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The pulse radiolysis of carbon monoxide
THE
PULSE
RADIOLYSIS
OF
CARBON
MONOXIDE
c. WILLIS Research Chemist-/y Branch, Chalk River Nuclear Laboratories, Atomic Energy of Caiiada Limited, Chalk River, Ontario, Canada
and C. DEVILLERS
*
Service de Chimie Physique, Centre d’Etudes B.P. No. 2. 91, Gif-sur-Yvette,
Received
Nucleaires France
de Saclay,
25 March 1968
Carbon monoxide has been irradiated with intense pulses of 1 MeV electrons and two transient absorption spectra have been observed. One of these consisted of a series of bands extending from 5000 - 8500 b and the second a single line at 2479 A. The species giving rise to the band system is probably C20@) and that to the altigle line ~(1s). It is proposed that carbon atoms are not the exclusive precursors of C20(=) in the system and a reaction involving electronically excited carbon mawxide is ucccrring. co*
+co
Carbon dioxide, carbon suboxide and carbon suboxide polymer are formed when carbon monoxide is irradiated with ionizing radiation [ 1,2] ** or photolyzed with ultraviolet light [2,3]. The molecule C20 has been proposed to be an intermediate but no direct evidence of its formation from carbon monoxide in these systems has previously been obtained. We have irradiated carbon monoxide with high intensit- electron pulses and have observed a transient oabsorption spectrum in the region 5000-8600 A. This spectrum can be attributed to the molecule C20 in a 32, SkkS. &o a single absorption line was observed at 2469 A and this is due to carbon atoms in a 1.9 state. The irradiations were carried out with 1 MeV electron pulses for a 705 Febetron system ***_ These electron pulses have a duration of 100 ns at currents up to 5000 A. The irradiated gas was contained in a cell equipped with an internal, multiple-reflection mirror system. Silver surfaced mirrors were used for wavelengths above 4000 ii and aluminum surfaced mirrors were used for wavelengths below this. The mean dose * Guest Scientist, Division of Pure Physics National
Research Council, Ottawa, 1967-68. ** and other references contained in these two publications. *** Manufactured by the Field Emission Corporation, McMinnville, Oregon.
-
c20(3Z)to.
in the cell was - 30 kilorads/pulse. The spectroscopic source was a capillary discharge Iamp with an energy of 100 jouLes and a discharge time tl = 2 ors. A 0.75 meter Spex f/6.8 Czerny?Zrner grating spectrograph was used with a dispersion of 10 A/mm in the first order. The absorption spectrum was photographed at various delay times after the electron pulse. The gases used were Matheson research grade. The carbon monoxide was passed through a narrow quartz tube at 800°C to remove traces of iron carbonyl present. The carbon dioxide was used directly after several freeze-pump-thaw cycles to remove any gases not conciensible at liquid nitrogen temperature. At short delays, an absorption spectrum was obtained which extended from 5000 - 8600 A. The region 6000 - 8600 A is shown in fig. 1. The absorption was most intense with the shortest delays we were able to obtain (- I ,Us) and decayed to half intensity in about 20 ,us. AH the bands in the spectrum decayed uniformly with time. The addition of 10-40s carbon dioxide (400 Torr carbon monoxide; 40-300 Torr carbon dioxide) had no effect either on the initial intensity of this absorption or on its decay time. Al-
though no other absorption spectrum was ebbserved in the region accessible with the silver surfaced mirrors, after many pulses a chin white deposit was observed to have been formed on the mirrors. This deposit reduced the reflectivity of
Volume
2. number
6800
Fig.
1
--
6700
6600
65-00
I
I
I
‘v-
7900
7800
7700
7600
I
I
8700
6600
1. Absorption
spectrum
I 8500
6400
..
6360
6200
I 8400
6100
+4GO
7200
I 8100
I
I
5300
I
1 7300
1.
7500
8200
in pulse irradiated carbon monoxide from 6000-8860 A: pressure: dak IN, slits: 100 @. 52 light passes across irradiated zone.
