UV laser photodissociation of CF2ClBr and CF2Br2 at 298 K: quantum yields of Cl, Br, and CF2

29 November 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 262 (1996) 669-674

UV laser photodissociation of CF 2C1Br and CF 2Br 2 at 298 K: quantum yields of C1, Br, and CF 2 Ranajit K. Talukdar a,b, Martin Hunter 1,a,b, Rachel F. Warren James B. Burkholder ~,b A.R. Ravishankara 3,a,b

2,a,b,

a NOAA, Aeronomy Laboratory, R / E / A L 2 . 3 2 5 Broadway, Boulder. CO 80303, USA b Cooperative Institute for Research in Environmental Sciences University of Colorado. Boulder. CO 80309. USA Received 9 August 1996

Abstract

In the photodissociation of CF2C1Br, the quantum yields for CI atom production were measured to be 1.03 + 0.14, 0.27 + 0.04, and 0.18 + 0.03, respectively, at 193, 222, and 248 nm, while those for Br atom were 1.04+ 0.13 at 193 nm, 0.86 + 0.11 at 222 nm, and 0.75 + 0.13 at 248 nm. Errors are 95% confidence limits and include estimated systematic errors. The CF2 quantum yields in 193 nm photolysis were measured to be 1.15 + 0.30 for CF2Br2 and 0.91 + 0.30 for CF2CIBr. These results indicate the quantum yield for photodissociation of CF2CIBr is unity. 1. Introduction

Fire suppression agents, CFzC1Br and CF2Br 2, have recently received significant attention because they deplete stratospheric ozone if released into the atmosphere [1]. Both CF2CIBr and CF2Br 2 are primarily removed from the atmosphere via UV photolysis [2]. The atmospheric photolysis rates depend on their absorption cross sections, the solar flux, and the absolute photolysis quantum yields for photodissociation. In our earlier work [2], we reported the absorption cross sections and atmospheric lifetimes assuming the quantum yield for dissociation to be unity.

Current address: The Phillips Laboratory, Hanscom Air Force Base, MA, USA. Current address: Centre for Environmental Technology, Imperial College of Science, Technology, and Medicine, 48 Prince's Gardens, London SW7 2PE, UK. 3 Also associated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA.

We showed photolysis at short wavelengths, between 190 and 210 nm, to be a significant contributor to the overall stratospheric photolysis rate. Since there is very limited absolute quantum yield data for these molecules, we have focused on measuring them. CF2C1Br and CF2Br 2 both show strong continuous absorption features in the UV, which result from several different electronic transitions. The photodissociation channels and the threshold wavelength (A, in nm) for their occurrence are: CF2CIBr + hv ~ CF2CI + Br,

A = 444,

(1)

--* CF z + CIBr,

A = 444,

(2)

CF2Br + C1,

A = 356,

(3)

CF 2 + C l + B r ,

A=245,

(4)

and CF2Br 2 + h v ~ C F 2 B r + B r ,

000%2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-261 4(96)0 11 57-8

CF 2 + B r + B r ,

A=446, A=285.

(5) (6)

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R.K. Talukdar et al. / Chemical Physics Letters 262 (1996) 669-674

For evaluating the impact of these compounds on stratospheric ozone, the most important parameter is not the branching to the different photolysis channels but the absolute quantum yield for photodissociation. However, the overall dissociation quantum yields can be evaluated from a knowledge of the quantum yields for the different processes. We have previously reported the Br atom quantum yields in the photodissociation of CF3Br and CF2Br 2 [3]. Here, we report the quantum yields for Cl and Br from CF2ClBr photolysis at 193, 222, and 248 nm. The quantum yields for CF 2 from the 193 nm photolysis of CF2ClBr and CF2Br 2 are also reported.

