J. Phys. Chem. 1993,97, 7597-7605
7597
Kinetic, Thermochemical, and Spectroscopic Study of C1203 James B. Burkholder,' R. L. Mauldin 111, R. J. Yokelson, S. Solomon, and A. R. Ravishankarat Aeronomy Laboratory, NOAA, 325 Broadway, Boulder, Colorado 80303, and The Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309 Received: February 18, 1993; In Final Form: April 26, I993
The UV absorption spectrum of C1203 between 220 and 320 nm was measured using time-resolved transient absorption. C1203 was produced following 193-nm pulsed laser photolysis of NzO/Clz/OClO/He or CF2C12/0ClO/N2 gas mixtures by reaction 1: C10 OClO M ClzO3 M. The absorption spectrum peaks at 267 nm with a cross section of (1.60 X lo-'' cm2 (2u error limits including estimated systematic errors). The rate coefficient for the forward reaction 1, kl,was measured at temperatures between 200 and 260 K at N2 number densities over the range (1.1-10.9) X 10l8molecules ~ m - ~The . data were fit using the Troe formalism, with an Fc fixed at 0.6,to yield ko(300) = (6.2 f 1.0)X cm6 molecule-2 s-', k, = (2.4f 1.2) X lo-" cm3 molecule-' s-l, and n = 4.7 f 0.6 (2u error limits). The equilibrium constant for reaction I, K,, was measured at five temperatures over the range 232-258 K. A second law analysis of this data along with data reported by Hayman and Cox [Chem. Phys. Lett. 1989,155, 11 yielded ASo= -21.2 f 4.5 cal mol-' K-1 and AHo = -11.1 f 1.2 kcal mol-' (2a error limits of the fit). These photochemical and kinetic results are compared with previously reported values. The kinetic, equilibrium, and photochemical data were included in a photochemical box model of the polar stratosphere to assess the role of C1203 in stratospheric chemistry. On the basis of the results of the model, it is concluded that ClzO3 does not play a significant role in the polar stratosphere.
2;;)
Iotroduction Reactivechlorineoxides such as C10 and Cl202 play important roles in the catalytic O3destruction cycles that take place in the Antarctic stratosphere during austral Sander et al.3 and Anderson et a1.4 have proposed that higher chlorine oxides such as C103, C1203,C1204,C1206,and C120.1 may be formed when the concentrations of C10, C1202, and OClO are elevated. These higher oxides of chlorine could be important if they are produced in sufficient amounts to act as temporary or long-term chlorine reservoirs or if they engage in O3 destruction cycles. High concentrationsof both ClO5 and OC106have been observed during winter and spring in the lower Antarctic stratosphere. Therefore, the first higher chlorine oxide that needs to be considered is C1203which is formed by the reaction C10
+ OClO + M
-
Cl2O,
+M
(1)
The product C1203is very weakly bound and can dissociate to the reactants
+
C1203 M
-
C10
+ OClO + M
(-1)
and establish the equilibrium C10
+ OClO + M * Cl,03 + M
(1) To evaluate the significance of C1203in the chemistry of the polar stratosphere, in general, and Antarctic stratosphere, in particular, we need kl, the rate coefficients for reaction 1, Kq, the equilibrium constant for process I, and the photochemical properties of C1203such as the absorption cross sections and the photolysis quantum yields. Recently, because of its possible importance in the chemistry of the polar stratosphere,several photochemicaland kinetic studies related to C1203 have been carried out. Hayman and Cox7 measured the UV absorption cross sections of C1203(a = (1.86 f 0.20)X 10-17 cm2 at the peak of the spectrum, 265 nm). They
* To whom correspondence should be sent at NOAA R/E/AL2, 325 Broadway, Boulder, CO 80303. Also associated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309. 0022-365419312097-7597$04.00/0
+
+
-
+
also reported the equilibriumconstant for reaction I. Parr et a1.8 reported the shape of the c1203 UV absorption spectrum and kl at 226 K. Burkholder et aL9 and Molina and Molinal (see Burkholder et al. for interpretation of their reported spectrum) have also reported UV absorption spectra of C1203. In their work, Burkholder et 01.9 estimated the peak absorption cross section at 265 nm to be -1.8 X 10-17 cm2, in good agreement with Hayman and COX.' The shape of the absorption spectrum measured by Burkholder et 01. however does not agree with that determined by Hayman and Cox. In addition, the absorption cross sections at wavelengths less than 250 nm given by Hayman and Cox are -40% higher than those reported by Parr et ul.? Burkholder et 01.: and Molina and Mo1ina.l Parr et al. have attributed this differences in cross sections and the shape of the spectrum to the formationof C1204in the experimentsof Hayman and Cox. Although both Cl2O3 and C1204 absorb in the same wavelength region, the absorption cross sections10 of C1204are 10 times smaller than those of C1203in this wavelength range. Therefore in the experiments of Hayman and Cox, the C1204 concentration would have to be greater than that of C1203to account for the spectral discrepancies. If significant amounts of C1204were formed in the experiments of Hayman and COX,^ their assumed stoichiometryfor C1203formationwould be invalid and hence their absorption cross section may be in error. Parr er a1.