4084
J. Phys. Chem. 1996, 100, 4084-4089
Photodissociation Processes of Ozone in the Huggins Band at 308-326 nm: Direct Observation of O(1D2) and O(3Pj) Products Kenshi Takahashi, Yutaka Matsumi,* and Masahiro Kawasaki Institute for Electronic Science, and Graduate School of EnVironmental Science, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: September 22, 1995; In Final Form: NoVember 30, 1995X
Both O(1D) and O(3Pj) atoms produced in the photodissociation of O3 were directly detected, using a vacuum ultraviolet laser-induced fluorescence technique. The photofragment yield spectra of the O(1D) and O(3Pj) atoms were recorded at 298 K, scanning the photodissociation laser wavelength across the range 308-326 nm in the Huggins band system of O3. The photofragment yield spectrum for O(3Pj) has the vibrational structure as appearing in the absorption spectrum in the Huggins band, while the spectrum for O(1D) is smooth and has no structure. The wavelength dependence of the quantum yield for O(1D) from the O3 photolysis exhibits a dip at every peak of the vibrational structure in the absorption spectrum in the Huggins band. The partial cross sections for the O(1D) production process in the photoabsorption of O3 were determined. The existence of the tail around 315 nm in the wavelength dependence of the quantum yield of O(1D) produced from O3 photolysis was verified. Our results indicate that the tail arises predominantly from the hot-band excitation of O3.
Introduction The ultraviolet (UV) absorption spectrum of ozone consists of two bands; a strong absorption band in the wavelength region 200-310 nm which is called the Hartley band (1B2 r X1A1) and the much weaker band system called the Huggins band in the range 310-360 nm.1 The Huggins band has a series of diffuse vibrational bands.2,3 The known spin-allowed dissociation channels in the O3 photolysis in the UV region are
O3 + hν f O(1D) + O2(a1∆g)
(1)
O3 + hν f O(3P) + O2(X3Σ-g)
(2)
The thermodynamic threshold of the dissociation wavelength for channel 1 is 310 nm,4 while that for channel 2 is about 1180 nm. Among the products, the O(1D) atoms are of special interest in atmospheric chemistry as they are very reactive. For example, the reaction of O(1D) with H2O molecules is the primary source of OH radicals which are the most important oxidizing species in the troposphere and lower stratosphere. In modeling troposphere chemistry, the quantum yield of the O(1D) atoms from the O3 photolysis is important.5,6 The wavelength range of interest is limited to 290-330 nm by the sunlight spectrum which penetrates into the troposphere and by the absorption property of O3. Many investigations have been reported on the wavelength dependence of the quantum yield of O(1D) from the photolysis of O3 in this wavelength range.7-12 All the investigations indicated that the quantum yield of O(1D) was almost constant for wavelengths λ e 305 nm and fell sharply around the threshold wavelength of 310 nm which is the thermodynamic threshold wavelength for the reaction channel 1. However, there is considerable controversy concerning the quantum yield of O(1D) at the long wavelengths around 315 nm. For 298 K, JPL recommended values4 are 310 nm do not have energy enough to dissociate to the O(1D) + O2(1∆g) products which are correlated with the excited state (1B2 and/or 21A1). Thus, it is reasonable that the vibrational structure in the absorption spectrum of the Huggins band appears in the photofragment yield spectrum for the O(3P2) atoms (Figure 1), which are produced by a predissociation process via the excited state (1B2 and/or 21A1). Michelsen et al.32 have suggested a contribution of the hotband process to the production of O(1D) in the wavelengths between 311 and 320 nm, based on a model calculation for temperature dependence of the O3 absorption spectrum. Ball et al.33 measured the kinetic energy release of the O2(1∆g) fragment from the O3 photolysis at 280-331 nm, using the technique of [2 + 1] resonance-enhanced multiphoton ionization and the time-of-fright spectroscopy. Their results showed that there was a contribution from the photolysis of vibrationally excited O3 molecules produced via spin-allowed process O(1D) + O(1∆g) in the wavelength range 309-331 nm. Our results indicate that the photoabsorption of O3 in this wavelength region of the Huggins band consists of at least two parts: one is cold absorption to the bound states which exclusively dissociate to the O(3P) + O2(3Σ-g) products. The other is the continuum absorption