- Email: [email protected]
Study of the recombination of chlorine atoms by flash photolysis
Volume 19, number 4
CHEMICALPHYSICS
STUDY OF THE RECOMBINATION
LETTERS
15 April 1973
OF CHLORLNE ATOMS BY FLASH PHOTOLYSIS
H. HIPPLER and J. TROE Institlrt de Chirnie Physique de I’Ecole Polyfechnique Fidhale de Lamanne, Lausmne, Swirzerhd Received 3 hfarch 1973
The recombination of chlorine atoms has been studied in its third order range by conventional f&h photolysis. llte rate constant wns measured in the inert gases He, Ne, Ar, Na, CO-J, CFd,CaFe, SiF4 and SFe at 300°K and in the pressure range 0.5-4 atm.
1. Introduction
2. Experimental
and results
The recombination of bromine [l-4] and of iodine [4-17-l atoms has been studied extensively using flash photolysis. An application of this method to the recombination of chlorine atcms, because of the smaller absorption coefficients of chlorine molecules, is somewhat more difficult because of detection problems. To our knowledge no rate data have yet been obtained by this method for chlorine. The available data on the rate constant of chlorine recombination at low temperatures with the third bodies He, Ar and Cl, have been determined in discharge-flow studies [II-151. Further information at temperatures between 500 and 600°K has been derived from a photochlorination study [ 16].The reverse dissociation of chlorine molecules was investigated in shock waves in the range 1600-300°K [ 1721]. A summary of the available results has been given in ref. [22]. In a recent series of experiments [IO, 1 I], using conventional and laser flash photolysis, we have followed the recombination of iodine atoms with 12 different inert gases in the pressure range O.l-:OOO atm and in the liquid phase. In this study, the transition from thirdorder recombinations in low-pressure gases to liquidphase recombinations was monitored in detail and information on the iodine-atom-inert-gas interaction was obtained. The present work starts the analog experiments on ciliorirre in the low-pressure third-order range up to about 4 atm and in several different inert gases. An extension to higher pressures will be described later.
The experiments have been performed by using a conventional 20 ~.lsec flash from a 2 W discharge. The flash lamp and the reaction cell (length 25 cm, inner diameter 1.6 cm), both made of Pyrex glass, were mounted parallel in a steel vessel. The inert gas was frIIed inside and outside the reaction cell. This allows working at pressures up to several hundred atmospheres. An aluminum reflector covered the inner wall of the steel vessel. After flashing, the reappearance of the chlorine molecules was monitored through quartz windows of the reaction cell by the light absorption at 313OA. As
light source, a high-pressure 200 W Xe-Hg arc lamp was used (Hanovia 901-B-1 1). By workins under these conditions, about 80% of the normaily scattered flash light was suppressed by absorption in the Pyrex walls. The changes of the chlorine absorption during the observed part of the reaction were of the order of 0.5-2 per cent of the initial absorption. The chlorine used was carefully purified. Chlorine partial pressures in the experiments were between 3 and 10 torr. The temperature of the experiments was 20-25°C. For the inert gases He, Ne, Ar, and N,, the pressure range used was l-4 atm, for the inert gasesSF6, SiF,, CF,, CO, and C,F, the range was OS-3 atm. At inert gas pressures above the lower limits given above, heating of the mixture by the flash gave a temperature increase of at most 6’C. Also in this pressure range, the contribution of chlorine molecules as third body in the recombination could be negIected under our conditions [12-l 53. .4t lower inert-gas pressures, .
607
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
..