* are the only investigators who have measured kl. Their value at 226 K between 4 to 28 Torr of N2 is kl = (2.8 f 2.2) X lO-3l cm6 molecule-2 s-l (the quoted errors are 2u error limits). More accurate measurements of kl over extended temperature and pressure ranges are essential to fully evaluate the rate of Clz03formation in the stratosphere. The equilibrium constant for process I, K,, over the temperature range 232-273 K was reported by Hayman and COX.^ They directly measured C10, OC10, and C1203 in an equilibrium mixture using UV absorption to deduce K,. A second law analysis of their data yielded the enthalpy change for this reaction, AIP = -14.8 f 2.9 kcal mol-1 and the entropy change, ASo = -36.1 f 12 cal mol-' K-1 at 257 K, the median temperature of their measurements. The uncertainty in their lowest temperature data leads to large uncertainties in Kq at the lower polar stratospheric
-
0 1993 American Chemical Society
Burkholder et al.
7598 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993
dielectric mirrors together decreased the probe beam intensity by approximately 20% over the range 210-360 nm but reduced the 193-nm laser light by a factor of 500. The laser was operated at 0.1 Hz so that the gases in the absorptioncell were completely flushed out between laser shots. The laser fluence in the cell was in the range 15-20 mJ. The probe beam transmitted through the cell was directed with a removable mirror onto the entrance slit of either a 0.28nm spectrometer with a diode array detector or a 0.25-m monochromator with a PMT detector. The monochromator/ PMT system provides much better time resolution than the diode array system and, therefore, was used for kinetic measurements. The diode array system acquires absorption data over a range of wavelengths simultaneously and was used for spectral measurements. The diode array spectrometer was fitted with a 600 groove/ mm grating and a 1024element diode array detector. The spectral bandwidth of the diode array was 150 nm and covered the range 210-360 nm, with a resolution of 0.6 nm. This wavelength range convenientlyincludesthe strong absorptionfeatures of C10, OC10, and C1203. Two different diode arrays, an intensified gated detector and ungated detector, were used during these measurements. The intensified gated diode array detector was used in a manner similar to a PMT with the added advantageof increased wavelength coverage. The gain of the detector was lo6. The intensified detector was used to measure spectra -250 ps after the photolysis pulse at reaction time increments as short as 50 ps. The ungated diodearray detector was used to measure spectra at reaction times longer than 2 ms, with a much longer exposure time of -20 ms. The ungated detector is preferred for measurements of slowly changing systems due to the significant reduction in the noise level over that of the intensified gated detector. The changes in absorptionwere calculated from spectra recorded before and after the photolysis laser pulse. Typically 100spectra werecoaddedto reduce the noise levels to an equivalent absorbance of h0.002 for the ungated detector and k0.005 for the intensified gated detector. The ungated diode array measurements were used to determine the relative shape of the C1203 absorption spectrum and the concentrations of ClO, OC10, and C1203 at equilibrium. The monochromator was used with 150-pm slits to give a resolution of 1nm. The signal output of the PMT was digitized by a signal averager. Sampling intervals ranging from 5-20 ps were used. Data acquisition was initiated 1 ms before the laser pulse to provide a baseline signal from which to calculate absorbances. Scattered laser radiation in the monochromator temporarily saturated the PMT, and it took -30 ps for it to completely recover. Therefore data recorded during this time was not used. Tht monochromator/PMT was used during the kinetic measurements of reaction 1 by following the transient absorbance at 265 nm where both Cl2O3 and C10 absorb strongly; but the differences in their cross section are quite large. Cl2O3 was produced in the absorption cell via reaction 1 using 193-nm photolysis of two different precursors to produce C10 radicals. Method 1: N20, (1.6-3.2) X 10l6 molecules ~ m - and ~ , C4, (1-3) X 101' molecules ~ m in- 250 ~ Torr of He was photolyzed with 15 to 20 mJ cm-2 of 193-nm radiation. C10 was produced by the following sequence of reactions:
-
Figure 1. Experimental apparatus used to measure kl, Kq,and the absorption cross section of Cl203.
