176
J. Phys. Chem. 1996, 100, 176-179
Vibrational Distribution of ClO Radicals Produced in the Reaction Cl + O3 f ClO + O2 Yutaka Matsumi,* Shuichi Nomura, and Masahiro Kawasaki Institute for Electronic Science and Graduate School of EnVironmental Sciences, Hokkaido UniVersity, Sapporo 060, Japan
Takashi Imamura Atmospheric EnVironment DiVision, National Institute for EnViromental Studies, Tsukuba 305, Japan ReceiVed: July 10, 1995; In Final Form: October 3, 1995X
The nascent vibrational distribution of ClO(X2Π) radicals produced in the ozone destruction reaction Cl + O3 f ClO + O2 has been measured, using vacuum-ultraviolet laser-induced fluorescence of the ClO(C2Σ-X2Π) transition. The nascent vibrational distribution of the ClO radicals is shown to be strongly inverted, with the relative ratio (V)0):(V)1):(V)2):(V)3):(V)4):(V)5) ) 0.8:1:1.3:2.4:2.9:2.7. The slope of the surprisal plots for the vibrational distribution of the product ClO is found to be (-8 ( 2). Vibrational relaxation processes for the ClO radicals in the X2Σ state by collisions are also studied. The upper limit values of the vibrational relaxation rate constants for ClO (V)1f0) by collisions with N2 and Ar molecules are estimated to be both 5 × 10-14 cm3 molecule-1 s-1.
Introduction Chlorine atoms can destroy ozone catalytically in the Earth’s atmosphere via the following reactions:1
Cl + O3 f ClO + O2 ClO + O f Cl + O2
∆H ) -38.6 kcal/mol
(1) (2)
a quantitative analysis for the vibrational distribution, they suggested an inverted distribution for the nascent ClO radicals. The recommended value for the rate constant of the reaction 1 is 1.2 × 10-11 cm3 molecule-1 s-1 at 298 K.9 In this study, we report the nascent vibrational distribution of ClO radicals produced by reaction 1 and upper limit values for the rate constants of the vibrational relaxation processes for ClO(X2Π,V)1) by collisions with N2 and Ar.
Net: O + O3 f 2O2
Experimental Section
Chlorofluorocarbons (CFCs), which are released into the atmosphere from the ground, can produce Cl atoms from their photolysis by the sunlight in the stratosphere. Since a close inverse correlation between the concentrations of O3 and ClO has been observed in the Arctic and Antarctic regions,2 it has been suggested that the catalytic destruction cycle involving ClO is a principal loss mechanism of ozone from the stratosphere. Despite the importance of these reactions, their dynamics are not well understood. This is because in the past there has been no sensitive and state-selective detection method for ClO radicals available. Recently, Matsumi et al.3,4 succeeded in detecting the laser-induced fluorescence of ClO radicals, using the C2Σ-X2Π transition at 170 nm. They have reported the nascent rotational and vibrational distributions of the ClO radicals produced in the reactions of O(1D) with HCl, CCl4, and chlorofluoromethanes.4 Also Baumga¨rtel and Gericke5 reported the detection of the ClO radicals produced in the photodissociation of OClO with a resonant two-photon excitation and fluorescence detection method using 350 nm laser light for the ClO C2Σ--X2Π transition. McGrath and Norrish6,7 observed ClO radicals vibrationally excited up to V ) 5 produced by reaction 1 using the flash photolysis-transient absorption spectroscopy. Choo and Leu8 have indicated that the yields of electronically excited O2 molecules, O2(1Σg+) and O2(1∆g), are less than 2.5% from the Cl + O3 reaction. Baumga¨rtel and Gericke5 reported the twophoton fluorescence excitation spectrum of ClO radicals produced in the reaction of Cl + O3. Although they did not present
