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The photolysis of CO at 193 nm in the liquid and gas phases
Journal of Photochemistry
THE PHOTOLYSIS PHASES+
P. B. ROUSSEL*
and Photobiology,
A: Chemistry, 46 (1989)
OF CO AT 193 nm IN THE LIQUID
and R.
159 - 167
AND GAS
A. BACK
Division of Chemistry, National Research Council of Canada, Ottawa, KlA (Received
June 28, 1988;
159
in revised form October
OR6 (Canada)
3,1988)
The photolysis of CO at 193 nm, which excites the u’ = 2 level of the Z(311) state, has been studied in the liquid phase at 77 K and the gas phase at room temperature. Products observed were COZ and C,O,, with quantum yields of CO2 of about 10m2 and 10S3 in the gas and the liquid respectively. Quantum yields of C302 were about 1% of the CO2 in both phases. Ozone was also a product and was shown to have been derived from traces (20 30 ppm) of O2 present in the CO. Mechanisms of product formation are discussed.
1. Introduction The photochemistry of carbon monoxide has been studied many times in the gas phase, usually at short wavelengths (147.0, 123.6 run) which excite the A( III) state [I - 43; a few studies have been made at 206.2 nm, which excites the Z(3EI) state [ 51. The final products in all cases were COZ, C302 and a polymer, with rather low quantum yields. Much less work has been done in liquid CO. Buschmann and Groth irradiated liquid CO at 77 K at wavelengths of 206.2, 147.0 and 123.6 nm and reported no measurable decomposition, but found sensitized decomposition of CH4 and C2H4 in CO-hydrocarbon mixtures [6] ; they later investigated the CO-C2H4 mixture in some detail at 206.2 nm. The high power available from an ArF excimer laser at 193 nm, which excites the u = 2 level of the Z state, offers the possibility of exploring the photochemistry of this state more easily than with the rather weak iodine resonance lamp at 206.2 nm used previously. The present paper describes a study of the photolysis of liquid and gaseous CO at 193 nm, together with some experiments at 206.2 nm for comparison. +NRCC
publication
SNRCC
Research
lOlO-6030/89/$3.50
no. 29473. Associate,
1984
- 1987. @ Elsevier Sequoia/Printed
in The Netherlands
160
2. Experimental details The reaction cell used in most experiments with liquid CO was a quartz cylinder, 2.5 cm in diameter and 10 cm long. The cell was extended at each end by an evacuated chamber, 10 cm long, for thermal isolation, and the cell was mounted in a Styrofoam bucket, which could be filled with liquid nitrogen, with the extensions protruding through the walls. The four planar cell windows were of Suprasil quartz. A second similar cell, but with a 0.4 cm light path, was used in some absorption measurements; it was also fitted with 0.4 cm i.d. sidearms with windows to permit measurements at right angles to the laser beam. Gas phase experiments were made in cylindrical quartz vessels 2.5 cm in diameter and 10 or 30 cm long with Suprasil windows. One 30 cm vessel had a thermosyphon loop attached, and when one vertical arm was cooled in liquid nitrogen, reagents and products were rapidly swept from the vicinity of the front window and products condensable at 77 K were removed from the gas phase. Carbon monoxide (Matheson, Ultra-high purity) was passed slowly through a series of cold traps at 77 K which removed traces of water and hydrocarbons (detected by mass spectrometry (MS)) and any possible metal carbonyls (which were never detected). Traces of CO* remained, together with 02, N2 and Ar; the sum of these impurities was shown by gas chromatograph analysis to be less than 100 ppm, while the concentration of O2 was measured to be 25 - 30 ppm. Some experiments in the gas phase were made using Matheson Research grade CO with an oxygen content less than 1 ppm. A Lumonics excimer laser (TE-860 series) was operated at 10 or 100 Hz with pulse energies typically about 40 mJ. An iodine resonance lamp driven by a microwave discharge was used, as described previously [7] ; about 90% of the light was at 206.2 nm, with minor components at 187.6, 184.4 and 183.0 nm; the latter were removed by a liquid water filter in some experiments. In the liquid phase experiments, CO was condensed in the cell as a liquid at 77 K at which it has a vapour pressure of about 400 Torr. The liquid was allowed to fill the cell and the sidearm on top of the cell to a level slightly above the liquid N2 coolant, thereby permitting a pressure slightly above the equilibrium value at 77 K and preventing formation of bubbles by the slight heating of the liquid by the laser beam. After irradiation of the CO, liquid or gas, the contents of the reaction vessel was pumped away slowly, as a gas, through a cold trap at 77 K while the cell was slowly allowed to warm to room temperature. Products condensed in the trap were then analysed by gas chromatography (GC) or by MS. GC analysis for COZ and C302 was made with a 1.5 m Porapac Q column programmed from room temperature to 100 “C, using both flame ionization and thermal conductivity detectors. Analysis of CJ02 by GC and by MS was carefully calibrated with a sample prepared by dehydration of malonic acid and purified [8].