temperatures. For example, Kq at 200 K calculated by excluding their lowest temperature data is 10times lower than that obtained by including that point. In light of the above uncertainties, a reexamination of Kq is warranted. In this paper we report UV absorption cross sections of C1203 between 220 and 320 nm, kl between 200 and 260 K at number densities of (1.1-10.9) X 10l8 molecules cm-3 of N2, and Kq between 232 and 258 K. We discuss the implications of these results to the chemistry of the polar stratosphere. Experimental Section
This work consists of three independent but interrelated measurements: (a) the UV absorption spectrum and absolute absorption cross sections of Cl2O3; (b) the rate coefficients, kl, for the formation of C1203;and (c) the equilibrium constant, KY for reaction I. The apparatus used in all these experiments is shown in Figure 1. The apparatus has been described recently by Trolier et al." and Mauldin et a1.I2 A brief outline of the apparatus focusing on the modifications made for the present work and the methods of data acquisition and analysis used here are given below. The apparatus was designed to measure transient absorption spectra of gas-phase species following pulsed photolysis. The apparatus consists of three basic components: (1) a long-path absorption cell (optical path length of 91 cm), (2) a pulsed photolysis light source, and (3) a UV probe light source and spectrometer/detector combination. The absorption cell was made of jacketed 30-mm4.d. Pyrex tubing, whose temperature was regulated by circulating methanol from a temperature controlled bath through the jacket. The absorption cell could be cooled to 200 K. The temperature of the cell was uniform over its length to h0.25 K. The 1-in.-diameter quartz windows of the cell were mounted inside the temperature regulated region to keep the entire absorption path at a constant temperature. N2 gas was blown over the outside of the windows to prevent condensation during low temperature measurements. The use of a pulsed ArF excimer laser (193 nm), instead of the previously used broad band Xe flash lamp,ll*12asthe photolysis light source represents the most significant change for the present experiments. Use of a laser was essential to make the chemistry in the system simpler by preventing photolysis of OC10. The pulsed laser beam, pulse duration of 20 ns, was copropagated with the cw UV probe beam from a 30-W D2 lamp through the absorption cell. An iris mounted near the entrance window of the absorption cell ensured overlap of the photolysis and probe beams. A dielectric mirror reflected the photolysis beam back through the cell. This dielectric mirror served to increase the photolysis fluence and also decrease the laser beam transmitted into the spectrometers. Another dielectric mirror was placed near the entrance slit of the monochromator to further reduce the scattered laser light which temporarily, -30 ps, blinded the photomultiplier tube (PMT). The recovery of the PMT was measured with no photolyte in the absorption cell. The two
-
-
-
+ hv N2+ O('D) O('D) + C1, C1+ ClO o ( ~ D )+ M o(3~) +M o(3~) + ci, c i + c i o c1+OClO c10 + c10 N,O
--
(2) (3) (4)
-
The concentration of OClO was in the range (3-10)
(5) X
(6) 1015~ m - ~ .
