J. Phys. Chem. 1995,99, 11830-11833
11830
Pressure Effect of Foreign Gases on the Herzberg Photoabsorption of Oxygen Yoshito Oshima, Yoko Okamoto, and Seiichiro Koda* Department of Chemical System Engineering, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan Received: March 1, 1995; In Final Form: May 22, 1 9 9 9
The effect of pressurized foreign gases on the photoabsorption of 0 2 in Herzberg bands and the Herzberg continuum was studied. The values of the cross section in 0 2 and 02/N2 mixtures under various pressures were in good agreement with the results in previous literature. The absorption of 0 2 in C02, methane, ethane, and ethylene was found to increase considerably with cross sections proportional to the number density of the foreign gases. The "density coefficient" of the absorption cross section was determined for individual foreign gases. A good correlation was found between the coefficient and the foreign gas ionization potential, which implies the contribution of charge-transfer-like interaction between 0 2 and the foreign gas for relaxing the dipole-forbidden transition.
Introduction In spite of a long history of the study of the photoabsorption augmentation of Herzberg bands and the Herzberg continuum by 0 2 itself and some foreign gases, the precise estimation of the cross section as a function of pressure is a recent concern, partly because it is necessary for estimating stratospheric ozone production and destruction rates. It was already known in the 1930s that the photoabsorption in the range 240-290 nm of oxygen molecules is enhanced when the pressure is highly raised.'s2 Finkelnburg2 observed diffuse bands in the range 240-290 nm distinct from the Herzberg I bands (A3CT X3C,) and showed that the absorption is proportional to the square of the oxygen pressure in the range 60-600 atm. These bands are thus named highpressure bands and have subsequently been identified with the Herzberg I11 system (A'3A, X3&) by detailed spectroscopic ana lyse^.^,^ It has also been known for many years that some foreign gases increase the intensity of the high-pressure In more recent decades, various photoabsorption studies in the Herzberg continuum and/or bands have been The mechanism of the absorption augmentation may be understood as the collision-induced relaxation of the selection rule for the dipole-forbidden Herzberg I11 system according to Blake and McCoy17 and Horowitz et a1.20 The same conclusion was obtained by Goodman and Brus2I and Kajihara et al.22 in the absorption and emission study of 0 2 immersed in solid matrices. No quantitative analysis, however, of the dependence on foreign gases other than N2 and Ar has been reported, because the main objective of most of the previous works has been to investigate the kinetics and mechanism of atmospheric ozone formation and destruction. In the present study, the effect of various pressurized foreign gases on the photoabsorption of 0 2 in the Herzberg bands and Herzberg continuum was quantitatively investigated. The motivation is 2-fold. One, measurements of the increase of the absorption cross section by a variety of foreign gases can provide a more precise understanding of the mechanism of the pressure effect. Two, the values of the cross section in the presence of foreign gases are necessary in order to analyze the kinetics of
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* To whom correspondence should be addressed. E-mail address:
[email protected]. Tel: 81-3-3812-2111 ext 7327. Fax: 813-5684-8402. @
Abstract published in Advance ACS Absrrucrs, July 1, 1995.
0022-365419512099-11830$09.0010
W-laser-induced oxidation in 02hydrocarbon mixtures under high pressure. We have investigated oxidation of ethane in suband supercritical phase 021ethane mixtures under irradiation with a KrF excimer laser at 248 nm.23 In spite of the fact that both 0 2 and ethane barely absorb any W laser light under ordinary pressures, relatively large amounts of oxidized products were obtained. The total-product yields increased with increase in the total pressure, while keeping the amount of 0 2 fixed, which may be attributed to the absorption augmentation in parallel with the increase of the total pressure. In this paper, the dependence of the absorption intensity of 0 2 in the range 230-280 nm on the density of 0 2 itself, N2, C02, methane, ethane, and ethylene will be reported.