161
The absorption spectrum of the reaction vessel and its contents could be measured by placing the vessel in the light path of a single-beam spectrophotometer, using either a 0.25 m Heath monochromator or a 0.5 m McPherson monochromator with resolutions of about 0.1 nm and 0.05 nm respectively. For measurements below 200 nm, the light path was flushed as much as possible with dry Nz.
3. Results 3.1. Absorption spectra The absorption spectrum of liquid CO obtained in the 0.4 cm cell is shown in Fig. 1 with the spectra of the two light sources superimposed. The first three Cameron bands, O-O, 1-O and 2-0, are clearly evident, with maxima at 207.4 nm, 200.2 nm and 193.6 nm, corresponding to vacuum wavenumbers of 48 201 cm-‘, 49 934 cm-’ and 51636 cm-’ respectively. The bands are much broader than in the gas phase, with no rotational structure, and are shifted to the red by about 275 cm-‘, very close to the shift observed in the spectrum of benzene in liquid CO [9]. The vibrational spacing in the I state, 1733 and 1702 cm -’ for the O-l and the l-2 intervals, is unchanged from the gas phase (0, = 1739, W,X, = 14.5 cm-l [lo]) within the accuracy of the present measurements. The iodine lamp and the ArF laser are seen to excite cleanly the u’ = 0 and u’ = 2 levels of the E state, with good overlap. An absorption coefficient of 0.13 M-l cm-l was estimated at 206.2 run, about twice that reported by Dunn et al. [5] and 30% less than that measured by Harteck et al. [5], both in the gas phase.
220
210
200
190
180
A b-4
Fig. 1. Spectrum of light from deuterium lamp transmitted by 4 mm of liquid CO at 77 K showing the first three Cameron bands. Vertical scale is transmitted intensity. Spectra of photolysis sources are superimposed. Narrow bands below 191 nm are from 02 gas in the optical path.
162
The ArF laser has an appreciable band width, and taking a typical spectral distribution reported for this [II], an average absorption coefficient of 0.16 M-l cm-” was estimated. This value is insensitive to the band shape assumed for the laser since it lies near the maximum of the v’ = 2 Cameron ba-nd (Fig. 1). 3.2. Pho tolysis products Carbon dioxide was the main product both in liquid CO and in the gas phase at 193 and 206.2 nm. Quantum yields of COZ at 193 nm, measured by GC, were about 10m3 in liquid CO and lo-* in the gas. Difficulty in removing traces of CO2 in the starting material, the low quantum yields, and inaccuracies in estimating the light absorbed make the uncertainties in these values rather large, perhaps +50%. Carbon suboxide was also observed as a product in both phases, with quantum yields measured by GC about 1% of the yields of CO*. Identification of COZ and C302 as products was confirmed by MS. Quantum yields of these products, within considerable scatter, showed no systematic variation with pressure or duration of photolysis. A third product of the photolysis was ozone, detected and measured by its UV absorption spectrum, in both liquid and gaseous CO with both
330
280
230
180
A b-4
Fig. 2. Spectrum of light from deuterium lamp transmitted by 10 cm of liquid CO at 77 K, before (3) and after (A) 30 min irradiation with the ArF laser. Absorption is due to 03. Vertical scale is transmitted intensity.