The Journal of Physical Chemistry, Vof. 97, No. 29, 1993 7599
Study of C1203
Method 2: CF2C12, (0.5-2) X 10l6 molecules cm-3, was photolyzed in the presence of OClO with 15-20 mJ cm-2of 193nm radiation. Here, C10 was produced by the following sequence of reactions:
+
CF2C12 hu
followed by
-
+
(74
+ 2C1
(7b)
CF2C1 C1 CF,
+
c1+OClO c10 c10 (6) In method l,O(lD) produced in the NzO photolysis with unit quantum yield” rapidly reacted with Cl2 which was present in a large excess (k3 = 2.8 X 10-l0cm3molecule-l s-l)I3 to produce C10 radicals. The C1 atom produced in reaction 3 reacts with OClO (k6 = 3.4 X 10-11 exp(l6OIT) 01113 molecule-’ s-l)13, to produce two more C10 radicals. Therefore, three C10 radicals were produced for each O(lD) formed by photolysis. Even if a fraction of the O(1D) was quenched to O(3P), its reaction with C12 was sufficiently fast to convert most of them to C10. This point is discussed later. At the reactant concentrationsused here, the C10 production was completewithin a few microseconds after the laser pulse. The concentrationof C10 in the absenceof OClO in method 1 is equal to that of the O(lD) produced. Therefore, in the C1203 cross section measurements the initial O(lD) concentration was determined by measuring the C10 produced in the absence of OC10. This method was used to measure the UV absorption spectrum and absolute cross sections of C1203. In method 2, C1 atoms formed as a primary photolysis product of CF2Clzreacted with OClO to produce C10 and subsequently C1203. Rebbert and Ausloos14have reported C1 atom yields for CF2C12 photolysis at discrete wavelengths between 213.9 and 147 nm. A linear interpolation of their results to 195 nm gives a C1 atom yield of 1.32, 32% of the photolysis goes via reaction 7b. The yield of C10 is not very sensitive to the quantum yield for C1, as discussed later. Each C1 atom produced by photolysis yields two C10 radicals through reaction 6. In addition, the CFzC1 photofragment also produces 3 more C10 radicals via the sequence CF2Cl
--
+ OClO
+M c1+OClO
CF2C10
+ C10
(8)
CF20+ C1+ M c10 + c 1 0
(9)
CF2C10
(6) The ratecoefficient for reaction 8 has not been measured, however, for the concentrationsof OClO used, the first-order rate constant for the loss of CFzCl is expected to be fast (k’s > lo5 s-l) on the time scale of our measurements. All our observations are consistent with this expectation. The rate coefficient for the decomposition of CFzClO to yield another C1 atom, kg, has been measured at 4-20 Torr to be >7 X lo5 s-1 and 6.4 X lo5 at 298 K and 5 X 104 at 238 K.15 Therefore, the decomposition of the CF2Clphotoproductleads to the formation of three C10 radicals. The decomposition is fast enough at the higher temperatures of our measurements to be complete before a significant fraction of C10 reacts with OC10, and hence, does not interfere with the measurements of kl. At the lowest temperature of our measurements, the decomposition of CFzClO is slower and may generate C10 on the time scale of its loss via reaction 1. In that event, secondary C10 generation may have a small effect on our measured values of kl; this will be discussed further later. The CF2photofragment formed in reaction 7b is relatively stable and is not expected to contribute to C10 or C1 formation. The room temperature rate coefficient for the reaction CF,
+ OClO
-
products
(10)
was measured in this study to be -7 X 10-14 cm3 molecule-’ s-l, which is sufficiently slow to not interfere with measurements of
kl or Ksp. For measurements of &lo,CF2 was produced in the presence of a known excess of OClO via 193-nm photolysis of CFzBr2 where its quantum yield is known to be unity? CF2Br,
+ hv
-
CF,
+ 2Br
