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Determination of quantum yields in the UV photolysis of COF2 and COFCl
22 January 1999
Chemical Physics Letters 299 Ž1999. 561–565
Determination of quantum yields in the UV photolysis of COF2 and COFCl Andreas Nolle ¨ a, Christopher Krumscheid b, Horst Heydtmann
b,)
a
b
Deutsches Fernerkundungsdatenzentrum (DFD), DLR Oberpfaffenhofen, D-82234 Wessling, Germany Institut fur ¨ Theoretische und Physikalische Chemie, J.W. Goethe UniÕersitat, ¨ Marie Curie Straße 11, D-60439 Frankfurt am Main, Germany Received 20 May 1998; in final form 2 October 1998
Abstract The photolysis quantum yields of carbonylfluoride ŽCOF2 . and carbonylchlorofluoride ŽCOFCl. were determined at various wavelengths using various light sources in the UV region. The following apparent quantum yields for pure COF2 were calculated from product analysis: F X Ž193 nm. s 0.47 " 0.03, F X Ž210 nm. s 0.57 " 0.05, and F X Ž220 nm. s 0.11 " 0.02. It is argued that, in the case of the laser pulse photolysis at 193 nm, the quantum yield of the elementary photolysis step is double the apparent quantum yield since COF2 is formed from COF radicals: F Ž193 nm. s 0.94 " 0.06. For COFCl the following apparent quantum yields were found: F X Ž210 nm. s 0.90 " 0.05 Žlamp., F X Ž210 nm. s 0.85 " 0.25 Žlaser., F X Ž222.5 nm. s 0.77 " 0.33, F X Ž230 nm. s 0.71 " 0.30 and F X Ž248 nm. s 0.52 " 0.14. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In the atmosphere, carbonylfluoride ŽCOF2 . and carbonylchlorofluoride ŽCOFCl. are the first stable decomposition products of various CFCs and HCFCs such as CFC-12 ŽCCl 2 F2 . or HCFC-22 ŽCHClF2 . and CFC-11 ŽCCl 3 F. or HCFC-141b ŽCH 3 CCl 2 F., respectively w1–3x. Quantum yields as well as absorption cross-sections are important photochemical parameters for determining the atmospheric lifetimes due to photolysis of atmospheric compounds. The quantum yield of COF2 at 206 nm was estimated by Molina and Molina to be F s 0.26 w4x. Unfortunately, the experimental arrangement for this determination was not published. The quantum yield of )
Corresponding author. Fax: q49 69 798 29445
the analogous compound COFCl at 193 nm was determined by Hermann et al. w5x using excimer laser photolysis to be 0.98 " 0.09. In this Letter we report the determination of quantum yields at three different wavelengths for COF2 and at four additional wavelengths for COFCl. In the troposphere, both COF2 and COFCl undergo rapid hydrolysis by reaction with water w6–8x. Under stratospheric conditions, however, the main decomposition pathways are photolysis w1–9x COF2 q hn ™ COF q F
Ž 1.
and reaction with OŽ1 D. w10x. The threshold wavelength for the photolysis of COF2 is about l s 220 nm w9x. For COFCl, carbon–chlorine and carbon–fluorine bond fissions are possible and photolysis can lead to
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 2 5 7 - 3
562
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565
formation of COF and COCl radicals, with branching ratios depending on the photolysis energy. However, the COCl radical dissociates thermally at ambient temperature w5x so the observable reactions are COFClq hn ™ COF q Cl
Ž 2.
COFClq hn ™ CO q F q Cl .
Ž 3.
The threshold wavelengths for these two processes q7 4 are calculated to be 318y5 1 and 231 " 4 nm, respectively, using the thermodynamic data from w11x. In this work, the apparent quantum yield is defined in the usual manner as the number of molecules in balance destroyed divided by the number of the absorbed quanta. This figure is often, but not always, identical to the quantum yield of the elementary photolysis steps Ž1., Ž2. plus Ž3.. The absorption cross-sections are needed in the evaluation of the quantum yields. They have been measured for COF2 by Nolle ¨ et al. in the wavelength range 168–230 nm at 296 K w12,13x. The absorption cross-sections of COFCl were also measured at different temperatures in the wavelength range 200–260 nm w14x and earlier by Chou et al. at room temperature in the range 186–226 nm w15x.