164
where CO* is the vibrationally excited ground state. Reaction (2) has been shown to be very efficient, with k2 = 1.4 X 1O’l M-’ s-’ for U’ = 2 1121, so that for @(CO*) = 10e2, k, = 1.4 X lo9 M-’ s-l. The two quanta of vibration excited at 193 nm apparently did not enhance @(C02) above its value at 206.2 nm. The yield of CO2 in liquid CO at 77 K was about ten times smaller than in the gas phase. The weak dipole moment [lo] in the ground state (0.122 D, C--O+) would tend to align the molecules initially in the right configuration for reaction (l), but the strong reversed dipole moment in the 2 state (1.374 D, C+-O+) would repel them from this configuration. The same repulsion would also occur in an encounter in the gas phase, and may account for the relatively low efficiency of reaction (1) in both phases. The lower quantum yield in the liquid at 77 K may be simply a temperature effect, reflecting a small activation energy needed to overcome this repulsion. Carbon suboxide in the present systems was presumably formed by the same reactions proposed in other studies [I - 51 of CO photochemistry: c+co
Lc20
(3)
M
c,o+co-co
3
2
For all species in their ground states, reaction (4) requires a change of spin and is relatively slow [133. Yields of C3O2 in previous studies of both the Z and the A states of CO have always been much less than those of C02, which has been attributed to loss of suboxide by polymerization, photolysis and other secondary reactions. Yields have been enhanced in several systems [4,5] by removal of products from the reaction zone by rapid circulation and trapping, but the highest yields achieved have never exceeded about 20% of the C02. In the present experiments, yields of C302 are much lower, about 1% of the CO?, and removal of products from the reaction zone by a thermosyphon which was at least as fast as that used effectively by Vikis [ 41 gave no enhancement of the C3O2 yield. This behaviour can be attributed to the much higher intensity of the ArF laser and the strong absorption of C3O2 at 193 nm [l]. Its photolysis at this wavelength is expected to produce CO + C,O, and 2C0 + C, and with the high concentration of CO present the C20 and C should sitnply reform C3O2 via reactions (3) and (4). To account for the loss of suboxide, rapid secondary reactions of C, C20, C3O2 and perhaps other species, such as those discussed by Okabe [l], must be invoked, leading to the formation of carbon suboxide polymer and carbon, reactions which would be favoured at the high intensity of the ArF laser. Reactions of 02, present at about 30 ppm, or of O3 generated from it, do not seem to have been a factor in the loss of C302, because experiments in the gas phase using high purity CO (0, < 1 ppm), which produced no detectable 03, gave the same low yield of suboxide.
165
The production of ozone from the O2 impurity in the CO was surprisingly efficient both in liquid CO and in the gas phase. Ozone formation in the photolysis of O2 itself, liquid or gas, is well known, and it has been observed before in solutions of O2 in liquid CO, but at much higher concentrations than in the present experiments [14]. The accepted mechanism of formation [l] is 02+hv
-
20(3p)
(5)
in which O2 is excited to the 31ZUstate in the Schumann-Runge bands, which predissociates to give two ground state atoms, followed by O(3P) + o* - M
03
(6)
Values of the molar decadic absorption coefficient of 6.158 M-l cm-’ and 0.727 M-’ cm-l were measured in liquid CO for CO and O2 respectively, and, assuming simple competitive absorption, quantum yields of 0s were calculated, using E,,, = 3030 M-l s-l for O3 [15], based on the light absorbed by the OZ. Values ranged from about 1.5 to 2.0, similar to those reported for the photolysis of pure oxygen in the gas phase El]. This high efficiency for ozone formation implies that the reaction O( 3P) + co 5