the mirrors but did not show any characteristic absorption. A second transient absorption spectrum was obseqed with aluminum surfaced mirrors at 2479 A using Kodak 103-O plates. This consisted of a single line. It was most intense at the shortest delays and decayed to half intensity in 50 - 100 p s in 700 Torr of carbon monoxide. Mass-spectrometric analysis of the gaseous products, condensible at 77OK, indicated the presence of carbon dioxide and carbon suboxide but no quantitative measurements of the yields have yet been made. Since all the bands of the absorption s-ectrum shown in fig. 1 decayed uniformly with time, they can be attributed to a single species. The following points indicate that this species is the molecule C,O: (i) Devillers has reported [4] a band at 6350 A for a species formed in the flash photolysis of carbon suboxide. On chemical grounds he attributed this band to the molecule 3C20. This band has aDstructure identical to the one we find at 6350 A, shown in fig. 1. (ii) More recently Ramsay and Devillers have observed [5] more bands of the spectrum due to the species formed in the flash photolysis cf carbon suboxide. These bands extend to 8600 A. On preliminary spectroscopic arguments they attribute the bands to the transition C20(311 - B2). The spectrum observed in their work [5] is identical tothat shown in fig. 1. (iii) The separation between the bands at 6350 A, 7300 A and 8600 A and between those at 6700 A and 7700 K is about 2000 cm-l_ This is very si52
May 1968
CHEMICALPHYSICSLETTERS
700 Torr.
plates:
Ko-
milar to the value of 1977.5 cm-1 given for one of the fundamental vibrational frequencies of the molecule C20 by Jacox, Milligan, Moll and Thompson [6]. These workers report a second frequency of 1074 cm-l which in our case may correspond to the separation of about 1200 cm-l for the bands at 8600 A and 7700 A and at 7300 A and 6700 A. The formation of C20 in irradiated carbon monoxide has been suggested [1,2] to be a result of the reaction of a carbon atom with carbon monoxide, co *co* co*--+ co
(1) c+o
+ co*
-
co2
c + co
-
c20
(2) + c
(3) (4)
where CO* is an energetic molecule of carbon monoxide in an ionized or electronically excited state. The single absorption line observed a; 2479 A can be assigned to carbon atoms in a 1s state IS). This undergoing the transition (1P agrees with an earlier study by Meaburn and Perner [7] who also observed this single line under conditions very similar to those used in the present work. These authors reported the half-life for the species to be w 100 ~1s. This long life of C(%) in carbon monoxide excludes the ossibility of its being the precursor of the C20( %Z:) observed in the present work. If C(1D) and C(3P), which may be formed in the ir-
Volume 2. number 1
CHEMICAL PHYSICS LETTERS
radiation, have half-lives as long as ~(1s). as seems likely, then these can also be eliminated as the precursor of the C20(3X) observed. Thus. although C20 may be formed in irradiated carbon monoxide by reactions (1) - (4), another source must also exist not involving the reaction of carbon atoms. There are two possibilities, either reactions involving ionic species or direct formation of C20 by reaction of an excited carbon monoxide molecule, co+co*-
c20 + 0.
co + cog.
-
C20(3C)
c O&J)
(5’)
(heat of formation ofC0 is -26.42 kcal/mole [8 [: of O(3FC; is +59 kcaljmole [8]; and of C20(3X) is +93 f 5 kcaljmole [9]). at least this amount of energy must be contained in the excited state of carbon monoxide in reaction (5). There are several non-dissociative [lO.ll] states of carbon monoxide above 9 eV and one or more of these could be involved.
(5)
In continuously irradiated carbon monoxide, the decomposition yield, G(-CO), is rapidly reduced by the addition of small quantities of carbon dioxide [1] and this reduction has been attributed tc the very efficient charge transfer reaction co+ + co2 -
2 CO(XlZ)
Xay I9G8
(6)
Even at more than 40% carbon dioxide in carbon monoxide, the yield of C20(3X), was unaffected. Thus. CO+, and species derived from this ion, cannot be precursors of the C20(3\;) observed in the present work. The most probable reaction leading to the formation of C20(3\;), then, is that Involving the reaction of an electronically excited carbon monoxide molecule. Since reaction (5’) is endothermic by 8.9 eV,
References [l] [2] [3] [4] t51 t61 171 t81 [91 1101 t111
S. C. Lind. Radiation Chemistry of Gases (Reinhold. New York. 1961). G. Liuti. C.Kunz and S. Dondes. J. Am.Chem.Soc. 89 (1967) 5542. W. tiroth. W. Pessara and H. J. Rommel. 2. Physik. Chemie (Frankfurt) 32 (1966) 192. C. Devillers. Comot. Rend. 262~ (19661 1455. D.A. Ramsay and C. Devillers. private communication. M. E. Jacox. D. E. MilliRan. S. G. &foll and frr. E. Thompson. J. Chem. Phys. 43 (2965) 3734. G. 31. Menburn and D. Perner. Sature 212 (19661 1042. National Buresu of Standards Circular 506. C.Willis and K.D. Bayes. J.I’hys.Chem. il (19Gii 3367. J. R. McNcshv and bl. Okahc. Advances in Photochemistry 3 (1964) 157. A.Skerbelc. V. D. Meyer and E. S. Lassettre. J. Chem. Phys. 44 (1966) 4069.