Figure 1 shows a schematic diagram of the experimental system. Cl atoms were generated by the photodissociation of Cl2 at 355 nm. A mixture of Cl2 and O3 was irradiated with the third harmonic (355 nm) of a Q-switched YAG laser (Quanta-Ray GCR190, 10 Hz, 20 mJ/pulse). The ClO radicals produced by the reaction were probed using vacuum ultraviolet laser-induced fluorescence (VUV LIF).3 The VUV laser light was generated by four-wave mixing (2ω1-ω2) in Xe gas (10 Torr),10 using two dye lasers (Lambda Physik, FL3002) both pumped by a single XeCl excimer laser (Lambda Physik, LEXTRA-50, 308 nm, 200 mJ/pulse). The output of the dye laser (Coumarin 307) was frequency-doubled by a BBO crystal for ω1. The wavelength of ω1 was 249.62 nm which was twophoton resonant with the Xe 6p[1/2]0 state. The wavelength of ω2 was tuned in the range 390-470 nm, using PBBO, stilbene 3, coumarin 120, and coumarin 2 laser dyes. The laser energies were about 0.5 and 5 mJ/pulse for ω1 and ω2, respectively. The generated VUV laser light entered the reaction cell through a LiF window. The output of the VUV light passing through the reaction cell was monitored with a VUV monochromator and a solar-blind photomultiplier. The VUV LIF of ClO (C2Σ--X2Π) was detected by a solar blind photomultiplier (Hamamatsu Photonics, R1259) and the signal of the photomultiplier was fed into a gated integrator (Stanford Research, SR250). The reaction cell was evacuated by a rotary pump (330 L/min) through a liquid nitrogen trap. For measurements of the nascent vibrational distribution, a fast gas-flow rate was required. Ar gas (2 Torr) was added to the mixture of reactant gases to increase the flow rate in the reaction cell and to thermalize the velocity of the reactant Cl atoms produced by the photodisso-
X
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0176$12.00/0
© 1996 American Chemical Society
Vibrational Distribution of ClO Radicals
J. Phys. Chem., Vol. 100, No. 1, 1996 177
Figure 1. Schematic diagram of the experimental apparatus for the laser-induced fluorescence detection of ClO(C2Σ--X2Π3/2) from Cl + O3.
ciation. Since the reactivity of ClO radicals was low, accumulation of ClO in the cell over successive laser pulses took place when the flow rate was slow. Checks for ClO accumulation were made every measurement by taking the spectrum under the same conditions but with the probe laser fired before the photodissociation laser. We also sometimes used a lower repetition rate (5 Hz) to avoid the accumulation of ClO radicals in the cell especially for the quantitative measurement of the V ) 0 state. O3 was prepared by passing O2 through a commercial ozonizer, collected on silica gel cooled to liquid nitrogen temperature and then stored in a glass bulb. Cl2, N2, and Ar were obtained commercially and used without further purification. The pressure in the reaction cell was monitored with a capacitance manometer. The partial pressure of Cl2 was maintained at 70 mTorr. Although the purity of the O3 reagent was not measured simultaneously, the partial pressure of O3 was estimated to be 70 mTorr from the measurements of the rise time of the ClO signal in the reaction of Cl + O3, using the above recommended rate constant for reaction 1. Results and Discussion Figure 2 shows typical laser excitation spectra of the ClO radicals produced from the Cl + O3 reaction, which are parts of the (V′, V) ) (0, 3) and (0, 4) vibronic bands of the C2Σ V′ - X2Π3/2 V transition. The measured spectra were compared with the simulations calculated using the molecular constants of ClO presented by Coxon et al.11,12 Matsumi et al.3 have indicated that predissociation processes are much faster than radiative decay for the V′ g 1 levels of the ClO C2Σ state, since the fluorescence intensity from the V′ g 1 levels was very weak. Therefore, for quantitative measurements of the vibrational distribution of the ClO radicals, the excitation spectra of the (0, V) bands of the C2Σ-X2Π transition were used. The Franck-Condon factors for the C2Σ-X2Π (0, V) bands4 are listed in Table 1. Unfavorable Franck-Condon factors for the transitions from V g 6 levels in the X state to the V ) 0 level in the C state precluded measurements of the distribution for vibrational states with V g 6, although the vibrational levels up to V ) 17 can be populated energetically in reaction 1. To examine the nascent vibrational distribution of ClO, we recorded fluorescence excitation spectra using a buffer gas (2 Torr of Ar) probing at delay times of 5-20 µs after the
Figure 2. Laser-induced fluorescence excitation spectra of (V′, V) ) (0, 3) and (0, 4) bands of ClO (C2Σ- V′ ) 0-X2Π3/2 V) from Cl + O3. [Cl2] ) 70 mTorr, [O3] ) 70 mTorr, and [Ar] ) 2 Torr. The time delay between the photolysis and probe is 20 µs. Numbers of the rotational assignments for the Q21 and Q1 branches are (J - 1/2).