co2
(7)
was not occurring in liquid CO at 77 K, a conclusion supported by the reported rates of reactions (6) and (7) in the gas phase [ 161, from which it may be estimated that k,/k, = 1011 at 77 K, and at 30 ppm of Oa reaction (6) would be more that lo6 times faster than reaction (7). The conclusion that ozone formation can be explained entirely by reactions (5) and (6) is supported by observations with the iodine lamp. When this lamp was unfiltered, 0, formation was observed in liquid CO with 30 ppm of 02, but when the shorter wavelengths which can photolyse O2 were filtered out, 0s formation ceased. Thus absorption by 02 seems essential for ozone formation in liquid CO at 77 K. Quantum yields of O3 in the gas phase are less easily explained, and there is more uncertainty in their measurement because the spectra of both CO and O2 in the gas phase consist of discrete rotational lines much narrower than the laser bandwidth. Absorption coefficients measured at low resolution cannot be used but were measured instead directly from the attenuation of the ArF laser beam, and values of 0.065 M-l cm-’ and 0.064 M-l cm-’ were obtained for O2 and CO respectively. Of necessity these measurements were made at gas pressures from 100 to 1000 Torr, and because the partial pressures of O2 present in the photolysis experiments (20 - 30 ppm) were very much lower, and because its spectrum is discrete, the absorption by O2 may have been underestimated. Quantum yields of ozone based on the light absorbed by 02, taking an absorption coefficient for ozone of 3030 M-l cm-’ at 255 nm [l, 151,
166
ranged from about IO to 50 for the irradiation of CO at about 760 Torr and 20 - 30 ppm of Oz. It is difficult to devise a plausible chain reaction that could account for such high quantum yields, and it also seems unlikely that uncertainty in the absorption by O1 could be large enough to explain them. There is a further problem, since it may be estimated [X6] that k6/k7= 145 at room temperature, so that with 30 ppm of O2 R6/R7 = 0.0044, and less than 1% of oxygen atoms can be expected to react via reaction (6) to form 03.
The high quantum yields, based on light absorbed by 02, suggest that light absorbed by CO, which was about 3 X IO4 times greater, was also contributing to 0s formation. Direct sensitization of O2 decomposition by energy transfer or reaction by CO { S(311)) does not help very much since its reported rate of reaction with O2 is only about twice that with CO [123, and given the approximately equal absorption coefficients at 193, this could at most give twice the decomposition of the direct photolysis. Sensitization by vibrationally excited CO* formed in reaction (2) is more hopeful, as CO* is probably much longer lived than the 2 state, and a sizeable fraction might have sufficient energy [17] to permit the reaction
co4 +oz---+
co2 + O(3P)
(8)
which is estimated to have an activation energy of 35 - 60 kcal mol-’ and a large frequency factor [16]. Another possible source of oxygen atoms is the reaction
(320 + 02 -
2co
+ O(3P)
(9)
for which a rate constant of about 10s M-’ s’-’ has been reported [IS], since the competing reaction (4) with CO is relatively slow [13]. Regeneration of C,O by photolysis of C302, discussed earlier, could enhance the probability of reaction (9). Reactions (8) and (9) do not circumvent the problem of the unfavourable competition between reactions (6) and (7); one solution might be the formation of CO,
co*+o* zco3 followed co3+0*-
(10)
by co1
+
03
(11)
Reaction (11) was suggested by Roper and Demore to account for ozone formation in the photolysis of CO-O1 mixtures at 77 K at a wavelength (254 nm) at which O2 could not be directly decomposed [ 191. The experimental uncertainties in our measurements do not permit a decision between these possible explanations, and further speculation is unwarranted.