53
PULSE
RADIOLYSIS
OF
CARBON
MONOXIDE
c. WILLIS Research Chemist-/y Branch, Chalk River Nuclear Laboratories, Atomic Energy of Caiiada Limited, Chalk River, Ontario, Canada
and C. DEVILLERS
*
Service de Chimie Physique, Centre d’Etudes B.P. No. 2. 91, Gif-sur-Yvette,
Received
Nucleaires France
de Saclay,
25 March 1968
Carbon monoxide has been irradiated with intense pulses of 1 MeV electrons and two transient absorption spectra have been observed. One of these consisted of a series of bands extending from 5000 - 8500 b and the second a single line at 2479 A. The species giving rise to the band system is probably C20@) and that to the altigle line ~(1s). It is proposed that carbon atoms are not the exclusive precursors of C20(=) in the system and a reaction involving electronically excited carbon mawxide is ucccrring. co*
+co
Carbon dioxide, carbon suboxide and carbon suboxide polymer are formed when carbon monoxide is irradiated with ionizing radiation [ 1,2] ** or photolyzed with ultraviolet light [2,3]. The molecule C20 has been proposed to be an intermediate but no direct evidence of its formation from carbon monoxide in these systems has previously been obtained. We have irradiated carbon monoxide with high intensit- electron pulses and have observed a transient oabsorption spectrum in the region 5000-8600 A. This spectrum can be attributed to the molecule C20 in a 32, SkkS. &o a single absorption line was observed at 2469 A and this is due to carbon atoms in a 1.9 state. The irradiations were carried out with 1 MeV electron pulses for a 705 Febetron system ***_ These electron pulses have a duration of 100 ns at currents up to 5000 A. The irradiated gas was contained in a cell equipped with an internal, multiple-reflection mirror system. Silver surfaced mirrors were used for wavelengths above 4000 ii and aluminum surfaced mirrors were used for wavelengths below this. The mean dose * Guest Scientist, Division of Pure Physics National
Research Council, Ottawa, 1967-68. ** and other references contained in these two publications. *** Manufactured by the Field Emission Corporation, McMinnville, Oregon.
-
c20(3Z)to.
in the cell was - 30 kilorads/pulse. The spectroscopic source was a capillary discharge Iamp with an energy of 100 jouLes and a discharge time tl = 2 ors. A 0.75 meter Spex f/6.8 Czerny?Zrner grating spectrograph was used with a dispersion of 10 A/mm in the first order. The absorption spectrum was photographed at various delay times after the electron pulse. The gases used were Matheson research grade. The carbon monoxide was passed through a narrow quartz tube at 800°C to remove traces of iron carbonyl present. The carbon dioxide was used directly after several freeze-pump-thaw cycles to remove any gases not conciensible at liquid nitrogen temperature. At short delays, an absorption spectrum was obtained which extended from 5000 - 8600 A. The region 6000 - 8600 A is shown in fig. 1. The absorption was most intense with the shortest delays we were able to obtain (- I ,Us) and decayed to half intensity in about 20 ,us. AH the bands in the spectrum decayed uniformly with time. The addition of 10-40s carbon dioxide (400 Torr carbon monoxide; 40-300 Torr carbon dioxide) had no effect either on the initial intensity of this absorption or on its decay time. Al-
though no other absorption spectrum was ebbserved in the region accessible with the silver surfaced mirrors, after many pulses a chin white deposit was observed to have been formed on the mirrors. This deposit reduced the reflectivity of
Volume
2. number
6800
Fig.
1
--
6700
6600
65-00
I
I
I
‘v-
7900
7800
7700
7600
I
I
8700
6600
1. Absorption
spectrum
I 8500
6400
..
6360
6200
I 8400
6100
+4GO
7200
I 8100
I
I
5300
I
1 7300
1.
7500
8200
in pulse irradiated carbon monoxide from 6000-8860 A: pressure: dak IN, slits: 100 @. 52 light passes across irradiated zone.