Experimental Section The absorption cell consists of a stainless-steel cylinder of a length of 5 cm and an inner diameter of 1 cm equipped with quartz windows (Nippon Silica Glass Co. Ltd.) of 1 cm thickness at both ends for light introduction. Each window was sealed with an O-ring. After the cell was evacuated, the sample gas was introduced into the cell up to a fixed pressure. Oxygen (Suzuki Shokan Co. Ltd., 99.7%), dried air (Suzuki Shokan Co. Ltd., dew point -42 "C), and C02 (Suzuki Shokan Co. Ltd., 99.7%) were used as received. Hydrocarbons methane (Takachiho Commercial Co. Ltd., 99%), ethane (Takachiho Commercial Co. Ltd., 99.9%), and ethylene (Takachiho Commercial Co. Ltd., 99.9%) were also used without further purification. In the case of 02/ CO:!or Ozhydrocarbon mixtures, the cell was at fist filled with 5 or 10 atm of 02, followed by the addition of the foreign gas up to a fixed pressure, and the mixture was left for about 30 min to achieve complete mixing. The total pressure range for the measurement was 30-92 atm for each sample. The cell was placed in the temperature-controlledbath. The density of the fluid under subcritical conditions was calculated by a state equation, taking compressibility for each fluid into consideration, while the BWR equation24 was applied to the mixture in calculating the density under the supercritical condition. The measurements of absorption spectra and cross sections were performed using a D:! lamp (Hamamatsu Photonics L544) as a light source at 306 K. Wavelengths shorter than 220 nm were optically filtered by an aqueous solution of Cos04 (1 cm 0 1995 American Chemical Society
Herzberg Photoabsorption of Oxygen
- 2
J. Phys. Chem., Val. 99, No. 31, 1995 11831
n 230
240
250
260
270
280
Wave length [nml Figure 1. Absorption spectrum of (a) 02/C02 mixture (311 K, partial pressure of 0 2 10 atm, total 90 atm) and (b) 02/C2H6mixture (312 K, partial pressure of 0 2 11 am, total 60 atm).
thickness) to minimize the formation of ozone. The transmitted light was resolved by a monochromator (Nikon P-20) with a typical bandpass width of 0.2 nm and detected by a photomultiplier (Hamamatsu Photonics R928). The quantitative measurements of the photoabsorption cross section were conducted with the wavelength fixed at 230 and 248 nm, where the intensity of the transmitted light was sampled for 5 s. By averaging the signal, we obtained the absorbance, In IdI, where ZO is the intensity of the light after passing through the vacuum cell, and I is the intensity of the light after passing through the cell containing the sample.
Results
1. Absorption Spectra under Pressurized Conditions and Their Assignment. Absorption spectra were measured under several different pressures (less than 90 atm in the total pressure) of the foreign gases with a fixed pressure of 0 2 (5 or 10 atm). In any mixture, the light transmittance decreased with the increase of the foreign gas pressure. The absorption spectra have a broad appearance, with increasing intensity with the decrease of the wavelength. In the case of N2 and C02, the spectra showed a very weak progression of diffuse bands in the range 240-280 nm, as shown in Figure l a for the 02/C02 mixtures. In the case of ethane, this progression was clearly observed in the range 240-280 nm, superimposed on a continuously rising background, as shown in Figure lb. Thus, the progression in the range 240-280 nm can be assigned to the Herzberg I11 bands. We assume that the increase of the absorption in the above wavelength range is mainly due to the increased Herzberg I11 bands andor Herzberg continuum (at shorter wavelength) in any foreign gases studied in the present work, even though no clear band structure could be observed in some foreign gases. More detailed consideration of the assignment will be shown in the Discussion section. 2. Absorption Cross Section. 2.1. Effect of 02 Densio on Absorption Cross Section. In order to quantitatively treat the enhancement of the absorption, the absorbance was measured at 230 and 248 nm as representative wavelengths in the bands and continuum range, respectively, changing the pressure from 14 to 55 atm at 309 K. The latter wavelength is the same as the KrF laser light in the previous photolysis The dependence of absorbance on the density of 0 2 is shown in Figure 2. The plot of the experimental results fits well on the parabolic curve, which suggests that Lambert-Beer's law (eq 1 with a constant u),
0, density
[lo" molecule C ~ ' ~ I
Figure 2. Dependence of absorbance on 0 2 density for the 0 2 photoabsorption at 230 nm (open circles) and 248 nm (filled circles).