2. Experimental Two apparatuses, one for detection of atomic species (CI and Br) and another for CF 2, were used during this study. The vacuum UV atomic resonance fluorescence technique was used to detect CI and Br while long-path absorption was used for CF 2. The details of apparatus, data acquisition and data analysis procedures have been presented in previous publications from our laboratory [3-6]. Therefore, this discussion will cover only the aspects needed to understand the present measurements. 2.1. Atomic resonance fluorescence The apparatus has been described in our paper on the Br atom quantum yield measurements in the photodissociation of bromo compounds [3]. The sensitivity for detection of C1 and Br atoms, via cw atomic resonance fluorescence, were = 3 × 108 atom cm -3 and = 1 × 109 atom cm -3, respectively, for 1 s integration with a signal-to-noise ratio of unity. Quantum yields of CI and Br atoms were measured at 298 K under 'slow-flow' conditions. A dilute mixture of the photolyte along with the carrier gas (He, N 2, or Ar) was flowed through the reaction cell. The pressure and flow velocity in the reaction cell were in the range of 20-300 Torr and 5-15 cm s - 1, respectively. The photolyte concentrations were in the range of ( 1 - 5 0 ) × 1014 molecule cm -3. In a typical experiment, CF2CIBr and a reference compound (CF2Br 2, CH3Br, or HCI) were pho-

tolyzed in back-to-back runs and the temporal profiles of CI and Br atoms were measured. The temporal profiles were analyzed to obtain 1, the initial signals. The photolytes, CFeCIBr, HC1, and CH3Br, absorbed up to 15% of the light used to excite Ci and Br atomic fluorescence. To correct for the attenuation, the resonance fluorescence signals were measured at various concentrations of the photolytes and extrapolated to zero concentration as discussed previously [3]. From the corrected initial intensities, the quantum yield was determined via the equation: ( (TA~A) CF2ClBr

ICF2CIBr

(~qSA)Ree

1Ref

(7)

Such experiments were carried out at 193, 222, and 248 nm. To measure the quantum yield at 222 or 248 nm relative to that at 193 nm, in back-to-back runs, a mixture of CF2C1Br and N 2 was photolyzed first by 193 nm radiation and then by 222 or 248 nm laser light. Care was taken to ensure that the laser beams traversed exactly the same volume in the cell and were of exactly the same dimensions. In such experiments, the quantum yield at 222 or 248 nm is given by: ~A(CF2CIBr)

~__

iA ~ ×

_ 193 ~r*

~UCF2CIBr

rh193

× ~'CF2C,Br"

(8)

CF2ClBr

The concentrations of all the photolytes were determined from the measured flow rates and the pressure in the photolysis cell. The concentration of HC1 was also measured by 184.9 nm absorption in a 100 cm long cell through which the reaction mixture was flowed prior to entering the reactor. The concentrations measured by UV absorption and flow agreed within 3%. 2.2. Transient UV absorption The absolute quantum yields for CF 2 from CF 2 Br 2 and CF2CIBr photolysis were measured by detecting transient UV absorption of CF 2 in the A ~ X band. Light from a 30 W D 2 lamp was co-propogated with a pulsed ArF excimer laser beam, 193 nm, in a 91 cm long cell (id = 2.2 cm) through which the gas mixture was slowly flowed. The photolysis laser fluence was in the range 2 - 4 mJ cm -2. The laser

671

R.K. Talukdar et al./ Chemical Physics Letters 262 (1996) 669-674

repetition rate was 0.1 Hz, which allowed the cell to be replenished with a fresh mixture between photolysis pulses. The UV probe beam was diverted to a monochromator photomuitiplier tube (PMT) combination to measure the temporal profiles of the absorptions at 248.7 or 258.3 nm, with an instrumental resolution of 0.67 nm. Alternatively, the beam was diverted to a diode array spectrometer to detect CF 2 or measure the concentration of photolytes via broadband UV absorption. Quantum yields of CF 2 from CF 2Br 2 and CF2CIBr were determined relative to that from C2F 4, which has previously been determined to be 2.0 at 193 nm [7,8]. The CF 2 absorption cross sections, although not needed to determine the quantum yields, were also determined in these experiments. The excimer laser fluence in the cell was measured by photolyzing a mixture of N 2 0 - O 2 - N 2 and monitoring the 0 3 yield via the sequence of reactions: N20 + hv 193 nm O(ID) + 02 O ( ' D ) + N 2 ~ O(3p) + N 2 O(3p) + 02

N2 ' O 3

(9) (10) (11)

