6661
J . Phys. Chem. 1989, 93, 6661-6665 water but different gases, except that the C 0 2 bands were evident in the C 0 2 solutions. The four k ( v ) spectra of the same type of water but different gases were averaged. The five averages for the different types of water agreed to f1.5% near the top of the OH stretching band and *OS% elsewhere. The OH stretching intensity decreased in the following order: tap, triple-distilled, distilled tap, building distilled, and deionized water. The five k ( v ) spectra of water from different sources but the same gas were averaged. The four averages for the different gases differed by f 1.3% near the top of the O H stretching band and i0.7% elsewhere, with the intensities decreasing in the order of dissolved gases: nitrogen, oxygen, carbon dioxide, argon. Finally, the 20 k(v) spectra for different water-gas combinations were averaged to yield a single k(u) spectrum from about 200 recorded spectra. In this final spectrum the k(u) values below 3700 cm-' agree to within 2% with those previously reported by us2 as the averages of values measured in 19849 and 1987.I Surprisingly, in the region that was previously' noted to be particularly variable, 3700-3300 cm-', the agreement between the final k(v) values of this 1988 work and those reported previously2 was particularly good, namely, 51%. Discussion Our most recent published spectrum2 is, thus, quite precise. Its accuracy was discussed previously2 and will be further tested by the results of future workers. There is no effect greater than i1.5% of either the gas content or the water purity on the intensity of the O H stretching band of water. The agreement between the average spectra of different sets is no better than that between the spectra in a set. This indicates a slight variation associated with rinsing, drying, and refilling the (9) Bertie, J. E.; Eysel, H. H. Appl. Speclrosc. 1985, 39, 392.
cell. That the percentage variation is much smaller for the bending band and the background absorption than for the O H stretching band indicates that most of the variation is not due to differences in coverage of the surface of the ATR rod. Because the intensity of the bending vibration is insensitive to the degree of hydrogen bonding' while that of the stretch is, of course, very sensitive,' it is natural to consider differences in microscopic surface contact as possibly limiting the precision. However, it may well be that the differences noted in the literature and this work are mainly due to random measurement error. The intensities of the bending band and background frequently were reproduced exactly in different spectra even though the O H stretching band varied by 5%. The OH stretching band absorbs very strongly. Thus, transmission measurements require very thin films which are very difficult to measure precisely. Further, the strong absorption allows only 1.6% reflection in the CIRCLE cell at the peakl,2 and yet reflects only -5% at an air-water interface5 Both of these reflectivities are more difficult to measure precisely than stronger reflection. So the absorption intensity is not easy to measure precisely, and therefore accurately, by any of these methods. In contrast, the bending mode can be measured well in transmission and by the CIRCLE method. A single ATR reflection inside a ZnSe rod in water should reflect about 29% at the peak of the OH stretching band of water, and so this may be a better method to use. Zolotarev3 has used single-reflectionATR, but with a spectrometer that was less precise than modern ones. Also, reflection from an air-water interface with the high signal-to-noise ratio available from an FT-IR spectrometer should yield better precision than was available to earlier workers.5
-
Acknowledgment. J.E.B. thanks the Natural Sciences and Engineering Research Council of Canada for their support of this work. Registry No. H20, 7732-18-5; Nlr 7727-37-9; 02, 7782-44-7; Ar, 7440-37-1; C02,124-38-9.
Infrared Spectroscopic Studies of Supercritical Fluid Solutions Jonathan P. Blitz: Clement R. Yonker,* and Richard D. Smith Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory,' Richland, Washington 99352 (Received: March 17, 1989)
Near- and mid-infrared spectroscopic studies of supercritical carbon dioxide and binary supercritical fluid solutions of carbon dioxide/water, argon/water, krypton/water, and xenon/water as a function of fluid density are reported. The intermolecular interactions for supercritical carbon dioxide and solvent environments indicated by the IR studies are compared to results obtained from UV-visible solvatochromic studies with supercritical carbon dioxide. Infrared shifts observed in the binary fluid mixtures with water are discussed in terms of the Onsager electrostatic theory. The results demonstrate that while solute shifts induced by noble gas solvents can be described by a simple model based upon dielectric contributions, this model inadequately explains results for water/carbon dioxide mixtures. Further information can be obtained by monitoring the rotational freedom of the solute and directly probing the solvent as a function of density by IR spectroscopy.