Table I Third-or&r rate constants for the recombination atoms in the presence of different chaperons
of chlorine
Gas
k‘ 11Or5 cm6 mole-’ set’ ]
Reference
He
1.5 1.4 1.7 2.0 4.25 4.37 7.5 15 13.5 20.4 20 56 55 21 57 21
113,221 this work this work 113,221 1141 1151 this work this work [ 13, 221 1141 1151 this work this work this work this work this work
NC
Ar
Nz (32
co1 CF4 cz
F6
SiF4 SF6
(313°K) + 0.2 (298°K) 2 0.2 (298’K) (313°K) * 0.8 (293°K) * 0.5 (298°K) i 1.5 (298°K) ?: 4 (298°K) (313°K) (293°K) r (298’K) ?I 6 (298°K) +-5 (298°K) +4 (298°K) + 10 (298°K) *4 (298°K)
considerable perturbations of the measurements were observed. These may partly be due to thermal effects. Further, the contribution of chlorine as third body may no longer be neglected. At pressures’above the limits given, the recombination rates were too rapid to be measured with the given arrangement. The rate canstants for the observed reaction Cl + Cl + M + Cl, + M are presented in the form k=(1/[C112[M])
d[Ci,]/dr.
The values for k obtained with different M are given in table 1 and compared with literature__ values. Ask has been found to be proportional to T-2.y for M = Ar [ 151, measurements at 293 and 3 13°K are easily compared. For M = He, our value is somewhat lower than the flowsystem value of ref. [ 131, for M = Ar our value is higher than that of refs. j14, 1.51.Compared to the recombination of iodine atoms, the same marked increase of k from He and Ne to polyatomic inert gases is observed. The relative increases are somewhat different and a boiling-temperature correlation [23] is less clear than for iodine. However,-the experimental accuracy of these chlorine experiments because of the larger tletection problems-probabiy is worse than in the iodine systern. -.’
15 April 19738
By photofragment spectroscopy 1241: it has been shown -hat under flashing at 347 1a chlorine groundstate atoms C1(2P3,2) are formed predominantly. As the absorption of the Pyrex walls of the reaction cell and the flash lamp cuts off the flash light below 3200A, it is rather probable that only recombination of groundstate atoms has been observed in the present work. Very short lived transient absorptions as observed during the flash in ref. [25] could not be detected in our experiments. In the observed pressure range, no deviations from a third-order rate law couId be observed yet. However, for M = Nz and P = 4 atm the second-order rate constant k[M] is already 2.5 X lOI cm3 mole-1 set-I. For iodine recombination, limiting high-pressure secondorder rate constants of about 2 X 1013 cm3 mole-l set-1 have been observed in refs. [IO, 111. Therefore, one may expect a change of reaction order for the chlorine recombination in N2 below 100 atm.
Acknowledgement Financial support of the Schweizerischer Nationalfonds is gratefully acknowledged.
References 111 J.K. Ip and G. Burns, Discussions Faraday Sot. 44 (1967) 241; J. Chem. Phys. 51 (1969) 3413,34X. 121 S.K. Chang, A.G. Clarke and G. Burns, J. Chem. Phys. 54 (1971) 1835. 131 B.A. DeGrsff and K.J. Lan8, J. Phys. Chem. 74 (1970) 4181. [4] R.L. Strong, J.C.W.Chien,P.E. Graf and J.E. Willard, J. Chem. Phys. 26 (1957) 1287. 151 E. Rabinovitch and W.C. Wood, J. Chem. Phys. 4 (1936) 497. 161MI. Christie, A.G. Harrison, R.G.W. Norrish and G. Porter, Proc. Roy. Sot. A 231 (1955) 446. 171 D.L. Bunker and N. Davidson, J. Am. Chem. Sot. 80 (1958) 508.5. 181 3. Britton, N. Davidson, W. Gchmann and G. Schott, J. Chem. Phys. 25 (1956) 804. 191 J.Troe and H.Gg. Wagner, Z. Physik. Chem. 55 (1967) 326. 1lOI H. Hippler, K. Luther and J. Troe, Chem. Phys. Letters 16 (1972) 174. IllI K.Luther, Doktorarbeit (1972), Ecole Polytechnique FedCrale de Lausanne, Switzerland; H. Hippler, K. Luther and J. Troe, Ber. Bunsenges. Physik. Chem., to be published.