Transient absorption signals from the PMT were recorded in 20 /xs bins. The temporal profiles following each laser pulse were corrected for variations in the photolysis laser fluence and coadded. Absorbances following 100 photolysis pulses were coadded. The probe light intensity ( I 0) was recorded prior to the photolysis laser pulse and was used as the reference for absorbance calculations. Thus, we measured the changes in absorption due to changes in composition of the gases in the cell induced by the photolysis pulse. The diode array system (1024 elements, 0.28 m spectrometer) covered a spectral range of 150 nm with a 0.7 nm resolution. At this resolution, the A - X band of CF 2 exhibits diffuse vibrational bands with the strongest peak at 249 nm [7,9,10]. The spectra of CF 2 from the photodissociation of CF2CIBr, CF 2 B r 2 , and C2F4 were recorded between 3 and 30 ms after the photolysis laser pulse by exposing the diode array to the probe beam using a mechanical shutter arrangement [5]. Quantum yields were measured at 298 K in 110 Torr of N 2 flowing through the cell at - - 2 0 cm s -~ Concentrations of CF2Br 2 and

CF2C1Br were measured by UV absorption using the diode array spectrometer. The C2F 4 concentration was calculated using the flow rates of C2F4 ( - 0 . 0 3 STP cm 3 s- l) and N 2 and the cell pressure. In back-to-back experiments, repeated three times, CF 2 from C2F 4 and X (X = CF2Br 2 or CF2C1Br) were measured while holding the laser fluence constant. Under these conditions the quantum yield for CF 2 from X is given by: 2 A ~ F - ' ( X ) [ C 2 F410-c~3'

193 X a'CF,(

)

A F (C2F4)

93

(12)

'

where A~F(X) is the absorbance measured at wavelength h due to CF 2 produced by photolysis of compound X with concentration [X], and 0 "193 is the absorption cross section of X at 193 nm. Absorptions were measured at h = 248.7 and 258.3 nm, atop the two most prominent structures in the CF 2 absorption spectrum. Eq. (8) is valid only if the photolyte does not absorb significantly compared to CF 2 at A, which was the case for C2F 4. For CF2Br 2 and CF2CIBr, however, a small correction to A~F(X) was necessary ( < 4%) to account for an increased transmission due to the photolyses of the parent compounds. Concentrations (in molecule cm -3) of C2F 4, CF2Br 2, and CF2C1Br were 2.8 × 10 ~5, -- 5.5 × 10 m5 and = 1.6 × 1016, respectively. Under these conditions, there was significant attenuation of the photolysis laser fluence through the length of the cell (40-75%) and it was necessary to correct A~F(X) for the resulting non-uniform distribution of CF 2 in the cell. Our overall uncertainty in CF 2 quantum yields is estimated to be 20% for measurements at 248.7 nm and 25% for 258.3 nm. These uncertainties include estimated systematic errors of 5% in photolyte concentrations (except for [C2F4], where it is 10%) and 5-15% in measurements of A~F(X), depending on

[x]. 2.3. Materials

The purities of gases used in this study were: > 99% for CF2Br 2 and CF2CIBr; 99% for C2F 4 (which contained 1% d-limonene as a polymeriza-

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R.K. Talukdar et aL / Chemical Physics Letters 262 (1996) 669-674

tion inhibitor); 99.995% for HC1; 99.99% for N 2 0 ; 99.9995% for 02; 99.9995% for N 2, 99.9995% for He; 99.999% for H2; 99.9% for CH3Br. CF2CIBr and CH3Br were subjected to several freezepump-thaw cycles before use. Stock mixtures of CF2CIBr and CF2Br 2 in He (1-2%) were prepared manometrically in glass bulbs for use in the atomic resonance fluorescence experiments. Their concentrations in the mixtures were also measured by UV absorption in the cell.