from the hot rovibronic levels in the X1A1 state to the repulsive limb of the excited state (1B2 and/or 21A1) which results in the dissociation to O(1D) + O2(1∆g). Moreover, according to our photofragment yield spectrum for O(1D) enlarged three times as shown in Figure 3, the slope changes below and above 321 nm. This slope change allows us to speculate the existence of two different photodissociation processes below and above 321 nm. The wavelength of 321 nm is near the threshold of contribution from hot-band excitation predicted by Mitchelsen et al.32 Recently, Ball et al.33 found that the O2(1∆g) fragment produced in the wavelengths between 320 and 331 nm had higher kinetic energy than those formed by photolysis at a shorter wavelength. Their results were consistent with the spin-forbidden dissociation pathway O(3P) + O2(1∆g). The existence of a spin-forbidden dissociation pathway has some grounding in theoretical calculations of the potential energy surfaces of O3.30 Several repulsive triplet surfaces correlating to O(3P) + O2(1∆g), O(3P) + O2(1Σ+g), and O(1D) + O2(3Σ-g) have been found to cross the D ˜ 1A′ singlet surface which itself correlates in Cs symmetry with the 1B2 state.30 In our experiment, thus, nonzero contribution
Photodissociation Processes of Ozone to the formation of O(1D) in the wavelength longer than 321 nm may be attributable to a spin-forbidden process O(1D) + O2(3Σ-g). The absolute values of the partial cross sections for the O(1D) production process are indicated in Figure 3 for our results as well as the data calculated from the quantum yield of the O(1D) atoms recommended by NASA/JPL4 and the total absorption cross sections presented by Molina and Molina.25 The slope of our photofragment yield spectrum for O(1D) spectrum changes drastically below and above 311 nm which is near the thermodynamical threshold wavelength for the dissociation channel 1. The yield sharply decreases below the dissociation limit wavelength and then slowly decreases above the one. The latter parts are not observed in the photofragment yield curve for O(1D) calculated with the JPL/NASA recommended data (open circles in Figure 3). This part results in the “tail” in the wavelength dependence of the quantum yield of O(1D) in Figure 6. This “tail” for 311-325 nm was reported in some previous investigations at room temperature.9-12 As shown in Figure 6, our results are in good agreement with the earlier results presented by Trolier and Weisenfeld.10 In the recent measurements of the quantum yield of O2(1∆g) by Ball et al.,11 the “tail” was also observed in the wavelength dependence of the O2(1∆g) yield from the photolysis of O3, using the technique of [2 + 1] resonance-enhanced multiphoton ionization. Michelsen et al.32 suggested a hot-band model for the evaluation of the quantum yield of O(1D) produced from the O3 photolysis, and concluded that the calculated yield was 0.2-0.3 for wavelength range 312-320 nm. The presence of “tail” in our study as shown in Figure 6 is consistent with those reports;9-12,32 in contrast, the NASA/JPL recommended quantum yield of O(1D) is negligible in wavelength region above 310 nm.4 Although the quantum yield of O(1D) produced at λ > 311 nm is relatively small, the process may be of considerable consequence in the atmosphere, particularly at lower stratosphere and troposphere where O3 in the atmosphere absorbs the majority of the incident ultraviolet light. For lower stratospheric and tropospheric conditions, only O3 is a precursor molecule for O(1D), and, for example, the O(1D) atom leads to the generation of OH radicals which is the very important oxidizing particle in the troposphere.34 Our results suggest that the “tail” in the wavelength range 310-321 nm comes largely from a contribution of hot-band excitation to the repulsive limb of the excited state (1B2 and/or 21A1) and that there is a possibility of the formation of O(1D) atom from the spin-forbidden dissociation process of O(1D) + O2(3Σ-g) at λ > 321 nm. It is not likely that this spin-forbidden channel contributes largely to the formation of O(1D) in the range 308-326 nm. Because if this spin-forbidden process were predominant and responsible for the formation of O(1D) in this range, the vibrational structure observed in the photoabsorption spectrum of O3 in the Huggins band should appear in the photofragment yield spectrum for O(1D). The temperature dependence of the quantum yield of O(1D) from the O3 photolysis has been also paid attention and discussed.35-38 Because the O(1D) formation in the “tail” region at λ > 310 nm is due to a hot-band excitation for dissociation to O(1D) + O2(1∆g), with decrease of the temperature the quantum yield of O(1D) may decrease more rapidly than the absorption coefficient in that wavelength range. Moreover, the dip structure in the O(1D) quantum yield at the peaks of the absorption spectrum (Figure 6) may become deeper at low temperatures, because the vibrational structure of the Huggins band at low temperature becomes more discrete than at 298 K and the absorption coefficient becomes smaller.2,24,39