TABLE 1: Value of a13 at 230 and 248 nm (unit: cms molecule-lP solvent 230 nm 248 nm none (neat 0 2 ) (1.57 f 0.04) x (3.71 f 0.10) x lo-" (7.24 f 0.13) x lo-" (1.47 f 0.13) x lo-" N2' CW (2.81 5 0.42) x (6.14 f 1.94) x lo-" C2W (3.29 f 0.27) x (1.10 f 0.13) x c2w (6.21 f 0.70) x (1.52 f 0.28) x c02c (1.40 f 0.09) x (3.45 f 0.29) x lo-" a The value of u1.S was obtained by statistical analysis with a 95% confidential limit. Measured by use of dried air. The partial pressure of 0 2 was kept constant at 10 atm. ~~
~~
could not be applied to this system. Thus, the observed cross section, 0,is considered to be represented by an expression of the form
= 0 0 + ~1,0,@0, where (TO is a pressure-independent absorption cross section, more than an 80% fraction of which was reported by Huestis et al. to come from Herzberg I between 243 and 258 nm,25and (TI,O~ is a "density coefficient" of the cross section proportional to the number density of 0 2 as the foreign gas. Substituting (T in eq 2 into eq 1, we can rewrite the dependence of absorbance on the density of 0 2 as eq 3. (3) Because the first term of eq 3 is negligibly small compared to the second term under high-pressure conditions, it was impossible to determine the value of a0 from the present experimental results. Therefore, the reported values of ao,l3 3.2 x cm2 at 230 nm and 0.3 x cm2 at 248 nm, were used for the least-squares calculation of (TI,O~ by fitting the parabolic curve to the experimental results. The determined values are shown in Table 1. Ditchbum and Young8 studied the pressure dependence of the absorption coefficient of 0 2 between 185 and 250 nm with a pressure of 0.2-5 atm. According to them, the values of (21.0~ at 230 and 248 nm were 3.8 x and 1.3 x cm2/atm, respectively, which are converted to 1.5 x and 5.3 x cm5/molecule, where the value at 248 nm was calculated by linear interpolation of those at 240 and 250 nm. Shardanand'3 also measured the values of (~1.0~ at pressures up to 82 atm, which were reported to be 1.6 x cm5/molecule at cm5/molecule at 248 nm. The results 230 nm and 4.2 x
Oshima et al.
11832 J. Phys. Chem., Vol. 99, No. 31, I995
41
0.41
0
2. 0
LddLLfA 2 0, Dens i t y
0
1
I
0
4
CO, dens i t y
[lo’ mo I
Figure 3. Dependence of absorbance on 0 2 density for the photoabsorption of air at 230 nm (open symbols) and 248 nm (filled symbols) (00, 310 K; OW, 314 K; AA, 319 K).
[lo2’mo I ecu Ie
cmP1
Figure 4. Dependence of absorbance on CO2 density for the photoabsorption of 02/C02 mixture at 230 nm (open symbols) and 248 nm (filled symbols) (00,306 K; OW, 314 K).
cross section can be expressed as eq 7. obtained in the present work are in good agreement with these values, suggesting that the dependence of the absorption cross section on the 0 2 density is valid under such high-pressure conditions as adopted here. 2.2. Collision-Induced Enhancement of Absorption Cross Section in O D 2 Mixtures. The absorption cross section of air under high-pressure conditions was measured at 230 and 248 nm to investigate the effect of N2 on the absorption of 0 2 . The relationship of the absorbance to the density of air shows a parabolic curve, as in Figure 3. In the case of air, the absorption cross section is considered to obey eq 4,
‘=
0‘
+ ‘l,O,@O, + ‘I,N,@N2
(4)
Because the N2/02 ratio is constant, eq 4 can be rewritten as eq 5 by using y (the ratio of the number of N2 molecules to that of 0 2 molecules), (T
= 0‘
+ (O1,O2 + Y0I,N,)@O2
(5)
Substituting (T in eq 4 into eq 1, we obtain the dependence of the observed absorbance of air on 0 2 density as eq 6. M I d O = (‘0l)@o2 + W I , O * + Y‘sI.Nz)~~@o*2 (6) With the cross sections of 02, 0‘, and 0 1 . 0obtained ~ in section 2.1, the fitting of the parabolic curve to the experimental results in Figure 3 by a least-squares method gives us the density coefficient of N2. The obtained values of ~ 7 1 for . ~ N2 ~ at 230 and 248 nm are also shown in Table 1. In the analysis of nitrogen-induced absorption of 0 2 by Shardanand,I4 01 ,N, for N2 was estimated to be 2.2 x cm5/molecule at 248 nm. The ratio of ( T I & to ~1.0,was 0.46 at 230 nm and 0.40 at 248 nm, which is in fair agreement with the value reported by Shardanand (0.47 at 245 nmI4). Nitrogen is less effective in increasing 0 2 absorption in both the Herzberg bands and Herzberg continuum than 0 2 itself. 2.3. Enhanced Absorption of 0 2 in 02/Foreign Gas Mixtures. As mentioned in the Introduction section, the density dependence of 0 2 absorption in mixtures of 0 2 with various foreign gases will give us valuable information to clarify the mechanism of the enhancement of the absorption. There has been no quantitative report of the effect of foreign gases other than N2 and Ar on 0 2 absorption. Assuming that any foreign gas, S , plays a role similar to N2 in the enhancement of 0 2 absorption, the absorption
0
=0‘ + ‘l,O,eo,
+ ‘,,s@s
(7)
Substituting 0 in eq 7 into eq 1 leads to eq 8.