3. Results 3.1. CI atom quantum yields Photolysis of HCI was used as the reference in 193 nm experiments. Following the production of C! atoms via photolysis of HCI, there was a secondary generation of C1 atoms via the reaction: H+HCI~H

2 +C1

(13)

The temporal profiles were analyzed, by fitting to a biexponential expression, to obtain the CI atom resonance fluorescence signal immediately after the photolysis and the time constant for the rise due to reaction (13). From the measured time constants for C1 atom rise as a function of [HCI], the room temperature rate coefficient for reaction (13) was determined to be ( 5 . 2 + 0 . 4 ) X 10 -j4 cm 3 molecule - l

s- ~, in good agreement with previous determinations [11,12]. The CI atom profiles measured following CF2C1Br were fit to a biexponential form to obtain the CI atom signal at t = 0. It should be noted that interpretation of the temporal profiles are not needed to obtain the quantum yields, but only the initial signal levels are necessary. The measured Cl atom quantum yield at 193 nm is listed Table 1. There was no suitable Ci atom reference for 222 and 248 nm photolysis. Therefore, the Cl atom quantum yields were measured relative to that at 193 nm, as described earlier. The measured quantum yields at 222 and 248 nm are also listed in Table 1. The measured quantum yields were invariant with buffer gas pressure by a factor of 6 and the photolysis laser fluence by a factor of 10.

3.2. Br atom quantum yields The temporal Br atom profiles following CF2CIBr photolysis were exponential under all conditions at all three photolysis wavelengths. The determination of the Br atom quantum yields at 193 and 248 nm was similar to that for the C1 atom at 193 nm. The Br atom quantum yields at 222 and 248 nm were measured relative to that from 193 photolysis. The measured quantum yields are listed in Table 1. The Br atom quantum yield at 248 nm measured relative to that at 193 nm agrees well with the value obtained

Table 1 Quantum yields for the production of CI and Br atoms in the photolysis of CF2CIBr as a function of wavelength at 298 K Photolysis wavelength (rim)

Reference molecule

Number of measurements

Buffer g a s / pressure (Torr)

Quantum yield a

~ci 193 222 248

HCI CF2CIB r b CF2CIB r b

3 2 3

He/50, N2/200 He/50, A r / 2 0 0 He/50, N2/300

1.03 + 0.14 0.27 4-0.04 0.I 8 + 0.03

qbBr 193 222 248 248

CHaBr CF2CIB r b CF2C1B r b CH3Br

3 3 4 4

He/50, N2/300 He/50, N2/300 He/50-300 He/50, N2/300

1.04 0.86 0.77 0.72

a Quoted error bars are at 95% confidence level and include precision and estimated systematic uncertainties, b The quantum yields at 222 and 248 nm photolysis of CF2CIBr were measured relative to that at 193 nm.

~ 0.13 ± 0.11 _+ 0.11 ~ 0.12

R.K. Talukdar et al. / Chemical Physics Letters 262 (1996) 669-674

using CH3Br photolysis at 248 nm as the reference. Since the Br atom yield from CF2C1Br at 193 nm is referenced to that from CH3Br, the relative measurement at 248 nm served as an internal consistency check. The Br atom quantum yields did not change when the laser fluence was varied from 0.5 to 4.5 mJ cm -2 at 248 nm, from 0.2 to 1.8 mJ cm -2 at 222 nm and from 0.15 to 1.2 mJ cm -2 at 193 nm, indicating an absence of any secondary and non-linear (multi-photon) processes.

3.3.

CF 2

quantum yields

The CF 2 absorption spectrum measured using the diode array spectrometer was in good agreement (after accounting for the differences in spectral resolution) with that reported by Sharpe et al. [7]. The C F 2 quantum yields were derived relative to that from C2F 4 photolysis, which is known to be 2. Quantum yields measured by monitoring 258.3 nm absorption were similar to that using 248.7 nm, but the signal-to-noise ratio was poorer. The CF 2 quantum yields are unity within the experimental precision of the measurements (Table 2). Absolute CF 2 absorption cross sections of O'CaF2= (2.0 + 0.2) X 10 -1 7 and (9.0 + 0.1) × 10 -18 cm 2 molecule-~ at 248.7 and 258.3 nm, respectively, were also determined. When the differences in the resolutions used are accounted for, our values agree with those reported by Sharpe et al. [7] and provides a self-consistency check of the quantum yield measurements.