167
References 1 H. Okabe, Photochemistry of Small Molecules, Wiley, New York, 1978, pp. 166, 177, 237, 320. 2 K. Falting, W. Groth and P. Harteck, 2. Phys. Chem. B, 41 (1938) 15. 3 W. E. Groth, W. Pessara and H. J. Rommel, 2. Phys. Chem. N.F., 32 (1962) 192. 4 A. C. Vikis, J. Photochem., 19 (1982) 1. 5 0. Dunn, P. Harteck and S. Dondes, J_ Phys. Chem., 77 (1973) 878. P. Harteck, R. R. Reeves, Jr., and B. A. Thompson, 2. Nuturforsch., Teil A, 19 (1964) 2. 6 H. W. Buschmann and W. Groth, Ber. Bunsenges. Phys. Chem., 72 (1968) 1067; 73 (1968) 859. 7 W. D. Wooley and R. A. Back, Can. J. Chem., 46 (1968) 295. T. Yokota and R. A. Back, Int. J. Chem. Kinet., 5 (1973) 37. 8 T. Morrow and W. D. McGrath, Tmns. Faraday Sot., 62 (1966) 3142. 9 E. R. Bernstein and J. Lee, J. Chem. Phys., 74 (1981) 3159. 10 K. P. Huber and G. Herzberg, Constunts of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979, p. 166. 11 M. P. Irion, Report 51, Max-Planck-Institut fiir Quontenoptik, 1981, p. 30. 12 K. Schofield, J. Phys. Chem. Ref. Data, 8 (1979) 723. 13 W. Bauer, K. H. Becker and R. Meuser, Ber. Bunsenges. Phys. Chem,, 89 (1985) 240. 14 J. R. McNesby, J. Chem. Phys., 31 (1959) 283. 15 W. B. DeMore and 0. Raper, J. Phys. Chem., 68 (1964) 412. 16 J. A. Kerr and S. J. Moss (eds.), Handbook of Bimolecukrr and Termolecular Gas Reactions, Vol. 1, CRC Press, Boca Raton, FL, 1981, p. 10. 17 Yu. Z. Ionikh, A. L. Kuranov, A. N. Lobanov and L. S. Starenkova, Opt. Spectrosc. (USSR), 60 (1986) 444. 18 V. M. Donnelly, W. M. Pitts and J. R. McDonald, Chem. Phys., 49 (1980) 289. 19 0. F. Raper and W. B. DeMore, J. Chem. Phys., 40 (1964) 1047.
THE PHOTOLYSIS PHASES+
P. B. ROUSSEL*
and Photobiology,
A: Chemistry, 46 (1989)
OF CO AT 193 nm IN THE LIQUID
and R.
159 - 167
AND GAS
A. BACK
Division of Chemistry, National Research Council of Canada, Ottawa, KlA (Received
June 28, 1988;
159
in revised form October
OR6 (Canada)
3,1988)
The photolysis of CO at 193 nm, which excites the u’ = 2 level of the Z(311) state, has been studied in the liquid phase at 77 K and the gas phase at room temperature. Products observed were COZ and C,O,, with quantum yields of CO2 of about 10m2 and 10S3 in the gas and the liquid respectively. Quantum yields of C302 were about 1% of the CO2 in both phases. Ozone was also a product and was shown to have been derived from traces (20 30 ppm) of O2 present in the CO. Mechanisms of product formation are discussed.
1. Introduction The photochemistry of carbon monoxide has been studied many times in the gas phase, usually at short wavelengths (147.0, 123.6 run) which excite the A( III) state [I - 43; a few studies have been made at 206.2 nm, which excites the Z(3EI) state [ 51. The final products in all cases were COZ, C302 and a polymer, with rather low quantum yields. Much less work has been done in liquid CO. Buschmann and Groth irradiated liquid CO at 77 K at wavelengths of 206.2, 147.0 and 123.6 nm and reported no measurable decomposition, but found sensitized decomposition of CH4 and C2H4 in CO-hydrocarbon mixtures [6] ; they later investigated the CO-C2H4 mixture in some detail at 206.2 nm. The high power available from an ArF excimer laser at 193 nm, which excites the u = 2 level of the Z state, offers the possibility of exploring the photochemistry of this state more easily than with the rather weak iodine resonance lamp at 206.2 nm used previously. The present paper describes a study of the photolysis of liquid and gaseous CO at 193 nm, together with some experiments at 206.2 nm for comparison. +NRCC
publication
SNRCC
Research
lOlO-6030/89/$3.50
no. 29473. Associate,
1984
- 1987. @ Elsevier Sequoia/Printed
in The Netherlands
160