With the partial pressure of 0 2 being fixed at 10 atm through a series of runs, the first and second terms of the right-hand side in eq 8 can be regarded as constant. The total absorbance will be linear in the density of the foreign gas S. The experimental result for C02 is shown in Figure 4. The total absorbance increases linearly with increase of C02 density below 5 x lo2’ molecule/cm3, and the slope of this straight line corresponds to 1.4 x and 3.5 x cm5/molecule for u l , at ~ 230 and 248 nm, respectively. The value of 01,sfor C02 is estimated to be as large as that for 02, which indicates that the effect of C02 on the absorption of 0 2 is as large as the self-effect of 0 2 under high-pressure conditions. Another important fact we observed in Figure 4 is that, above the C02 density of 5 x 1O2I molecule/cm3, there is a deviation from the straight line in the total absorbance. The point from which the plot starts to deviate from the straight line corresponds to 84 atm, 306 K, and this condition is regarded as a supercritical one. It is known that, due to the fluctuation of the structure of the fluid,26light scattering is significant when the fluid is under supercritical conditions. Therefore, for 02/C02 mixtures, light scattering should be taken into consideration in the measurement of collision-induced absorption of 0 2 under supercritical conditions. The separation of true absorption from scattering in the supercritical region has not been attempted in the present work. The absorption augmentation for other gases-methane, ethane, and ethylene-could also be analyzed according to eq 8. The derived density coefficients for each foreign gas are tabulated in Table 1.
Discussion To understand the effect of individual foreign gases, the density coefficients, 01,sat 230 and 248 nm, have been plotted against the average polarizability, the 6 parameter of LennardJones potential, and the ionization potential of the foreign gases. In these correlations, twice the 01,svalue for 0 2 is compared with other ul ,S values, noting that the binary collision frequency among oxygen molecules is half of what is derived from the square of eo,. Among the studied characteristics, only the ionization potential gives a good correlation, as illustrated in Figure 5. The correlation is similar for the 230 and 248 nm
Herzberg Photoabsorption of Oxygen
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CP,
J. Phys. Chem., Vol. 99, No. 31, 1995 11833 combination of the density-independent and density-dependent term. In the case of 0 2 in C02, the enhanced absorption is measured under s u b and supercritical conditions. A discontinuous increase of apparent total absorption is observed near the critical point of the 02/C02 mixture, due to the light-scattering contribution. The density coefficient correlates well with the ionization potential of the foreign gas, which implies that a charge-transferlike interaction between the foreign gas and 0 2 plays an important role in relaxing the dipole-forbiddentransition, A‘’A,
L6 x’z;. -
’07 0
I on izat ion potent i a I [eVl
Figure 5. Relationship between ug,~and ionization potential (open circles for 230 nm and filled circles for 248 nm).
Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture of Japan (Grant No.
absorption cross sections. This fact shows that the mechanism of the enhancement of the absorption is the same for band and continuum absorption. According to previous researcher^,'^**^ it is certain that the mechanism of the absorption augmentation in the Herzberg bands region is the collision-inducedrelaxation of the selection rule for the dipole-forbidden transition. Thus, the Herzberg III system (in both the band and continuum regions) can be enhanced, which is reasonable considering that the A”A, - X3& is forbidden due to the fact AA = 2. On the contrary, the symmetry forbiddenness in the case of the A3Z: - X’C; transition is not expected to be affected by the third neighboring molecule. The spin selection rule, which prohibits the c’Z; - X’C; transition, is also barely affected by the present small molecules. The fact that foreign molecules with lower ionization potentials have larger enhancement effects suggests that the symmetry of the electron distribution of the oxygen molecule is noticeably perturbed by a charge-transfer-like interaction between 0 2 and the foreign molecule during the collision. We have investigated the oxidation of ethane in sub- and supercritical phase Odethane mixtures, under irradiation with a KrF excimer laser at 248 nm.23 The total-product yields increased with the increase in total pressure. Now it is clear that the absorption of the laser light also increases with the increase in total pressure. The oxidation reaction is considered to be initiated by a certain reaction of 0 2 in the Herzberg ungerade states, particularly A‘’A,. A detailed analysis of the subsequent reactions is underway.
04238 103).
+-
Conclusion By measuring the effect of foreign gas density on the collision-induced absorption of 0 2 in neat 0 2 and in various mixtures under high-pressure conditions, the cross section in Lambert-Beer’s law is confirmed to be expressed as a linear
References and Notes (1) Wulf, 0.R. Proc. Natl. Acad. Sci., USA. 1928, 14, 614. (2) Finkelnburg, W. Z. Phys. 1934, 90,1. (3) Herzberg, G.Can. J . Phys. 1953, 31, 657. (4) Coquart, B.; Ramsay, D. A. Can. J . Phys. 1986, 64, 726. (5) Salow, H.; Steiner, W. Nature 1934, 134, 463. (6) Salow, H.; Steiner, W. 2.Phys. 1936, 99, 137. (7) Evans, D.F. J . Chem. Soc. 1960, Part 11, 1735. (8) Ditchburn, R. W.; Young, P. A. J. Armos. Terr. Phys. 1962, 24, 127. (9) Blake, A. J.; Carver, J. H.;Haddad, G. N. J. Quant. Spectrosc. Radiat. Transfer 1966, 6, 451. (10) Shardanand, Phys. Rev. 1969, 186, 5. (11) Hasson, V.;Nicholls, R. W. J . Phys. B: Arm. Mol. Phys. 1971, 4 , 1789. (12) Ogawa, M. J. Chem. Phys. 1971, 54, 2550. (13) Shardanand; b a d Rao, A. D. J . Quant. Spectrosc. Radiat. Transfer 1977, 17, 433. (14) Shardanand, J . Quant. Spectrosc. Radiat. Transfer 1977, 18, 525. (15) Shardanand, J. Quant. Specfrosc. Radiat. Transfer 1978, 20, 265. (16) Johnston, H. S.; Paige, M.; Yao, F. J . Geophys. Res. 1984, 89, 11661. (17) Blake, A. J.; McCoy, D. G. J . Quant. Spectrosc. Radiat. Transfer 1987, 38, 113. (18) Horowitz, A.; von Helden, G.; Schneider, W.; Simon, F. G.; Crutzen, P. J.; Moortaat, G. K. J . Phvs. Chem. 1988, 92,4956. (19) Horowitz, A.:Schneider, W.; Moortgat, G. K. J . Phys. Chem. 1989, 93, 7859. (20) Horowitz, A.;Schneider, W.; Moortgat, G. K. J . Phys. Chem. 1990, 94 - ., 2904 - - .. (21) Goodman, J.; BNS, L. E. J . Chem. Phys. 1977, 67, 1482. (22) Kajihara, H.;Okamura, T.; Okada, F.; Koda, S. Laser Chem. 1995, IS, 83. (23) Iguchi, K.; Oshima, Y.; Koda, S. J. Photochem. Photobiol. A: Chem. 1994, 80, 439. (24) Benedict, M.; Webb, G. B.; Rubin, L. C. J . Chem. Phys. 1940, 8, 334. (25) Huestis, D.L.;Copeland, R. A.; Knutsen, K.; Slanger, T. G. Can. J. Phys. 1994, 72, 1109. (26) White, J. A,; Maccabee, B. S. Phys. Rev. Lett. 1971,26 (24), 1468. JP950582K