2. Experimental 2.1. COF2 [13] The COF2 sample was commercially obtained from Heraeus Feinchemikalien Karlsruhe, Germany Žpurity f 96%.. The contaminations could be identified in the IR absorption spectra and the main impurities were phosgene ŽCOCl 2 . and CO 2 . Because of the comparably high absorption cross-section of COCl 2 in the UV, it was necessary to purify the sample by repeated trap-to-trap low temperature distillation. The quantum yield studies of COF2 were performed with different cells placed into a FTIR spectrometer ŽBiorad FTS 20r80. after irradiation. For the lamp photolysis experiments, a long path minicell which incorporates a three mirror multiple reflection system was used which allows IR light paths from 1.2 to 7.2 m. The mini-cell was made of borosilicate glass with two KBr windows and one
MgF2 window. For the laser photolysis experiments at 193 nm, a stainless steel cross cell equipped with MgF2 windows and KBr windows was used. Both cells were connected with a standard greaseless vacuum apparatus. The pressure was measured with Baratron gauges ŽMKS.. As photolytic light sources, a pulsed ArF excimer laser ŽLambda Physik EMG 103. at 193 nm and a Hg medium pressure lamp combined with a powerful monochromator for l s 210 and 220 nm were used. All photolysis experiments were performed at room temperature Ž298 K.. The absolute light intensity of the laser was measured with a joulemeter ŽGentec ED-500. connected to a digital storage oscilloscope ŽHameg HM 408.. The laser photolysis experiments were performed with initial pressures of 1000, 2000 and 3000 Pa. For each partial pressure of COF2 , we performed 12 measurements. The averaged output pulse energy of the excimer ArF laser was 150 mJ. A pulse frequency of 1 Hz and 20–250 pulses per photolysis were used. This photolysis condition led to a decomposition of COF2 in the range of 1.5–9%, as determined by FTIR spectroscopy. For the lamp photolysis experiments, the light intensity within the photolysis cell was measured at the beginning of each run by actinometry using COCl 2 ŽF s 1. w16,17x. The intensity of the light source for the wavelength centered at l s 210 " 2.5 nm was between 1.57 = 10 15 and 4.4 = 10 15 photonsrs. For the wavelength centered at 220 " 2.5 nm the light intensity was between 3.8 = 10 15 and 4.8 = 10 15 photonsrs. The absorption cross-sections of COF2 were taken from the work of Nolle ¨ et al. w12x. The lamp photolysis experiments were performed with initial pressures of COF2 from 210 to 630 Pa and in some experiments an inert gas was added. The irradiation time varied between 60 and 120 minutes. In both types of experiments, the numbers of photolysed COF2 molecules were determined by comparison of characteristic IR peaks Ž n 1 at 1928 cmy1 and n 6 at 774 cmy1 . using the spectra taken before and after the photolysis. The quantum yields were determined by application of the Beer–Lambert law. 2.2. COFCl For the COFCl, measurements, the same procedure was followed as for COF2 and only differing steps are mentioned. The laser photolysis of COFCl
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565
Fig. 1. Results of the quantum yield experiments for COF2 without added inert gas ŽB sexcimer, l sexcimer Žapparent quantum yield., ' s lamp..
were all done with the described cross cell. The cell was filled with initial pressures of about 500 Pa. The substance used was synthesized in the laboratory of Prof. H. Willner, University of Hannover, since it is not commercially available. Impurities were Cl 2 , SO 2 and COCl 2 Ž0.2% each.. For the photolysis of COFCl at 248 nm, the above described excimer laser was used with KrF as the laser medium. Due to the small absorption cross-section of s s 9.7 = 10y2 2 Žcm2 moleculey1 ., up to 7000 pulses were necessary for a decomposition of about 5% even with an average output energy of 150 mJ per pulse. With pulse frequencies of 20 Hz, the samples were irradiated for about 5 minutes. The photolyses at the remaining wavelengths were done with an excimer pumped dye laser system ŽLambda
563
Physik LPX 605i Ž308 nm XeCl., Lambda Physik LPD 3000. using the dyes Stilben 3 Ž210 nm. and Coumarin 47 Ž222.5 and 230 nm. with frequency doubling. The advantage of increasing absorption cross-sections to shorter wavelengths by factors of about 20–100 was compensated by the smaller output energy of the laser by a factor of 10y3 , resulting in pulse numbers up to 10 5 for degradations of about 1.5%. With a pulse frequency of 20 Hz, the samples were irradiated for up to 1.5 h. At 210 nm, not only laser experiments but also lamp experiments were accomplished w13x. The analysis of the photolysis products was performed as described above. The COFCl degradation was followed by observing the IR bands n 2 at 1876 cmy1 and n4 at 1095 cmy1 . Experiments with COCl 2 always yielded the correct quantum yield of F s 1.