TABLE 1: Nascent Vibrational Distribution of ClO Radicals Produced in the Reaction of Cl + O3 ClO (V)
rel populationa
FCFb (0, V)
0 1 2 3 4 5 6
0.8 ( 0.2 1 1.3 ( 0.2 2.4 ( 0.4 2.9 ( 0.6 2.7 ( 0.7 c
0.3093 0.3379 0.2067 0.0939 0.0354 0.0118 0.0036
a Normalized at V ) 1. b Franck-Condon factor for the ClO C2ΣX2Π (0, V) transition. Taken from ref 4. c Not measured.
photodissociation of Cl2. The rotational structure and intensity distribution of the (0, V) band spectra obtained experimentally were well reproduced by simulations assuming a rotational temperature of 300 K and a line width of 0.6 cm-1. Under these experimental conditions, the rotational distribution of the product ClO was relaxed to room temperature by collisions with the buffer gas, while the collisional relaxation of the vibrational distribution was negligibly small. The intensity ratios among the (0, V) vibronic bands in the excitation spectra were not changed by varying the delay time in the range 5-20 µs.
178 J. Phys. Chem., Vol. 100, No. 1, 1996
Matsumi et al.
Figure 3. Nascent vibrational populations of ClO radicals produced from reaction of Cl + O3, which are normalized at the V ) 1 state.
According to the vibrational relaxation rate for V ) 1 f V ) 0 which will be described below, the nascent vibrational population is degraded minimally by collisions (less than 15%) at delay time of 5-20 µs under these experimental conditions. It should be noted that the Cl atoms generated by the photodissociation of Cl2 should be translationally relaxed by collisions with the buffer gas before the reaction and therefore most of the reaction between Cl and O3 take place under thermal conditions. The intensity of the vibronic bands was analyzed using the formula
Figure 4. Surprisal plots for the vibrational populations of ClO radicals produced from reaction of Cl + O3. fV is the fraction of the total available energy in the product vibration. The slope of the solid line for the linear surprisal, λV, is -8.
∞
I(0,V) ∝ ν0Vq0,VNV
q0V′′ν0,V′′3Q0,V′′ ∑ V′′)0
(3)
where I(0,V) is the fluorescence intensity of the vibronic band C2Σ-X2Π(0,V) in the excitation spectra, ν0V is the frequency of the vibronic band (0, V), q0V is the Franck-Condon factor for the (0, V) transition, NV is the population in the Vth vibrational state, and Q0V′′ is the efficiency of the photomultiplier at the emission frequency for the (0, V′′) vibronic band. The values of Q0V′′ were taken from the manufacture supplied frequency response curve of the photomultiplier. The populations of the both 2Π3/2 and 2Π1/2 spin-orbit states in the ClO X2Π state were taken into account. The nascent vibrational distribution obtained of the ClO radicals produced in the reaction of Cl + O3 is listed in Table 1 and plotted in Figure 3, which are normalized at the V ) 1 state. The nascent vibrational distribution of the ClO radicals in the reaction Cl + O3 is strongly inverted. Figure 4 shows the surprisal plots13 for the nascent ClO. The prior distribution of the vibrational states in the ClO products are calculated using the rigid rotor harmonic oscillator approximation:14 0
P (V|E) )
(E - EV)7/2
∑V (E - EV)
(4) 7/2
where E is the available energy (38.6 kcal mol-1). The slope of the surprisal plots λV for the ClO radicals from reaction 1 is (-8 ( 2). If a linear surprisal is assumed and extrapolated to vibrational states with V > 5, the nascent distribution peaks at V ) 8 ( 2. Farantos and Murrell15 have performed semiempirical calculations of the ground state potential for ClO3 together with
Figure 5. Time evolution of the vibrational population for the ClO radicals after the photolysis of Cl2. [Cl2] ) 70 mTorr, [O3] ) 70 mTorr, and [N2] ) 2 Torr. Smooth curves are simulated with the rate of 8 × 103 s-1 for the V ) 1 f 0 relaxation.