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
[ 121 J.W. Linnett and M.H. Both, Nature 199 (1963) 1181. [ 131 L.W. Bader and E,A. Ogyzlo, Nature 201 (1964) 491. 1141 E. Hutton and M. Wright, Trans. Faraday Sot. 61 (1065) 78; E. Hutton, Nature 203 (1964) 835. [ 15 J M.A.A. Clyne and D.H. Stedman, Trans. Faraday Sot. 64 (1968) 2698. 1161 G. Chiltz, R. Eckling, P. Goldfinger, G. Huybrechts, G. Martens and G. Simocns, Bull. Sot. Chim. Be&. 71 (1962) 747. [17] H. Hinoka and R Hardwick, 3. Chem. Phys. 36 (1962) 1715. 1181 T.A. Jacobs and R.R Geidt, J. Chem. Phys. 39 (1963) 749.
15 April 1973
1191 M. von Thiel, DJ. S,eery and D.J. B&ton,
J. Phys.
Chem. 69 (1965) 834. [20] R.W. Die&
andW.J. Felmlee, i.Chem. Phys. 39 (1963) 2115. [21] .R_A, Carbaretta and H.B. Palmer, J. Chem. Phys 46 (1967) 1333;47 (1967) 2202. [22] A.C. Lloyd, Intern. J. Chem. Kinetics 3 (1971) 39. 1231 K.E. Russelsand J. Simons, Proc. Roy Sot. A217 (1953) 279. 1241 G.E. Busch, R.T. Mahoney, R.J. Morse and K.R. Wilson, I. Chem. Phys. 51 (1969) 449. [ 251 A.G. Briss and R.G. Norrish, Proc. Roy Sac. A276 (1963) 51.
ERRATUM
S. Kielich, Depolarization of light scattering by atonlic and molecular solutions with strongly anisotropic translational-orientational fluctuations, Chem. Phys. Letters (1971) 516. Eq. (10) should have the following form (resulting term with riyjyk should be corrected to 3yiyjyk):
+
2YjYi(3aj
f
3aj
+
yi +
yj)
10
from formula (A.2), where the
K;” ,
with the radial-angular correlation parameter JiiRA in the form (11) containing cos2 oqi (and not cos2 Bqj). By effecting the interchange i +j and j + i in formula (1 l), corrected as above, one obtains the form of the parameter JltA.
609
CHEMICALPHYSICS
STUDY OF THE RECOMBINATION
LETTERS
15 April 1973
OF CHLORLNE ATOMS BY FLASH PHOTOLYSIS
H. HIPPLER and J. TROE Institlrt de Chirnie Physique de I’Ecole Polyfechnique Fidhale de Lamanne, Lausmne, Swirzerhd Received 3 hfarch 1973
The recombination of chlorine atoms has been studied in its third order range by conventional f&h photolysis. llte rate constant wns measured in the inert gases He, Ne, Ar, Na, CO-J, CFd,CaFe, SiF4 and SFe at 300°K and in the pressure range 0.5-4 atm.
1. Introduction
2. Experimental
and results
The recombination of bromine [l-4] and of iodine [4-17-l atoms has been studied extensively using flash photolysis. An application of this method to the recombination of chlorine atcms, because of the smaller absorption coefficients of chlorine molecules, is somewhat more difficult because of detection problems. To our knowledge no rate data have yet been obtained by this method for chlorine. The available data on the rate constant of chlorine recombination at low temperatures with the third bodies He, Ar and Cl, have been determined in discharge-flow studies [II-151. Further information at temperatures between 500 and 600°K has been derived from a photochlorination study [ 16].The reverse dissociation of chlorine molecules was investigated in shock waves in the range 1600-300°K [ 1721]. A summary of the available results has been given in ref. [22]. In a recent series of experiments [IO, 1 I], using conventional and laser flash photolysis, we have followed the recombination of iodine atoms with 12 different inert gases in the pressure range O.l-:OOO atm and in the liquid phase. In this study, the transition from thirdorder recombinations in low-pressure gases to liquidphase recombinations was monitored in detail and information on the iodine-atom-inert-gas interaction was obtained. The present work starts the analog experiments on ciliorirre in the low-pressure third-order range up to about 4 atm and in several different inert gases. An extension to higher pressures will be described later.