3. Results and discussion 3.1. COF2 Due to the different techniques used for the laser and the lamp photolysis experiments, separate discussions for the experimental arrangements are necessary. Ža. For l s 193 nm, we determined a mean quantum yield of F X s 0.47 " 0.03. The deviation is
Table 1 Quantum yields for COF2 with and without inert gas. Inert gas experiments are marked by asterisk. At 193 nm only the apparent quantum yield is given
l
F
Žnm.
Hg lamp
laser
193 210
0.58"0.05 0.57"0.05) 0.07"0.03 0.11"0.02)
0.47"0.03 – – -
220
Fig. 2. Results of the quantum yield experiments for COFCl. The primary quantum yields are identical with the apparent quantum yields. The value at 193 nm is taken from Ref. w5x ŽB sexcimer, ' s lamp, I sdye..
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565
564 Table 2 Quantum yields for COFCl
l
F
Žnm.
Hg lamp
laser
210 222.5 230 248
0.90"0.05 – – –
0.85"0.25 0.77"0.33 0.71"0.30 0.52"0.14
within the limits of the estimated systematic error of 12%. Under our experimental conditions, because of the relatively high concentrations of COF formed, the self-reaction of these COF radicals via COF q COF ™ COF2 q CO
Ž 4.
was possible. The proof for this is the formation of COF2 in the photolysis of COFCl performed in the same manner w5x. The rate constant of reaction Ž4. was investigated by Behr et al. and Maricq et al. w18,19x. We assume that all the COF radicals quantitatively react this way and then the quantum yield for Ž1. becomes F s 0.94 " 0.06. The quantum yield reported by Molina and Molina w4x is too low. Žb. Opposed to the laser flash photolysis results, the quantum yields measured in the lamp experiments are true values. Because of the continued irradiation with a low intensity, no sufficient concentration of COF radicals can build up. Therefore, reaction Ž4. is of minor importance. Other processes, especially wall reactions, destroy the COF radicals. This result is supported by the photolysis experiments with COFCl in this work. In these, under the same experimental conditions, no COF2 could be detected. The addition of an inert gas ŽN2 . in the range 600–1000 hPa did not change the quantum yield values considerably. The mean values of all quantum yield experiments with COF2 are shown in Fig. 1 and summarized in Table 1. The quantum yields obtained for the elementary process Ž1. Žsquare and triangles in Fig. 1. show a similar fall off with increasing wavelength, as reported for formaldehyde ŽCH 2 O. by Horowitz et al. w20x. 3.2. COFCl The only monitored product of the COFCl photolysis was COF2 stemming from steps Ž2. and Ž4., and
this occurred exclusively in the excimer and dye laser experiments. Due to the small degradation, signals of other products like CO that should have been detectable by IR spectroscopy did not exceed the noise level. The quantum yields were calculated simply from the degradation of the parent substance. These apparent values can be taken as the quantum yields of the elementary steps Ž2. plus Ž3. since the parent molecules cannot be reformed. The large errors of the quantum yields of the dye laser photolysis are mainly due to problems with the determination of the decreasing laser energy during the long irradiations. The results are shown in Fig. 2 and Table 2. They are at 210 nm in good agreement with F Ž193 nm. s 0.98 " 0.09 reported in an earlier paper from our laboratory w5x. The quantum yields show a similar dependence on wavelength to those of CH 2 O measured by Horowitz and Calvert w20x and of COF2 reported in this work. It is remarkable that at 248 nm, the quantum yield is quite low considering that the threshold wavelength of CCl bond fission is not even close. Branching ratios for reactions Ž2. and Ž3. cannot be extracted from our results. The analytical data for COF2 , considering the error limits, do not require us to assume the occurrence of reaction Ž3..