2. Experimental details The reaction cell used in most experiments with liquid CO was a quartz cylinder, 2.5 cm in diameter and 10 cm long. The cell was extended at each end by an evacuated chamber, 10 cm long, for thermal isolation, and the cell was mounted in a Styrofoam bucket, which could be filled with liquid nitrogen, with the extensions protruding through the walls. The four planar cell windows were of Suprasil quartz. A second similar cell, but with a 0.4 cm light path, was used in some absorption measurements; it was also fitted with 0.4 cm i.d. sidearms with windows to permit measurements at right angles to the laser beam. Gas phase experiments were made in cylindrical quartz vessels 2.5 cm in diameter and 10 or 30 cm long with Suprasil windows. One 30 cm vessel had a thermosyphon loop attached, and when one vertical arm was cooled in liquid nitrogen, reagents and products were rapidly swept from the vicinity of the front window and products condensable at 77 K were removed from the gas phase. Carbon monoxide (Matheson, Ultra-high purity) was passed slowly through a series of cold traps at 77 K which removed traces of water and hydrocarbons (detected by mass spectrometry (MS)) and any possible metal carbonyls (which were never detected). Traces of CO* remained, together with 02, N2 and Ar; the sum of these impurities was shown by gas chromatograph analysis to be less than 100 ppm, while the concentration of O2 was measured to be 25 - 30 ppm. Some experiments in the gas phase were made using Matheson Research grade CO with an oxygen content less than 1 ppm. A Lumonics excimer laser (TE-860 series) was operated at 10 or 100 Hz with pulse energies typically about 40 mJ. An iodine resonance lamp driven by a microwave discharge was used, as described previously [7] ; about 90% of the light was at 206.2 nm, with minor components at 187.6, 184.4 and 183.0 nm; the latter were removed by a liquid water filter in some experiments. In the liquid phase experiments, CO was condensed in the cell as a liquid at 77 K at which it has a vapour pressure of about 400 Torr. The liquid was allowed to fill the cell and the sidearm on top of the cell to a level slightly above the liquid N2 coolant, thereby permitting a pressure slightly above the equilibrium value at 77 K and preventing formation of bubbles by the slight heating of the liquid by the laser beam. After irradiation of the CO, liquid or gas, the contents of the reaction vessel was pumped away slowly, as a gas, through a cold trap at 77 K while the cell was slowly allowed to warm to room temperature. Products condensed in the trap were then analysed by gas chromatography (GC) or by MS. GC analysis for COZ and C302 was made with a 1.5 m Porapac Q column programmed from room temperature to 100 “C, using both flame ionization and thermal conductivity detectors. Analysis of CJ02 by GC and by MS was carefully calibrated with a sample prepared by dehydration of malonic acid and purified [8].
161
The absorption spectrum of the reaction vessel and its contents could be measured by placing the vessel in the light path of a single-beam spectrophotometer, using either a 0.25 m Heath monochromator or a 0.5 m McPherson monochromator with resolutions of about 0.1 nm and 0.05 nm respectively. For measurements below 200 nm, the light path was flushed as much as possible with dry Nz.
3. Results 3.1. Absorption spectra The absorption spectrum of liquid CO obtained in the 0.4 cm cell is shown in Fig. 1 with the spectra of the two light sources superimposed. The first three Cameron bands, O-O, 1-O and 2-0, are clearly evident, with maxima at 207.4 nm, 200.2 nm and 193.6 nm, corresponding to vacuum wavenumbers of 48 201 cm-‘, 49 934 cm-’ and 51636 cm-’ respectively. The bands are much broader than in the gas phase, with no rotational structure, and are shifted to the red by about 275 cm-‘, very close to the shift observed in the spectrum of benzene in liquid CO [9]. The vibrational spacing in the I state, 1733 and 1702 cm -’ for the O-l and the l-2 intervals, is unchanged from the gas phase (0, = 1739, W,X, = 14.5 cm-l [lo]) within the accuracy of the present measurements. The iodine lamp and the ArF laser are seen to excite cleanly the u’ = 0 and u’ = 2 levels of the E state, with good overlap. An absorption coefficient of 0.13 M-l cm-l was estimated at 206.2 run, about twice that reported by Dunn et al. [5] and 30% less than that measured by Harteck et al. [5], both in the gas phase.
220
210
200
190
180
A b-4
Fig. 1. Spectrum of light from deuterium lamp transmitted by 4 mm of liquid CO at 77 K showing the first three Cameron bands. Vertical scale is transmitted intensity. Spectra of photolysis sources are superimposed. Narrow bands below 191 nm are from 02 gas in the optical path.