J. Phys. Chem., Vol. 100, No. 10, 1996 4089 Acknowledgment. This work is partly supported by a Grantin-Aid in Priority Field of “Free Radical Science” from the Ministry of Education, Science and Culture, Japan (Y.M.). We would like to thank Dr. Takashi Imamura of National Institute for Environmental Science and Dr. Gus Hancock of Oxford University for their helpful discussions. Y.M. thanks British Council for their financial support to visit UK and to have a chance to discuss with Dr. Gus Hancock. We are also grateful to Mr. Masahiro Kishigami for his help on the experiments. References and Notes (1) Steinfeld, J. I.; Adler-Golden, S. M.; Gallagher, J. W. J. Phys. Chem. Ref. Data 1987, 16, 911. (2) Simons, J. W.; Paur, R. J.; Webster III, H. A.; Bair, E. J. J. Chem. Phys. 1973, 59, 1203. (3) Katayama, D. H. J. Chem. Phys. 1979, 71, 815. (4) DeMore, W. B.; Sander, S. P.; Howard, C. J.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Hampson, R. F.; Kurylo, M. J.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, No. 11; JPL Publication 94-26, NASA: Pasadena, 1994. (5) Salawitch, R. J.; et al. Geophys. Res. Lett. 1994, 21, 2547. (6) Muller, M.; Kraus, A.; Hofzumahaus, A. Geophys. Res. Lett. 1995, 22, 679. (7) Moortgat, G. K.; Warneck, P. Z. Naturforsch. 1975, 30A, 835. (8) Arnold, I; Comes, F. J.; Moortgat, G. K. Chem. Phys. 1977, 24, 211. (9) Brock, J. C.; Watson, R. T. Chem. Phys. 1980, 46, 477. (10) Trolier, M.; Weisenfeld, J. R. J. Geophys. Res. 1988, 93, 7119. (11) Ball, S. M.; Hancock, G.; Murphy, I. J.; Rayner, S. P. Geophys. Res. Lett. 1993, 20, 2063. (12) Armerding, A.; Comes, F. J.; Schulke, B. J. Phys. Chem. 1995, 99, 3137. (13) Amimoto, S. T.; Force, A. P.; Wiesenfeld, J. R.; Young, R. H. J. Chem. Phys. 1974, 73, 1244. (14) Brock, J. C.; Watson, R. T. Chem. Phys. Lett. 1980, 71, 371. (15) Greenblatt, G. D.; Weisenfeld, J. R. J. Chem. Phys. 1983, 78, 4924. (16) Shamsuddin, S. M.; Inagaki, Y.; Matsumi, Y; Kawasaki, M. Can. J. Chem. 1994, 72, 637. (17) Hilber, G.; Lago, A.; Wallenstein, R. J. Opt. Soc. Am. 1987, B4, 1753. (18) Hilbig, R.; Wallenstein, R. Appl. Opt. 1982, 21, 913. (19) Matsumi, Y.; Shamsuddin, S. M.; Sato, Y.; Kawasaki, M. J. Chem. Phys. 1994, 101, 9610. (20) Gerstenkorn, S; Luc, P. Atlas du Spectre D’Absorption de la Molecule D’Iode; C.N.R.S.: Paris, 1978. (21) Sinha, A.; Imre, D.; Goble Jr., J. H.; Kinsey, J. L. J. Chem. Phys. 1986, 84, 6108. (22) Freeman, D. E.; Yoshino, K.; Esmond, J. R.; Parkinson, W. H. Planet. Space Sci. 1984, 32, 239. (23) Abe, M.; Sato, Y.; Inagaki, Y.; Matsumi, Y.; Kawasaki, M. J. Chem. Phys. 1994, 101, 5647. (24) Davidson, J. A.; Schiff, H. I., Brown, T. J.; Streit, G. E.; Howard, C. J. J. Chem. Phys. 1978, 69, 1213. (25) Molina, L. T.; Molina, M. J. J. Geophys. Res. 1986, 91, 14501. (26) Katayama, D. H. J. Chem. Phys. 1986, 85, 6809. (27) LeQue´re´, F.; Leforestier, C. Chem. Phys. Lett. 1992, 189, 537. (28) Brand, J. C.; Cross, K. J.; Hoy, A. R. Can. J. Phys. 1978, 56, 327. (29) Banichevich, A.; Peyerimhoff, S. D. Chem. Phys. 1993, 174, 93. (30) Banichevich, A.; Peyerimhoff, S. D.; Grein, F. Chem. Phys. 1993, 178, 155. (31) Joens, J. A. J. Chem. Phys. 1994, 101, 5431. (32) Michelsen, H. A.; Salawitch, R. J.; Wennberg, P. O.; Anderson, J. G. Geophys. Res. Lett. 1994, 21, 2227. (33) Ball, S. M.; Hancock, G; Pinot de Moira, J. C.; Sadowski, C. M.; Winterbottom, F. Chem. Phys. Lett. 1995, 245, 1. (34) Wayne, R. P. Chemistry of Atmospheres, 2nd ed.; Oxford University Press: Oxford, 1991. (35) Kajimoto, O.; Cvetanovic´, R. J. Chem. Phys. Lett. 1976, 37, 533. (36) Moortgat, G. K.; Kudszus, E.; Warnick, P. J. Chem. Soc., Faraday Trans. 2 1977, 73, 1216. (37) Ball, S. M.; Hancock, G. Geophys. Res. Lett. 1995, 22, 1213. (38) Adler-Golden, S. M.; Schweitzer, E. L.; Steinfeld, J. I. J. Chem. Phys. 1982, 76, 2201. (39) Brion, J.; Chakir, A.; Daumont, D.; Malicet, J.; Parisse, C. Chem. Phys. Lett. 1993, 213, 610.
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