Acknowledgements The authors wish to thank the German Ministry of Research and Development ŽBMFT. for financial support ŽProject No. 01 LO 91115r6.. We thank also Prof. H. Willner, Hannover, Germany for a sample of pure carbonylchlorofluoride.
References w1x P.J. Crutzen, I.S.A. Isaksen, J.R. McAffee, J. Geophys. Res. 83 Ž1978. 345. w2x N.D. Sze, Geophys. Res. Let. 5 Ž1978. 781. w3x World Meteorological Organization, Report No. 20, Scientific Assessment of Statospheric Ozone: 1989, Vol. 2 Ž1989. WMO Genf. w4x M.J. Molina, L.T. Molina, presented at the 182nd national meeting of the American Chemical Society, New York, 1982. w5x M. Hermann, A. Nolle, ¨ H. Heydtmann, Chem. Phys. Let. 226 Ž1994. 559.
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565 w6x P.H. Wine, W.L. Chameides, WMO Report No. 20, Vol. 2, AFEAS Report, 1990, p. 271. w7x W.J. DeBruyn, S.X. Duan, X.Q. Shi, P. Davidovits, Geophys. Res. Let. 19 Ž1992. 1939. w8x D.R. Hanson, A.R. Ravishankara, Geophys. Res. Let. 18 Ž1991. 1699. w9x R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson Jr., J.A. Kerr, J. Troe, J. Phys. Chem. Ref. Data 21 Ž1992. 1125. w10x P.H. Wine, A.R. Ravishankara, Chem. Phys. Let. 96 Ž1983. 129. w11x W.B. DeMore, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, C.E. Kolb, M.J. Molina, JPL Publ. 97-4, Evaluation Number 12, NASArJet Propulsion Laboratory, Pasadena, CA, 1997. w12x A. Nolle, H. Heydtmann, R. Meller, W. Schneider, G.K. ¨ Moortgat, Geophys. Res. Let. 19 Ž1992. 281.
565
w13x A. Nolle, ¨ Ph.D. Thesis, University of Frankfurt, 1996. w14x A. Nolle, H. Heydtmann, R. Meller, G.K. Moortgat, Geo¨ phys. Res. Let. 20 Ž1993. 707. w15x G.C. Chou, G. Crescentini, H. Vera-Ruiz, W.S. Smith, F.S. Rowland, Results presented at the 173rd American Chemical Society National Meeting, New Orleans, LA, March 1977. w16x M.H.J. Wijnen, J. Am. Chem. Soc. 83 Ž1961. 3014. w17x J.G. Calvert, J.N. Pitts Jr., Photochemistry, John Wiley, New York, 1966. w18x P. Behr, K. Goldbach, H. Heydtmann, Int. J. Chem. Kinet. 25 Ž1993. 957. w19x M.M. Maricq, J.J. Szente, G.A. Khitrov, J.S. Francisco, Chem. Phys. Let. 199 Ž1992. 71. w20x A. Horowitz, J.G. Calvert, Int. J. Chem. Kinet. 10 Ž1978. 805.