162
The ArF laser has an appreciable band width, and taking a typical spectral distribution reported for this [II], an average absorption coefficient of 0.16 M-l cm-” was estimated. This value is insensitive to the band shape assumed for the laser since it lies near the maximum of the v’ = 2 Cameron ba-nd (Fig. 1). 3.2. Pho tolysis products Carbon dioxide was the main product both in liquid CO and in the gas phase at 193 and 206.2 nm. Quantum yields of COZ at 193 nm, measured by GC, were about 10m3 in liquid CO and lo-* in the gas. Difficulty in removing traces of CO2 in the starting material, the low quantum yields, and inaccuracies in estimating the light absorbed make the uncertainties in these values rather large, perhaps +50%. Carbon suboxide was also observed as a product in both phases, with quantum yields measured by GC about 1% of the yields of CO*. Identification of COZ and C302 as products was confirmed by MS. Quantum yields of these products, within considerable scatter, showed no systematic variation with pressure or duration of photolysis. A third product of the photolysis was ozone, detected and measured by its UV absorption spectrum, in both liquid and gaseous CO with both
330
280
230
180
A b-4
Fig. 2. Spectrum of light from deuterium lamp transmitted by 10 cm of liquid CO at 77 K, before (3) and after (A) 30 min irradiation with the ArF laser. Absorption is due to 03. Vertical scale is transmitted intensity.
164
where CO* is the vibrationally excited ground state. Reaction (2) has been shown to be very efficient, with k2 = 1.4 X 1O’l M-’ s-’ for U’ = 2 1121, so that for @(CO*) = 10e2, k, = 1.4 X lo9 M-’ s-l. The two quanta of vibration excited at 193 nm apparently did not enhance @(C02) above its value at 206.2 nm. The yield of CO2 in liquid CO at 77 K was about ten times smaller than in the gas phase. The weak dipole moment [lo] in the ground state (0.122 D, C--O+) would tend to align the molecules initially in the right configuration for reaction (l), but the strong reversed dipole moment in the 2 state (1.374 D, C+-O+) would repel them from this configuration. The same repulsion would also occur in an encounter in the gas phase, and may account for the relatively low efficiency of reaction (1) in both phases. The lower quantum yield in the liquid at 77 K may be simply a temperature effect, reflecting a small activation energy needed to overcome this repulsion. Carbon suboxide in the present systems was presumably formed by the same reactions proposed in other studies [I - 51 of CO photochemistry: c+co
Lc20
(3)
M
c,o+co-co
3
2
For all species in their ground states, reaction (4) requires a change of spin and is relatively slow [133. Yields of C3O2 in previous studies of both the Z and the A states of CO have always been much less than those of C02, which has been attributed to loss of suboxide by polymerization, photolysis and other secondary reactions. Yields have been enhanced in several systems [4,5] by removal of products from the reaction zone by rapid circulation and trapping, but the highest yields achieved have never exceeded about 20% of the C02. In the present experiments, yields of C302 are much lower, about 1% of the CO?, and removal of products from the reaction zone by a thermosyphon which was at least as fast as that used effectively by Vikis [ 41 gave no enhancement of the C3O2 yield. This behaviour can be attributed to the much higher intensity of the ArF laser and the strong absorption of C3O2 at 193 nm [l]. Its photolysis at this wavelength is expected to produce CO + C,O, and 2C0 + C, and with the high concentration of CO present the C20 and C should sitnply reform C3O2 via reactions (3) and (4). To account for the loss of suboxide, rapid secondary reactions of C, C20, C3O2 and perhaps other species, such as those discussed by Okabe [l], must be invoked, leading to the formation of carbon suboxide polymer and carbon, reactions which would be favoured at the high intensity of the ArF laser. Reactions of 02, present at about 30 ppm, or of O3 generated from it, do not seem to have been a factor in the loss of C302, because experiments in the gas phase using high purity CO (0, < 1 ppm), which produced no detectable 03, gave the same low yield of suboxide.