the mirrors but did not show any characteristic absorption. A second transient absorption spectrum was obseqed with aluminum surfaced mirrors at 2479 A using Kodak 103-O plates. This consisted of a single line. It was most intense at the shortest delays and decayed to half intensity in 50 - 100 p s in 700 Torr of carbon monoxide. Mass-spectrometric analysis of the gaseous products, condensible at 77OK, indicated the presence of carbon dioxide and carbon suboxide but no quantitative measurements of the yields have yet been made. Since all the bands of the absorption s-ectrum shown in fig. 1 decayed uniformly with time, they can be attributed to a single species. The following points indicate that this species is the molecule C,O: (i) Devillers has reported [4] a band at 6350 A for a species formed in the flash photolysis of carbon suboxide. On chemical grounds he attributed this band to the molecule 3C20. This band has aDstructure identical to the one we find at 6350 A, shown in fig. 1. (ii) More recently Ramsay and Devillers have observed [5] more bands of the spectrum due to the species formed in the flash photolysis cf carbon suboxide. These bands extend to 8600 A. On preliminary spectroscopic arguments they attribute the bands to the transition C20(311 - B2). The spectrum observed in their work [5] is identical tothat shown in fig. 1. (iii) The separation between the bands at 6350 A, 7300 A and 8600 A and between those at 6700 A and 7700 K is about 2000 cm-l_ This is very si52
May 1968
CHEMICALPHYSICSLETTERS
700 Torr.
plates:
Ko-
milar to the value of 1977.5 cm-1 given for one of the fundamental vibrational frequencies of the molecule C20 by Jacox, Milligan, Moll and Thompson [6]. These workers report a second frequency of 1074 cm-l which in our case may correspond to the separation of about 1200 cm-l for the bands at 8600 A and 7700 A and at 7300 A and 6700 A. The formation of C20 in irradiated carbon monoxide has been suggested [1,2] to be a result of the reaction of a carbon atom with carbon monoxide, co *co* co*--+ co
(1) c+o
+ co*
-
co2
c + co
-
c20
(2) + c
(3) (4)
where CO* is an energetic molecule of carbon monoxide in an ionized or electronically excited state. The single absorption line observed a; 2479 A can be assigned to carbon atoms in a 1s state IS). This undergoing the transition (1P agrees with an earlier study by Meaburn and Perner [7] who also observed this single line under conditions very similar to those used in the present work. These authors reported the half-life for the species to be w 100 ~1s. This long life of C(%) in carbon monoxide excludes the ossibility of its being the precursor of the C20( %Z:) observed in the present work. If C(1D) and C(3P), which may be formed in the ir-
Volume 2. number 1
CHEMICAL PHYSICS LETTERS
radiation, have half-lives as long as ~(1s). as seems likely, then these can also be eliminated as the precursor of the C20(3X) observed. Thus. although C20 may be formed in irradiated carbon monoxide by reactions (1) - (4), another source must also exist not involving the reaction of carbon atoms. There are two possibilities, either reactions involving ionic species or direct formation of C20 by reaction of an excited carbon monoxide molecule, co+co*-
c20 + 0.
co + cog.
-
C20(3C)
c O&J)
(5’)
(heat of formation ofC0 is -26.42 kcal/mole [8 [: of O(3FC; is +59 kcaljmole [8]; and of C20(3X) is +93 f 5 kcaljmole [9]). at least this amount of energy must be contained in the excited state of carbon monoxide in reaction (5). There are several non-dissociative [lO.ll] states of carbon monoxide above 9 eV and one or more of these could be involved.
(5)
In continuously irradiated carbon monoxide, the decomposition yield, G(-CO), is rapidly reduced by the addition of small quantities of carbon dioxide [1] and this reduction has been attributed tc the very efficient charge transfer reaction co+ + co2 -
2 CO(XlZ)
Xay I9G8
(6)
Even at more than 40% carbon dioxide in carbon monoxide, the yield of C20(3X), was unaffected. Thus. CO+, and species derived from this ion, cannot be precursors of the C20(3\;) observed in the present work. The most probable reaction leading to the formation of C20(3\;), then, is that Involving the reaction of an electronically excited carbon monoxide molecule. Since reaction (5’) is endothermic by 8.9 eV,
References [l] [2] [3] [4] t51 t61 171 t81 [91 1101 t111
S. C. Lind. Radiation Chemistry of Gases (Reinhold. New York. 1961). G. Liuti. C.Kunz and S. Dondes. J. Am.Chem.Soc. 89 (1967) 5542. W. tiroth. W. Pessara and H. J. Rommel. 2. Physik. Chemie (Frankfurt) 32 (1966) 192. C. Devillers. Comot. Rend. 262~ (19661 1455. D.A. Ramsay and C. Devillers. private communication. M. E. Jacox. D. E. MilliRan. S. G. &foll and frr. E. Thompson. J. Chem. Phys. 43 (2965) 3734. G. 31. Menburn and D. Perner. Sature 212 (19661 1042. National Buresu of Standards Circular 506. C.Willis and K.D. Bayes. J.I’hys.Chem. il (19Gii 3367. J. R. McNcshv and bl. Okahc. Advances in Photochemistry 3 (1964) 157. A.Skerbelc. V. D. Meyer and E. S. Lassettre. J. Chem. Phys. 44 (1966) 4069.
53