Chemical Physics Letters 299 Ž1999. 561–565
Determination of quantum yields in the UV photolysis of COF2 and COFCl Andreas Nolle ¨ a, Christopher Krumscheid b, Horst Heydtmann
b,)
a
b
Deutsches Fernerkundungsdatenzentrum (DFD), DLR Oberpfaffenhofen, D-82234 Wessling, Germany Institut fur ¨ Theoretische und Physikalische Chemie, J.W. Goethe UniÕersitat, ¨ Marie Curie Straße 11, D-60439 Frankfurt am Main, Germany Received 20 May 1998; in final form 2 October 1998
Abstract The photolysis quantum yields of carbonylfluoride ŽCOF2 . and carbonylchlorofluoride ŽCOFCl. were determined at various wavelengths using various light sources in the UV region. The following apparent quantum yields for pure COF2 were calculated from product analysis: F X Ž193 nm. s 0.47 " 0.03, F X Ž210 nm. s 0.57 " 0.05, and F X Ž220 nm. s 0.11 " 0.02. It is argued that, in the case of the laser pulse photolysis at 193 nm, the quantum yield of the elementary photolysis step is double the apparent quantum yield since COF2 is formed from COF radicals: F Ž193 nm. s 0.94 " 0.06. For COFCl the following apparent quantum yields were found: F X Ž210 nm. s 0.90 " 0.05 Žlamp., F X Ž210 nm. s 0.85 " 0.25 Žlaser., F X Ž222.5 nm. s 0.77 " 0.33, F X Ž230 nm. s 0.71 " 0.30 and F X Ž248 nm. s 0.52 " 0.14. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In the atmosphere, carbonylfluoride ŽCOF2 . and carbonylchlorofluoride ŽCOFCl. are the first stable decomposition products of various CFCs and HCFCs such as CFC-12 ŽCCl 2 F2 . or HCFC-22 ŽCHClF2 . and CFC-11 ŽCCl 3 F. or HCFC-141b ŽCH 3 CCl 2 F., respectively w1–3x. Quantum yields as well as absorption cross-sections are important photochemical parameters for determining the atmospheric lifetimes due to photolysis of atmospheric compounds. The quantum yield of COF2 at 206 nm was estimated by Molina and Molina to be F s 0.26 w4x. Unfortunately, the experimental arrangement for this determination was not published. The quantum yield of )
Corresponding author. Fax: q49 69 798 29445
the analogous compound COFCl at 193 nm was determined by Hermann et al. w5x using excimer laser photolysis to be 0.98 " 0.09. In this Letter we report the determination of quantum yields at three different wavelengths for COF2 and at four additional wavelengths for COFCl. In the troposphere, both COF2 and COFCl undergo rapid hydrolysis by reaction with water w6–8x. Under stratospheric conditions, however, the main decomposition pathways are photolysis w1–9x COF2 q hn ™ COF q F
Ž 1.
and reaction with OŽ1 D. w10x. The threshold wavelength for the photolysis of COF2 is about l s 220 nm w9x. For COFCl, carbon–chlorine and carbon–fluorine bond fissions are possible and photolysis can lead to
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 2 5 7 - 3
562
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565
formation of COF and COCl radicals, with branching ratios depending on the photolysis energy. However, the COCl radical dissociates thermally at ambient temperature w5x so the observable reactions are COFClq hn ™ COF q Cl
Ž 2.
COFClq hn ™ CO q F q Cl .
Ž 3.
The threshold wavelengths for these two processes q7 4 are calculated to be 318y5 1 and 231 " 4 nm, respectively, using the thermodynamic data from w11x. In this work, the apparent quantum yield is defined in the usual manner as the number of molecules in balance destroyed divided by the number of the absorbed quanta. This figure is often, but not always, identical to the quantum yield of the elementary photolysis steps Ž1., Ž2. plus Ž3.. The absorption cross-sections are needed in the evaluation of the quantum yields. They have been measured for COF2 by Nolle ¨ et al. in the wavelength range 168–230 nm at 296 K w12,13x. The absorption cross-sections of COFCl were also measured at different temperatures in the wavelength range 200–260 nm w14x and earlier by Chou et al. at room temperature in the range 186–226 nm w15x.