(11) CF2 was monitored by UV absorption at 249 nm.3’ The CF20 formed in reactions 9 and 10 is chemically stable and does not interfere with our measurements of kl. Also, the UV absorption spectrum of CF2O is weak13 and, therefore, does not interfere with the transient absorption measurements. Method 2 was used to measure the equilibrium constant, the spectrum of C1203, and kl, the rate coefficient for the formation of C1203. a 2 0 3 W Absorption Spectrum and Cross-Section Measurement. The UV absorption spectrum of C1203was measured over the wavelength range 220-360 nm between 200 and 260 K. Spectra were measured at short reaction times following the laser pulse using the intensified gated diode array detector and at longer times using the ungated detector. Quantitativeabsorption cross section measurements were made using method 1 as the C10 source with C10 concentrationsin the range (0.3-5.0) X 1013molecules ~ m - ~Absorption . at 265 nm was measured using the monochromator/PMT combination in the presence and in the absence of OClO in the cell. In the absence of OClO the absorption at 265 nm is due solely to C10. The C10 concentration was quantified using absorption cross sections previously determined using this apparatus.” In the presence of OC10, the C10 radicals are converted to C1203via reaction 1. C1203 absorbs at 265 nm, but with a cross section that is different from that of C10. Therefore, reaction 1 was also observable when monitoring 265-nm radiation. These measurements were made at low temperatures, 200 and 223 K, to ensure complete (>99%) conversion of C10 to C1203. Once reaction 1 had gone to completion, as shown by the temporal profile at 265 nm, the absolute UV absorption cross section for C1203 was calculated using the measured C10 concentration and the stoichiometryof reaction 1. The UV absorptionspectra recorded with the diode array spectrometer were used to determine the absorption cross sections at other wavelengths relative to that-at 265 nm. Measurements of Rate Coefficient: 4. The forward rate coefficient of reaction 1, kl, was measured at four temperatures between 200 and 260 K over the N2 number density range (1.110.9) X 1018 molecules cm-3. All kinetic measurements were made using method 2 to produce C10. Method 1 was not used because it requires He (N2 quenches O(1D)) and large concentrations of Clz, 1.2 X 10l8molecules ~ m - ~Measurements . of kl in Nz are required for atmospheric applications. The high concentrations of Cl2 mandates the determination of the thirdbody efficiency of C12 which can be avoided by using method 2. Transient UV absorption at 265 nm, monochromator/PMT, was used to measure kl. This wavelength was chosen because it is near the peak of the strong C1203absorption feature. Even though C10 absorbs at 265 nm, its cross section is -3 times smallerthan that of Clz03,this wavelength provides the maximum sensitivity for monitoring the reaction. The formation C1203 and loss of C10 were monitored under pseudo-first-orderconditionsinC10 with [OClO] > 50 X [ClO]o. Thetimedependence of the absorption is given by
-
where A, is the absorption at infinite time (i.e., when reaction has gone to completion) and Al is the absorption at time t , 1 is the path length, [ClO]o is the initial C10 concentration, ~ ~ 1 2 0 , and uclo are the absorption cross sections of C1203 and C10 at the wavelength of the measurement, k’l = kl[OClO][M], and kL1 = k-l[M]. Theslopeofaplotofln(A,-At) vstyields-(k’l kL1). From the measured values of k’1 + k’_l as a function
+
Burkholder et al.
7600 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 2.0
of [OClO], the values of kl[M] were obtained. Measurements made at various [MI are then used in the determination of kl. It should be pointed out that the absolute absorbance observed depends on the differencein the C10 and C1203 absorption cross sections while the determination of kl does not require knowing the absorption cross sections of either C10 or Cl2O3; it merely requires the knowledge of the stoichiometry of reaction 1. The [ C l o ] ~and [OClO] were varied over the range (0.75-1.33) X 1013 and (0.41-4.6) X 1015 molecules cm-3, respectively, in measuring kl . EquilibrhnConstantMeasurements. The equilibrium constant for reaction I
Kq = [Cl,OJ/[ClO] [OCIO]
220
was measured at five temperatures over the range 232-258 K. The equilibrium concentration of each species in eq 13, C10, OC10, and C1203,was measured quantitatively by UV absorption. The absorptioncross sectionsfor C10 and OClO needed for these calculations were taken from the literature,11J2while that for C1203 was measured here. The equilibrium constants were measured under conditions similar to those used to obtain kl. The total pressure in the cell was maintained at -250 Torr of N2 to ensure rapid equilibration (in