The experiments have been performed by using a conventional 20 ~.lsec flash from a 2 W discharge. The flash lamp and the reaction cell (length 25 cm, inner diameter 1.6 cm), both made of Pyrex glass, were mounted parallel in a steel vessel. The inert gas was frIIed inside and outside the reaction cell. This allows working at pressures up to several hundred atmospheres. An aluminum reflector covered the inner wall of the steel vessel. After flashing, the reappearance of the chlorine molecules was monitored through quartz windows of the reaction cell by the light absorption at 313OA. As
light source, a high-pressure 200 W Xe-Hg arc lamp was used (Hanovia 901-B-1 1). By workins under these conditions, about 80% of the normaily scattered flash light was suppressed by absorption in the Pyrex walls. The changes of the chlorine absorption during the observed part of the reaction were of the order of 0.5-2 per cent of the initial absorption. The chlorine used was carefully purified. Chlorine partial pressures in the experiments were between 3 and 10 torr. The temperature of the experiments was 20-25°C. For the inert gases He, Ne, Ar, and N,, the pressure range used was l-4 atm, for the inert gasesSF6, SiF,, CF,, CO, and C,F, the range was OS-3 atm. At inert gas pressures above the lower limits given above, heating of the mixture by the flash gave a temperature increase of at most 6’C. Also in this pressure range, the contribution of chlorine molecules as third body in the recombination could be negIected under our conditions [12-l 53. .4t lower inert-gas pressures, .
607
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
..
Table I Third-or&r rate constants for the recombination atoms in the presence of different chaperons
of chlorine
Gas
k‘ 11Or5 cm6 mole-’ set’ ]
Reference
He
1.5 1.4 1.7 2.0 4.25 4.37 7.5 15 13.5 20.4 20 56 55 21 57 21
113,221 this work this work 113,221 1141 1151 this work this work [ 13, 221 1141 1151 this work this work this work this work this work
NC
Ar
Nz (32
co1 CF4 cz
F6
SiF4 SF6
(313°K) + 0.2 (298°K) 2 0.2 (298’K) (313°K) * 0.8 (293°K) * 0.5 (298°K) i 1.5 (298°K) ?: 4 (298°K) (313°K) (293°K) r (298’K) ?I 6 (298°K) +-5 (298°K) +4 (298°K) + 10 (298°K) *4 (298°K)
considerable perturbations of the measurements were observed. These may partly be due to thermal effects. Further, the contribution of chlorine as third body may no longer be neglected. At pressures’above the limits given, the recombination rates were too rapid to be measured with the given arrangement. The rate canstants for the observed reaction Cl + Cl + M + Cl, + M are presented in the form k=(1/[C112[M])
d[Ci,]/dr.
The values for k obtained with different M are given in table 1 and compared with literature__ values. Ask has been found to be proportional to T-2.y for M = Ar [ 151, measurements at 293 and 3 13°K are easily compared. For M = He, our value is somewhat lower than the flowsystem value of ref. [ 131, for M = Ar our value is higher than that of refs. j14, 1.51.Compared to the recombination of iodine atoms, the same marked increase of k from He and Ne to polyatomic inert gases is observed. The relative increases are somewhat different and a boiling-temperature correlation [23] is less clear than for iodine. However,-the experimental accuracy of these chlorine experiments because of the larger tletection problems-probabiy is worse than in the iodine systern. -.’
15 April 19738
By photofragment spectroscopy 1241: it has been shown -hat under flashing at 347 1a chlorine groundstate atoms C1(2P3,2) are formed predominantly. As the absorption of the Pyrex walls of the reaction cell and the flash lamp cuts off the flash light below 3200A, it is rather probable that only recombination of groundstate atoms has been observed in the present work. Very short lived transient absorptions as observed during the flash in ref. [25] could not be detected in our experiments. In the observed pressure range, no deviations from a third-order rate law couId be observed yet. However, for M = Nz and P = 4 atm the second-order rate constant k[M] is already 2.5 X lOI cm3 mole-1 set-I. For iodine recombination, limiting high-pressure secondorder rate constants of about 2 X 1013 cm3 mole-l set-1 have been observed in refs. [IO, 111. Therefore, one may expect a change of reaction order for the chlorine recombination in N2 below 100 atm.