165
The production of ozone from the O2 impurity in the CO was surprisingly efficient both in liquid CO and in the gas phase. Ozone formation in the photolysis of O2 itself, liquid or gas, is well known, and it has been observed before in solutions of O2 in liquid CO, but at much higher concentrations than in the present experiments [14]. The accepted mechanism of formation [l] is 02+hv
-
20(3p)
(5)
in which O2 is excited to the 31ZUstate in the Schumann-Runge bands, which predissociates to give two ground state atoms, followed by O(3P) + o* - M
03
(6)
Values of the molar decadic absorption coefficient of 6.158 M-l cm-’ and 0.727 M-’ cm-l were measured in liquid CO for CO and O2 respectively, and, assuming simple competitive absorption, quantum yields of 0s were calculated, using E,,, = 3030 M-l s-l for O3 [15], based on the light absorbed by the OZ. Values ranged from about 1.5 to 2.0, similar to those reported for the photolysis of pure oxygen in the gas phase El]. This high efficiency for ozone formation implies that the reaction O( 3P) + co 5
co2
(7)
was not occurring in liquid CO at 77 K, a conclusion supported by the reported rates of reactions (6) and (7) in the gas phase [ 161, from which it may be estimated that k,/k, = 1011 at 77 K, and at 30 ppm of Oa reaction (6) would be more that lo6 times faster than reaction (7). The conclusion that ozone formation can be explained entirely by reactions (5) and (6) is supported by observations with the iodine lamp. When this lamp was unfiltered, 0, formation was observed in liquid CO with 30 ppm of 02, but when the shorter wavelengths which can photolyse O2 were filtered out, 0s formation ceased. Thus absorption by 02 seems essential for ozone formation in liquid CO at 77 K. Quantum yields of O3 in the gas phase are less easily explained, and there is more uncertainty in their measurement because the spectra of both CO and O2 in the gas phase consist of discrete rotational lines much narrower than the laser bandwidth. Absorption coefficients measured at low resolution cannot be used but were measured instead directly from the attenuation of the ArF laser beam, and values of 0.065 M-l cm-’ and 0.064 M-l cm-’ were obtained for O2 and CO respectively. Of necessity these measurements were made at gas pressures from 100 to 1000 Torr, and because the partial pressures of O2 present in the photolysis experiments (20 - 30 ppm) were very much lower, and because its spectrum is discrete, the absorption by O2 may have been underestimated. Quantum yields of ozone based on the light absorbed by 02, taking an absorption coefficient for ozone of 3030 M-l cm-’ at 255 nm [l, 151,
166
ranged from about IO to 50 for the irradiation of CO at about 760 Torr and 20 - 30 ppm of Oz. It is difficult to devise a plausible chain reaction that could account for such high quantum yields, and it also seems unlikely that uncertainty in the absorption by O1 could be large enough to explain them. There is a further problem, since it may be estimated [X6] that k6/k7= 145 at room temperature, so that with 30 ppm of O2 R6/R7 = 0.0044, and less than 1% of oxygen atoms can be expected to react via reaction (6) to form 03.
The high quantum yields, based on light absorbed by 02, suggest that light absorbed by CO, which was about 3 X IO4 times greater, was also contributing to 0s formation. Direct sensitization of O2 decomposition by energy transfer or reaction by CO { S(311)) does not help very much since its reported rate of reaction with O2 is only about twice that with CO [123, and given the approximately equal absorption coefficients at 193, this could at most give twice the decomposition of the direct photolysis. Sensitization by vibrationally excited CO* formed in reaction (2) is more hopeful, as CO* is probably much longer lived than the 2 state, and a sizeable fraction might have sufficient energy [17] to permit the reaction
co4 +oz---+
co2 + O(3P)
(8)
which is estimated to have an activation energy of 35 - 60 kcal mol-’ and a large frequency factor [16]. Another possible source of oxygen atoms is the reaction
(320 + 02 -
2co
+ O(3P)
(9)
for which a rate constant of about 10s M-’ s’-’ has been reported [IS], since the competing reaction (4) with CO is relatively slow [13]. Regeneration of C,O by photolysis of C302, discussed earlier, could enhance the probability of reaction (9). Reactions (8) and (9) do not circumvent the problem of the unfavourable competition between reactions (6) and (7); one solution might be the formation of CO,
co*+o* zco3 followed co3+0*-
(10)
by co1
+
03
(11)
Reaction (11) was suggested by Roper and Demore to account for ozone formation in the photolysis of CO-O1 mixtures at 77 K at a wavelength (254 nm) at which O2 could not be directly decomposed [ 191. The experimental uncertainties in our measurements do not permit a decision between these possible explanations, and further speculation is unwarranted.
167
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