Introduction During the past few years it has been demonstrated that new insights into the fundamental nature of solvation may be obtained through the study of supercritical fluid solutions.'-3 The utility of an approach using supercritical fluids, from an experimental viewpoint, arises from the fact that the physicochemical properties of fluids can be continuously varied from those of a gas to those
*Towhom correspondence should be addressed. 'Current address: Quantum Chemical Corp. US1 Division, Research Division, 1275 Section Road, Cincinnati, OH 45222. *Operated by Battelle Memorial Institute. 0022-3654/89/2093-6661$01.50/0
of a liquid. A large range in solvent properties can be obtained for a single solvent by varying temperature and/or pressure of the fluid, a capability not obtainable with liquids. This allows studies with one fluid solvent covering a large range of solvent strengths, whereas a similar study with liquid systems would (1) Yonker, C. R.; Frye, S . L.; Kalkwarf, D. R.; Smith, R. D. J. Phys. Chem. 1986, 90, 3022. (2) Yonker, C. R.; Smith, R. D. J . Phys. Chem. 1988, 92, 2374. (3) Kim, S . ; Johnston, K. P. In Supercritical Fluids, Chemical and Engineering Principles and Applications; Squires, T. G., Paulaitis, M. E., Eds.; American Chemical Society: Washington, DC, 1987; ACS Symp. Ser. No. 329, Chapter 4.
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require a number of different solvents and lead to uncertainties in interpretation. Previous work has focused on solvation in fluids through the study of UV-visible solvatochromic characteristics of various probemolecules.'-' These studies investigated changes in the electronic structure of the probe molecule as a function of solvent environment. This experimental approach, although useful, has some inherent limitations. First, most supercritical fluid solvents do not contain useful UV-visible chromophores, so it is not possible to probe solvent absorptions directly. Second, spectroscopic probing of electronic absorption in dense gases and liquids has an inherently low information content compared to vibrational spectroscopies. Finally, the use of solute probes always introduces uncertainties concerning possible specific probe-solvent interactions. Surprisingly little work has been reported using vibrational spectroscopy to study supercritical fluids. Franck's work applying vibrational spectroscopic techniques to the study of supercritical water and water/carbon dioxide mixtures is the most impressive.e6 Raman spectroscopic studies in the supercritical regime for selected solvents have led to reports on pressure-induced frequency shifts from forbidden transition^.^^^ Recent advances in FTIR spectroscopy and the potential of probing solute and solvent absorptions concurrently and directly make this an appealing technique for application to supercritical fluid solutions. Since 1980, most FTIR spectroscopy of supercritical fluids has been directed toward analytical applications, i.e., as an analytical detection method for supercritical fluid chromat~graphy.~-" Fundamental studies of the nature of supercritical solutions by FTIR spectroscopy have received little attention to date and have been largely limited to the observation of changes in two bands resulting from Fermi resonance as a function of pressure in supercritical carbon dioxide" and shifts in the frequency of maximum absorbance of a C-H stretching mode of anthracene at various temperatures as a function of density in CO2.I2 In this article, we report FTIR spectroscopic studies of pure carbon dioxide and binary mixtures of water with carbon dioxide, argon, krypton, and xenon. Studies of water dissolved in supercritical fluids are of intrinsic importance in the understanding of molecular interactions between polar and nonpolar molecules. Results are discussed in terms of the Onsager electrostatic theory of the reaction field for the polar solute (water) and the nonpolar continuum s01vent.I~ The study of water dissolved in a dense noble gas is a stringent test of the validity of the Onsager electrostatic theory. These studies are relevant to recent studies of reverse micelles in supercritical fluids where large amounts of water can be solvated in reverse micelle phases for these fluids.14J5 In order to understand the nature of water in reverse micelles with continuous phases such as xenon16 or fluids such as carbon dioxide, a fundamental understanding of the nature of water in a pure fluid is essential. Experimental Section A Nicolet 740 FTIR spectrometer (Nicolet Analytical Instruments) purged with nitrogen was used to acquire all infrared spectra. Near-infrared work was accomplished with a visible Franck, E. U. Phys. Chem. Earth 1981,I3&14, 65. Franck, E. U.;Roth, K. Discuss. Faraday SOC.1967,43, 108. Franck, E.U.Pure Appl. Chem. 1970,24,13. Schindler, W.; Zerda, T. W.; Jonas, J. J . Chem. Phys. 1984,81,4306. Versmold, H.; Zimmermann, U. Z. Phys. Chem. (Munich) 1987,155, 11. (9)Shafer, K.H.;Griffiths, P. R. Anal. Chem. 1983,55, 1939. (10)Olesik, S. V.; French, S. B.; Novotny, M. Chromatographia 1984,18, 489. (1 1) Johnson, C. C.; Jordan, J . W.; Taylor, L. T.; Vidrine, P. W. Chromarographia 1985,20, 717. (12)zerda, T. W.; Wiegand, B.; Jonas, J. J. Chem. Eng. Data 1986,31, 274. (13)Onsager, L. J . Am. Chem. SOC.1936,58, 1486. (14)Fulton, J. L.;Smith, R. D. J. Phys. Chem. 1988,92,2903. (151 Blitz. J. P.: Fulton. J. L.: Smith. R. D. J. Phvs. Chem. 1988.92.2707. (16)Fulton, J. L.;Blitz, J. P:;Tingey, J. M.; S&th, R. D. J . Phys. Chem. 1989,93,4198.