Acknowledgement Financial support of the Schweizerischer Nationalfonds is gratefully acknowledged.
References 111 J.K. Ip and G. Burns, Discussions Faraday Sot. 44 (1967) 241; J. Chem. Phys. 51 (1969) 3413,34X. 121 S.K. Chang, A.G. Clarke and G. Burns, J. Chem. Phys. 54 (1971) 1835. 131 B.A. DeGrsff and K.J. Lan8, J. Phys. Chem. 74 (1970) 4181. [4] R.L. Strong, J.C.W.Chien,P.E. Graf and J.E. Willard, J. Chem. Phys. 26 (1957) 1287. 151 E. Rabinovitch and W.C. Wood, J. Chem. Phys. 4 (1936) 497. 161MI. Christie, A.G. Harrison, R.G.W. Norrish and G. Porter, Proc. Roy. Sot. A 231 (1955) 446. 171 D.L. Bunker and N. Davidson, J. Am. Chem. Sot. 80 (1958) 508.5. 181 3. Britton, N. Davidson, W. Gchmann and G. Schott, J. Chem. Phys. 25 (1956) 804. 191 J.Troe and H.Gg. Wagner, Z. Physik. Chem. 55 (1967) 326. 1lOI H. Hippler, K. Luther and J. Troe, Chem. Phys. Letters 16 (1972) 174. IllI K.Luther, Doktorarbeit (1972), Ecole Polytechnique FedCrale de Lausanne, Switzerland; H. Hippler, K. Luther and J. Troe, Ber. Bunsenges. Physik. Chem., to be published.
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
[ 121 J.W. Linnett and M.H. Both, Nature 199 (1963) 1181. [ 131 L.W. Bader and E,A. Ogyzlo, Nature 201 (1964) 491. 1141 E. Hutton and M. Wright, Trans. Faraday Sot. 61 (1065) 78; E. Hutton, Nature 203 (1964) 835. [ 15 J M.A.A. Clyne and D.H. Stedman, Trans. Faraday Sot. 64 (1968) 2698. 1161 G. Chiltz, R. Eckling, P. Goldfinger, G. Huybrechts, G. Martens and G. Simocns, Bull. Sot. Chim. Be&. 71 (1962) 747. [17] H. Hinoka and R Hardwick, 3. Chem. Phys. 36 (1962) 1715. 1181 T.A. Jacobs and R.R Geidt, J. Chem. Phys. 39 (1963) 749.
15 April 1973
1191 M. von Thiel, DJ. S,eery and D.J. B&ton,
J. Phys.
Chem. 69 (1965) 834. [20] R.W. Die&
andW.J. Felmlee, i.Chem. Phys. 39 (1963) 2115. [21] .R_A, Carbaretta and H.B. Palmer, J. Chem. Phys 46 (1967) 1333;47 (1967) 2202. [22] A.C. Lloyd, Intern. J. Chem. Kinetics 3 (1971) 39. 1231 K.E. Russelsand J. Simons, Proc. Roy Sot. A217 (1953) 279. 1241 G.E. Busch, R.T. Mahoney, R.J. Morse and K.R. Wilson, I. Chem. Phys. 51 (1969) 449. [ 251 A.G. Briss and R.G. Norrish, Proc. Roy Sac. A276 (1963) 51.
ERRATUM
S. Kielich, Depolarization of light scattering by atonlic and molecular solutions with strongly anisotropic translational-orientational fluctuations, Chem. Phys. Letters (1971) 516. Eq. (10) should have the following form (resulting term with riyjyk should be corrected to 3yiyjyk):
+
2YjYi(3aj
f
3aj
+
yi +
yj)
10
from formula (A.2), where the
K;” ,
with the radial-angular correlation parameter JiiRA in the form (11) containing cos2 oqi (and not cos2 Bqj). By effecting the interchange i +j and j + i in formula (1 l), corrected as above, one obtains the form of the parameter JltA.
609