIy with some dynamical constraints operative. Visible region phot~i~~ation of 0, has also been studied by the CARS techmque (Moore et al., 1983).The disposition of the energy into translation, rotation and vibration was interpreted in terms of some simple models of the dissociation dynamics. Figure 8 shows one of the interesting results: there is a population inversion of u = 3 with respect to u = 2 of O,(‘Zs). Jnterpretation of dynamical behaviour will demand, of course, reliable potential surfaces for the interaction. In the context of testing potential energy surfaces, emission studies ffmre er al, 1982, 1984) of the dissociating motecufe offtr a valwbfe source ofexperimental information. Molecules that photodissociate, such as OJ, are not normally thought ofas fluorescent, because spontaneous emission is orders of magnitude slower than dissociation. However, with a high enough intensity of illumination, the few photons that are emitted can be detected. The emission process can be considered either as fluorescence or as resonantly enhanced Raman scattering; the point of consequence is that because the photons that areemitted refer to the shortest possible time-scale, while the two photofragments are still cfose to each other, they probe the unbound potential surfaces on which dissociation occurs. Conventional spectroscopic techniques afford no possibility of studying such surfaces. The frequencies of the fluorescence spectra are characteristic of the vibrational spacing of the electronic state from which absorption arises, while the intensities reflect the dynamics on the upper surface. Becausethe molecule is dissociating, it passes through infinite displacements,

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ozone

thus allowing effective Franck-Condon overlap over a wide range of lower-state vibrational levels.In the case of OS, vibrational states as close as 500 cm- ’ to the dissociation limit arc seen (Imre et al., 1982), as iliustrated in Fig. 9. However, an even more fascinating aspect of Fig. 9 is that levels containing the bending mode (vt) are not invoJved at alI, nor are levels confining odd quanta of the an~s~rn~ri~ stretch (vs). One can conclude immediately that the bending mode contributes little to the initial stages of dissociation. The absence of the odd v3levelsis attributed to the way in which the ground-state wave packet is transferred to the excited state in the absorption event. Spreading of the wave packet along the coordinate of the v3 vibration maintains symmetry, causing the overlap with the antisymmetric odd quanta to vanish. More subtle information can be extracted from the data (Jmre er al., 1984),and detailed analyses of the dynamics may ultimately provide the sofutions to the remaining problems of O3 phot~~i~ry that both physical chemists and atmospheric scientists await. Full d~onvoiution of the upper and lower surface information will demand a combination of &ever experiment with clever theory.

7. CONCLUSION The photodissociation of O3 is of crucial importance in the Earth’s atmospheric chemistry. In the U.V. region, photoiysis leads to the production of energyrich O(‘D) that itself drives much chemistry, including that of the OH radical. An unders~nd~~g of the dependence on wavebngth of tJre quantum yield for O(‘D) formation is recognized as essential in quantitative modeling of atmospheric chemical behaviour. The molecular fragments of the U.V. photoiysis can also be excited. Such excited molecules not only contribute to the natural airglow, but emissions from them are now routinely used to derive atmospheric

02fZ~l phobfragment internal energy flux distribution for Dnotatysis at 18 797 cm“ f 532 nm 1

Fig. 8. internal energy Rux distribution in the O&T ) fragment from photalysis of 0, at 2 = 532 nm. Reproduced from h&ore et nl. (1983).

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R. P. WAYNE

I

Fig. 9. Photoemission

spectrum of 0, excited at i. = 266 nm (resolution = 0.8 nm FWHM). inset shows higher resolution (0.035 nm FWHM) scan of the 100 peak. The Raman fundamental ofground state 0, appears weakly, and is labelled on the figure. Reproduced from lmre et ol. (1984). Copyright 1984, American Chemical Society.

concentrations of the precursor Oj. Once again, correct calculation ofconcentrations depends critically on an accurate knowledge of the efficiency with which the excited molecules are born, and of how that efficiency varies with wavelength. Laboratory chemical experiments have provided the quantitative information on which atmospheric modelling depends. These experiments have shown that there is a threshold wavelength, at 1 r 310 nm, for formation of O(‘D). At longer wavelengths, ground state atomic oxygen, O(‘P), is formed. The threshold corresponds to the thermochemical energy limit for production of two singlet (“paired spin”) fragments, O(‘D) + O1(‘As). At longer wavelengths,spincan only be conserved by formation of two triplet products, O(‘P)+ O,(32,), which are the electronic ground states. Current accepted thinking in atmospheric chemistry ascribes a quantum efficiency of unity for formation of the two singlet products of O3 photolysis at wavelengths short of the 1% 310 nm threshold region, and zero at wavelengths longer than it. The transition between the two photolysis regimes is not abrupt, because internal energy in the O3 can make some contribution to the total cncrgy nccdcd for dissociThe exact shape of the quantum ation. yield-wavelength curves is particularly significant in atmospheric calculations. since it falls in a spectral region where both solar intensity and the 0, absorption cross section vary rapidly with wavelength. Several laboratory studies, described in this review, have elucidated the shape of the curve with considerable precision. In spite of the outline of the O3 photodissociation phenomenon that has emerged from the laboratory studies, there remain some disquieting features, especially from the point of view of applications to atmos-

pheric models. The principal thesis of this review is that the laboratory studies already cast doubt on the way in which the photochemical data are used. Those studies afford ample evidence that the idea ofa transition from singlet to triplet products, each formed with unify eficiency beyond the threshold region, is actually wrong. First, at wavelengths well short (240-270 nm) of the threshold (z 310 nm), the quantum yield for O(‘D) formation does not exceed 0.9. The value mighl be unity at wavelengths nearer 300-310 nm, which are atmospherically more important, but, on the available evidence, it also might remain at 0.9. Secondly, the same arguments may apply to the e!Iiciency of Oz(’ AJ formation, so that assumption of unity production efficiency for all wavelengths shorter than 310 nm may lead to an underestimate of atmospheric O3 concentrations derived from airglow intensities. Thirdly, and in contrast, the experimental evidence also shows that O,(‘A,)continues to be formed with high efficiency at wavelengths at least as long as 334 nm, a situation excluded by the generally-accepted view of spinconserved photolysis. Production of 02(‘As) in the longer wavelength region would mean an additional contribution to the Infrared Atmospheric Band intcnsity beyond that othcrwisc expcctcd. Although the conclusions set out above might be strengthened by further experiments designed to look expressly for the ‘unexpected’ atomic and molecular products, and to assess the eficiency of their production in the threshold region, one major obstacle to further progress is the evidently limited understanding of the photodissociation process. The elements of the physical chemistry involved are clear enough, but interpretation of the details of dissociation is still awaited. Why dissociation should proceed in a certain way is one of the questions addressed by the study of photodissociation dynamics. Sophisticated dynamical

Review: the photcchemistry of ozone

theories might not at first sight seem relevant to atmospheric studies, but they most surely are since successful interpretation at the more detailed level implies an improved understanding of the overall photolytic process. This review therefore outlines the kinds of experiment currently in progress, and the part the results could play in extending the theory of experiments and

dissociation.

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