classical trajectory calculations on that surface for reaction 1. They predicted that V ) 1 was the most probable vibrational state and levels up to V ) 8 were populated. Our results indicate that the nascent ClO radicals are highly vibrationally excited as they predicted, although the most probable vibrational state is higher than their predicted value. The time evolution of the population for each vibrational level was measured by scanning the delay time between the photolysis and probe lasers, with the probe laser wavelength fixed at the bandhead peak position for each vibrational band. Figure 5 shows the time evolution curves of the vibrational population for ClO V ) 0-3, using 2 Torr of N2 as the buffer gas. The relative populations were calibrated by measuring the excitation spectra at several time delays and using eq 3. It was impossible to determine all of the relaxation constants for all the vibrational states from the time profiles, because the initial population of the V g 6 states are not known. Therefore, we analyzed only the relaxation between the V ) 0 and V ) 1 states. Since the populations of the V > 1 vibrational states are almost zero after 400 µs (Figure 5), only the populations of the
Vibrational Distribution of ClO Radicals
J. Phys. Chem., Vol. 100, No. 1, 1996 179
V ) 0 and 1 levels should be taken into account at t g 400 µs for the analysis, that is
ClO(V)1) + M f ClO(V)0) + M,
krelax
(5)
ClO(V)0) + M f ClO(V)1) + M,
kact
(6)
where krelax is the vibrational relaxation rate constant, kact is the activation rate constant, and M is O3, Cl2 and/or N2. The differential rate equations for the V ) 0 and 1 levels after 400 µs are expressed as
d[ClO(V)1)]/dt ) -krelax[M][ClO(V)1)] + kact[M][ClO(V)0)] d[ClO(V)0)]/dt ) krelax[M][ClO(V)1)] kact[M][ClO(V)0)] (7) where [ClO(V)0)] and [ClO(V)1)] are the concentration of ClO radicals in the V ) 0 and V ) 1 vibrational levels, respectively. It was assumed that the detailed balance principle holds between krelax and kact:
kact ) krelax exp[-EV/kT]
(8)
where EV is the vibrational spacing (854 cm-1) and T is room temperature (298 K). The simultaneous differential equations 7 for ClO V ) 0 and 1 were solved using a Runge-Kutta method. The values of krelax[M] were treated as parameters so as to reproduce the time evolution curves obtained experimentally for t > 400 µs. The smooth curves in Figure 5 assume a vibrational relaxation rate, krelax[M] ) 8 × 103 s-1. With various pressures of the N2 buffer gas (1-5 Torr) and at constant pressures of O3 (70 mTorr) and Cl2 (70 mTorr), we measured the time evolution curves of the ClO vibrational states and calculated the relaxation rates for the V ) 1 f 0 relaxation. These vibrational relaxation rates were found to be independent of the N2 pressure, krelax[M] ) (8 ( 1) × 103 s-1. When 2 Torr of Ar was used as the buffer gas, it was found that the rate was also krelax[M] ) 8 × 103 s-1. We therefore conclude that the vibrational relaxation process is predominantly caused by collisions with O3 and/or Cl2 molecules under our experimental conditions. The upper limit values of the vibrational relaxation rate constants krelax by collisions with N2 and Ar molecules are estimated to be both 5 × 10-14 cm3 molecule-1 s-1. The spacings of vibrational energy levels are 1135, 716, and 1089 cm-1 for O3, 560 cm-1 for Cl2, and 2359 cm-1 for N2, while that for ClO is 854 cm-1. The vibrational relaxation process of ClO by collisions with Ar can take place only by vibrational-translational (V-T) energy transfer. The vibrational spacing of N2 is far different from that of ClO. These may be the reasons why the vibrational relaxation rates for V ) 1 f V ) 0 by N2 and Ar are slow. It is likely that the fast relaxation process observed is attributed to the V-V energy transfer by collisions with O3 (∼4 × 10-12 cm3 molecule-1 s-1). Vanderzanden and Birks18 observed oxygen atoms in the reaction system Cl + O3. The appreciable role of vibrationally excited ClO radicals in the Cl + O3 reaction system has been pointed out. Choo and Leu8 suggested two possible schemes for O atom production: vibrationally excited ClO(V) radicals produced by the reaction of Cl + O3 reacting with O3 molecules or with Cl atoms:
ClO(V) + O3 f ClO + O2 + O or
(9)
ClO(V>2) + Cl f Cl2 + O
(10)
Burkholder et al.19 in a study of infrared line intensities of the ClO radical have presented evidence in support of reaction 10. In their experiments under excess Cl atom conditions, vibrationally excited ClO radicals produced in the Cl + O3 reaction react with Cl atoms to give Cl2 and O which remove additional ClO radicals. They pointed out the possibility for systematic error from the assumption of a 1:1 stoichiometry for [ClO]: [O3]0, when the Cl + O3 reaction were used as a quantitative source of ClO radicals for kinetic and spectroscopic studies. According to the LLNL 2-D model of the troposphere and stratosphere,9 the concentration of ClO is about 108 molecules cm-3 at 40 km altitude. ClO radicals in the stratosphere are removed by reaction 2 and photolyzed by the sunlight. The concentration of O atoms at 40 km is about 109 molecules cm-3 and the rate constant for reaction 2 is reported to be 3.8 × 10-11 cm3 molecule-1 s-1, which yields a rate of 4 × 10-2 s-1 for reaction 2. The photolysis rate coefficient of ClO at 40 km reported to be 3 × 10-3 s-1. The pressure of N2 is about 2 Torr at 40 km. As described above, we have obtained the upper limit value of 5 × 10-14 cm3 molecule-1 s-1 for the vibrational relaxation by collisions with N2, which corresponds to a reaction rate of 3 × 103 s under these conditions. This upper value of the relaxation rate is much faster than the above reaction rate of ground state of ClO(V)0) in the stratosphere. However, if the actual relaxation rate constant is 103-105 times smaller than the upper value, vibrationally excited ClO radicals could survive in the stratospheric conditions. Furthermore, if the rates of reactions involving vibrationally excited ClO such as (9) and (10) are fast, the reactions of vibrationally excited ClO radicals may be important in the atmosphere. 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.). References and Notes (1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (2) Anderson, J. G.; Toohey, D. W.; Brune, W. H. Science 1991, 251, 29. (3) Matsumi, Y.; Shamsuddin, S. M.; Kawasaki, M. J. Chem. Phys. 1994, 101, 8262. (4) Matsumi, Y; Shamsuddin, S. M. J. Chem. Phys. 1995, 103, 4490. (5) Baumga¨rtel, S.; Gericke, K.-H. Chem. Phys. Lett. 1994, 227, 461. (6) McGrath, W. D.; Norrish, R. G. W. Z. Physik. Chem. (Munich) 1958, 15, 245. (7) McGrath, W. D.; Norrish, R. G. W. Proc. R. Soc. 1960, A254, 317. (8) Choo, K. Y.; Leu, M. T. J. Phys. Chem. 1985, 89, 4832. (9) 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. (10) Hilbig, R.; Wallenstein, R, IEEE J. Quantum Electron. 1983, 19, 194. (11) Coxon, J. A. Can. J. Phys. 1979, 57, 1538. (12) Coxon, J. A.; Jones, W. E.; Skolnik, E. G. Can. J. Phys. 1976, 54, 1043. (13) Levine, R. D.; Bernstein, R. B. Acc. Chem. Res. 1974, 7, 393. (14) Zamir, E.; Levine, R. D. Chem. Phys. 1980, 52, 253. (15) Farantos, S. C.; Murrell, J. N. Int. J. Quantum Chem. 1978, 14, 659. (16) Barbe, A.; Secroun, S.; Jouve, P. J. Mol. Spectrosc. 1974, 49, 171. (17) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules; Van Nostrand: New York, 1979. (18) Vanderzanden, J. W.; Birks, J. W. Chem. Phys. Lett. 1982, 88, 109. (19) Burkholder, J. B.; Hammer P. D.; Howard, C. J. J. Geophys. Res. 1989, 94, 2225.
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