4. Discussion

The Br atom decay rate did not vary with the CF2CIBr concentration and we place an upper limit on the reaction rate coefficient of Br atoms with CF2CIBr of < 5 X 10 -14 cm 3 molecule -t s - I . Table 2 Absolute CF 2 quantum yields from CF2Br 2 and CF2CIBr photolysis at 193 nm Wavelength (nm)

248.7 258.3

Quantum yield CF 2 Br 2

CF2CIBr

1.11 + 0.22 a 1.18 + 0.30

0.79 + 0.20 1.03 + 0.29

a Error limits are 20" and include estimates of systematic errors.

673

The measured CI atom temporal profiles were somewhat different at each photolysis wavelength. The invariance of the measured CI atom temporal profile with changes in CF2C1Br concentration placed an upper limit on the rate coefficients for the reaction of C1 atoms with CFeCIBr of < 1 × 10 - 1 4 cm 3 molecule-t s-~. CF 2 radicals produced from C 2F4 photolysis were lost via its self reaction, as shown by their temporal profiles which obeyed second-order kinetics. The loss rates of CF 2 produced in CF2CIBr and CFzBr 2 photolysis were significantly faster, even at lower initial CF 2 concentrations, than that observed in C 2F4 photolysis. This enhanced rate is due to additional reactions in the system. However, this chemistry was not pursued further. Variations in the laser fluence, concentration of the photolyte, and gas pressure had no effect on the measured quantum yields. Therefore, we are confident that we are reporting primary quantum yields. The general observation is that both CF2Br 2 and CF2CIBr dissociate to give CF 2 and halogen atoms around 200 nm. Further, the quantum yield for dissociation is unity even at a few hundred Torr pressure. The CI( 2P3/2) to CI(2PI/2 ) equilibration is expected to be complete within 10 txs at 50 Tort of He [13]. In the case of Br atoms, 3 Torr of H 2 was added to the reactor to rapidly quench any Br( 2P~/2 ) to Bff 2P3/2), the ground state. Hence, no distinction is made between the spin-orbit states of the halogen atoms in this study. To our knowledge, direct measurements of the quantum yields for CF 2, C1, or Br from CF2CIBr and of CF 2 from CF2Br 2 have not been reported previously. However, several studies have reported relative product yields. Baum and Huber [14] reported the Br atom yield to be six times larger than that of C1 in the photolysis of CF2CIBr at 193 nm, implying the formation of a CF2CI photofragment. They used single-collision conditions and showed that the energetically excited CF2C1 fragment decayed into C1 + C F 2. Our unit quantum yield for C F 2 a t 193 nm indicates that a CF2CI photofragment, if formed, was not collisionally stabilized. Therefore, our results are not inconsistent with those of Baum and Huber [14]. In an end products determination, Taylor et al. [15] measured the yields of CF2CI 2, CF2CICF2C1, CF2Br 2 and. Br 2 following the 248 nm (Hg lamp)

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R.K. Talukdar et a l . / Chemical Physics Letters 262 (1996) 669-674

gas phase photolysis of CF2C1Br. To account for the observed products and their yields, they proposed two channels to be active: (1) Br atom formation, with qb 1> 0.78 and (2) BrCI formation with ~ = 0.013. Our Br atom yield of 0.75 +__0.13 agrees with their value. As mentioned earlier, the photodissociation quantum yield of CF2CIBr and C F 2 Br 2 are of interest for atmospheric purposes. Here, we have shown that at short wavelengths, i.e. - - 2 0 0 nm, both these molecules completely dissociate. Therefore, the assumption of unit dissociation is good for the stratosphere, where short-wavelength photolysis predominates. However, we have not determined the quantum yield for the photolysis in tropospheric wavelengths, i.e. > 290 nm. Laboratory measurements of the quantum yields at long wavelengths would be beneficial. In the interim a unit quantum yield for photodissociation of CF2C1Br and CF2Br 2 in model calculations is recommended. Note, however, that this assumption of unit dissociation quantum yield does not introduce a major error in the atmospheric lifetime of CF2CIBr, whose loss is mostly in the stratosphere. In the case of C F 2 Br 2, if the photodissociation quantum yield is less than unity for wavelengths > 290 nm, its atmospheric lifetime would increase by at most 50%.

Acknowledgement W e thank R. Yokelson for contributions during the initial stages of these experiments. This work

was funded in part by the N A S A Upper Atmospheric Chemistry Research Program.

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