2. Experimental 2.1. COF2 [13] The COF2 sample was commercially obtained from Heraeus Feinchemikalien Karlsruhe, Germany Žpurity f 96%.. The contaminations could be identified in the IR absorption spectra and the main impurities were phosgene ŽCOCl 2 . and CO 2 . Because of the comparably high absorption cross-section of COCl 2 in the UV, it was necessary to purify the sample by repeated trap-to-trap low temperature distillation. The quantum yield studies of COF2 were performed with different cells placed into a FTIR spectrometer ŽBiorad FTS 20r80. after irradiation. For the lamp photolysis experiments, a long path minicell which incorporates a three mirror multiple reflection system was used which allows IR light paths from 1.2 to 7.2 m. The mini-cell was made of borosilicate glass with two KBr windows and one
MgF2 window. For the laser photolysis experiments at 193 nm, a stainless steel cross cell equipped with MgF2 windows and KBr windows was used. Both cells were connected with a standard greaseless vacuum apparatus. The pressure was measured with Baratron gauges ŽMKS.. As photolytic light sources, a pulsed ArF excimer laser ŽLambda Physik EMG 103. at 193 nm and a Hg medium pressure lamp combined with a powerful monochromator for l s 210 and 220 nm were used. All photolysis experiments were performed at room temperature Ž298 K.. The absolute light intensity of the laser was measured with a joulemeter ŽGentec ED-500. connected to a digital storage oscilloscope ŽHameg HM 408.. The laser photolysis experiments were performed with initial pressures of 1000, 2000 and 3000 Pa. For each partial pressure of COF2 , we performed 12 measurements. The averaged output pulse energy of the excimer ArF laser was 150 mJ. A pulse frequency of 1 Hz and 20–250 pulses per photolysis were used. This photolysis condition led to a decomposition of COF2 in the range of 1.5–9%, as determined by FTIR spectroscopy. For the lamp photolysis experiments, the light intensity within the photolysis cell was measured at the beginning of each run by actinometry using COCl 2 ŽF s 1. w16,17x. The intensity of the light source for the wavelength centered at l s 210 " 2.5 nm was between 1.57 = 10 15 and 4.4 = 10 15 photonsrs. For the wavelength centered at 220 " 2.5 nm the light intensity was between 3.8 = 10 15 and 4.8 = 10 15 photonsrs. The absorption cross-sections of COF2 were taken from the work of Nolle ¨ et al. w12x. The lamp photolysis experiments were performed with initial pressures of COF2 from 210 to 630 Pa and in some experiments an inert gas was added. The irradiation time varied between 60 and 120 minutes. In both types of experiments, the numbers of photolysed COF2 molecules were determined by comparison of characteristic IR peaks Ž n 1 at 1928 cmy1 and n 6 at 774 cmy1 . using the spectra taken before and after the photolysis. The quantum yields were determined by application of the Beer–Lambert law. 2.2. COFCl For the COFCl, measurements, the same procedure was followed as for COF2 and only differing steps are mentioned. The laser photolysis of COFCl
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565
Fig. 1. Results of the quantum yield experiments for COF2 without added inert gas ŽB sexcimer, l sexcimer Žapparent quantum yield., ' s lamp..
were all done with the described cross cell. The cell was filled with initial pressures of about 500 Pa. The substance used was synthesized in the laboratory of Prof. H. Willner, University of Hannover, since it is not commercially available. Impurities were Cl 2 , SO 2 and COCl 2 Ž0.2% each.. For the photolysis of COFCl at 248 nm, the above described excimer laser was used with KrF as the laser medium. Due to the small absorption cross-section of s s 9.7 = 10y2 2 Žcm2 moleculey1 ., up to 7000 pulses were necessary for a decomposition of about 5% even with an average output energy of 150 mJ per pulse. With pulse frequencies of 20 Hz, the samples were irradiated for about 5 minutes. The photolyses at the remaining wavelengths were done with an excimer pumped dye laser system ŽLambda
563
Physik LPX 605i Ž308 nm XeCl., Lambda Physik LPD 3000. using the dyes Stilben 3 Ž210 nm. and Coumarin 47 Ž222.5 and 230 nm. with frequency doubling. The advantage of increasing absorption cross-sections to shorter wavelengths by factors of about 20–100 was compensated by the smaller output energy of the laser by a factor of 10y3 , resulting in pulse numbers up to 10 5 for degradations of about 1.5%. With a pulse frequency of 20 Hz, the samples were irradiated for up to 1.5 h. At 210 nm, not only laser experiments but also lamp experiments were accomplished w13x. The analysis of the photolysis products was performed as described above. The COFCl degradation was followed by observing the IR bands n 2 at 1876 cmy1 and n4 at 1095 cmy1 . Experiments with COCl 2 always yielded the correct quantum yield of F s 1.
3. Results and discussion 3.1. COF2 Due to the different techniques used for the laser and the lamp photolysis experiments, separate discussions for the experimental arrangements are necessary. Ža. For l s 193 nm, we determined a mean quantum yield of F X s 0.47 " 0.03. The deviation is
Table 1 Quantum yields for COF2 with and without inert gas. Inert gas experiments are marked by asterisk. At 193 nm only the apparent quantum yield is given
l
F
Žnm.
Hg lamp
laser
193 210
0.58"0.05 0.57"0.05) 0.07"0.03 0.11"0.02)
0.47"0.03 – – -
220
Fig. 2. Results of the quantum yield experiments for COFCl. The primary quantum yields are identical with the apparent quantum yields. The value at 193 nm is taken from Ref. w5x ŽB sexcimer, ' s lamp, I sdye..
A. Nolle ¨ et al.r Chemical Physics Letters 299 (1999) 561–565
564 Table 2 Quantum yields for COFCl
l
F
Žnm.
Hg lamp
laser
210 222.5 230 248
0.90"0.05 – – –
0.85"0.25 0.77"0.33 0.71"0.30 0.52"0.14
within the limits of the estimated systematic error of 12%. Under our experimental conditions, because of the relatively high concentrations of COF formed, the self-reaction of these COF radicals via COF q COF ™ COF2 q CO
Ž 4.
was possible. The proof for this is the formation of COF2 in the photolysis of COFCl performed in the same manner w5x. The rate constant of reaction Ž4. was investigated by Behr et al. and Maricq et al. w18,19x. We assume that all the COF radicals quantitatively react this way and then the quantum yield for Ž1. becomes F s 0.94 " 0.06. The quantum yield reported by Molina and Molina w4x is too low. Žb. Opposed to the laser flash photolysis results, the quantum yields measured in the lamp experiments are true values. Because of the continued irradiation with a low intensity, no sufficient concentration of COF radicals can build up. Therefore, reaction Ž4. is of minor importance. Other processes, especially wall reactions, destroy the COF radicals. This result is supported by the photolysis experiments with COFCl in this work. In these, under the same experimental conditions, no COF2 could be detected. The addition of an inert gas ŽN2 . in the range 600–1000 hPa did not change the quantum yield values considerably. The mean values of all quantum yield experiments with COF2 are shown in Fig. 1 and summarized in Table 1. The quantum yields obtained for the elementary process Ž1. Žsquare and triangles in Fig. 1. show a similar fall off with increasing wavelength, as reported for formaldehyde ŽCH 2 O. by Horowitz et al. w20x. 3.2. COFCl The only monitored product of the COFCl photolysis was COF2 stemming from steps Ž2. and Ž4., and
this occurred exclusively in the excimer and dye laser experiments. Due to the small degradation, signals of other products like CO that should have been detectable by IR spectroscopy did not exceed the noise level. The quantum yields were calculated simply from the degradation of the parent substance. These apparent values can be taken as the quantum yields of the elementary steps Ž2. plus Ž3. since the parent molecules cannot be reformed. The large errors of the quantum yields of the dye laser photolysis are mainly due to problems with the determination of the decreasing laser energy during the long irradiations. The results are shown in Fig. 2 and Table 2. They are at 210 nm in good agreement with F Ž193 nm. s 0.98 " 0.09 reported in an earlier paper from our laboratory w5x. The quantum yields show a similar dependence on wavelength to those of CH 2 O measured by Horowitz and Calvert w20x and of COF2 reported in this work. It is remarkable that at 248 nm, the quantum yield is quite low considering that the threshold wavelength of CCl bond fission is not even close. Branching ratios for reactions Ž2. and Ž3. cannot be extracted from our results. The analytical data for COF2 , considering the error limits, do not require us to assume the occurrence of reaction Ž3..
Acknowledgements The authors wish to thank the German Ministry of Research and Development ŽBMFT. for financial support ŽProject No. 01 LO 91115r6.. We thank also Prof. H. Willner, Hannover, Germany for a sample of pure carbonylchlorofluoride.
References w1x P.J. Crutzen, I.S.A. Isaksen, J.R. McAffee, J. Geophys. Res. 83 Ž1978. 345. w2x N.D. Sze, Geophys. Res. Let. 5 Ž1978. 781. w3x World Meteorological Organization, Report No. 20, Scientific Assessment of Statospheric Ozone: 1989, Vol. 2 Ž1989. WMO Genf. w4x M.J. Molina, L.T. Molina, presented at the 182nd national meeting of the American Chemical Society, New York, 1982. w5x M. Hermann, A. Nolle, ¨ H. Heydtmann, Chem. Phys. Let. 226 Ž1994. 559.
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