Density (g/Cm3)
Figure 1. Plot of the frequency shift of ( 3 4 versus density (g/cm3) for (0)supercrtical C02, 50 OC,and (W) subcritical C02,26 " C .
source, quartz beam splitter, and lead selenide detector. Spectra were acquired at 4-cm-I nominal resolution with coaddition of 256 scans. Mid-infrared work was done with a globar source, KBr-on-germanium beam splitter, and a liquid nitrogen cooled wide-band mercury-cadmium-telluride detector. Spectra were acquired at 1-cm-' nominal resolution with coaddition of 128 scans. Resulting interferograms were truncated by using Happ-Genzel apodization and ratioed to an open beam background spectrum. The long path length (7.5 or 5 cm) infrared cells equipped with sapphire windows have been previously described.17 SFC grade carbon dioxide (Scott Specialty Gases), research grade xenon (Linde), research purity krypton (Scott Specialty Gases), and UHP grade argon were used as received. Distilled deionized water was used in all cases. A Varian 8500 syringe pump was filled with pure carbon dioxide, argon, krypton, or xenon. The syringe pump was connected to the infrared cell and a high-pressure, hand-operated syringe pump (HIP, 60000 psi). Prior to introduction of the fluid, known volumes of water were loaded into the IR cell. After the cell was flushed and filled with fluid to the desired pressure, the solution was stirred with a Teflon-coated, magnetically coupled stir bar. The insulated cell was electrically heated to f0.5OC of the desired temperature by using a three-mode controller with a platinum resistance temperature probe (Omega, No. N2001). The solution pressure was measured with a high-pressure transducer (Precise Sensors, No. 45 1- 100000). Experiments of water in carbon dioxide were done in singlephase systems (Le., all water was dissolved). Carbon dioxide band maxima and bandwidths were obtained by using standard Nicolet software. The v2 + v 3 combination band position of water in carbon dioxide and the v 3 (asymmetric 0-H stretching) fundamental band position in the noble gases were monitored by locating the frequency of maximum absorbance. All studies with the dense noble gases were with two-phase systems. Results and Discussion Figure 1 shows the frequency of maximum absorbance as a function of density for the symmetric stretch overtone ( 3 4 of carbon dioxide.I8 The main feature is a shift to lower frequency as density increases, which is caused by the increased intermolecular interactions occurring in the bulk carbon d i o ~ i d e . ' ~ ~ ~ ~ Figure 1 shows both subcritical (26 "C) and supercritical data for carbon dioxide. These results are similar to those determined by UV-visible spectroscopic studies of subcritical and supercritical (17) Blitz, J. P.;Fulton, J. L.; Smith, R. D. Appl. Specfrosc., in press. ( 1 8) Herzberg, G. In Molecular Spectra and Molecular Structure II.
Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1945;pp 274,281. (19)Wiederhehr, R. R.; Drickamer, H. G.J . Chem. Phys. 1958,28,311. (20)Benson, A. M.;Drickamer, H. G. J . Chem. Phys. 1957,27, 1164.
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6663
IR Studies of Supercritical Fluid Solutions
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0'7
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b Density (g/cm3)
Figure 3. Plot of full width at half-maximum (fwhm) of ( 3 4 versus density for supercritical C02 at 50 OC.
79
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WAVENUMBER
(owl)
Figure 2. Plots of the infrared absorbance spectra ( 3 4 of C 0 2 for (a) supercritical C 0 2and (b) for COz pressurized by argon, both at 50 OC.
carbon dioxide using the solvatochromic probe method.lv2 Comparable densities yielded no temperature-dependent frequency shifts of the probe over this temperature range, consistent with previous conclusions that solvent properties are primarily dictated by fluid density.' Near-infrared spectra of the 3v3 band of carbon dioxide as a function of pressure are shown in Figure 2a. Loss in rotational structure is evident as pressure is increased, but the relative roles of sample pressure (collisional broadening) or COz-C02 intermolecular interactions are not resolved. In order to decouple these two effects, pure carbon dioxide was charged into the IR cell to a pressure of 40 bar. The cell was then further pressurized with the inert gas argon. Argon-carbon dioxide intermolecular interactions should be minimal: therefore, any changes seen in the spectra can be attributed to collisional broadening. Figure 2b shows that rotational freedom is maintained at subsequently higher pressures, even though the overall system density increases. This is evident in the rotational fine structure seen in the argon/carbon dioxide system at pressures greater than 103 bar. This leads to the conclusion that the results of the pure carbon dioxide experiment are largely a manifestation of specific solvent-solvent intermolecular interactions. Figure 3 gives the full width at half-height for the 3v3 band of COz as a function of fluid density at 50 O C over the pressure range of 95-370 bar. The bandwidth decrease can be related to a loss in rotational freedom of carbon dioxide. As density increases, the full width at half-height decreases rapidly until a density of approximately 0.5 g/cm3 is reached. Beyond this density the full width a t half-height remains relatively constant. This change in bandwidths is due to increased COz intermolecular interactions at increased density. Changes in molecular interactions contribute to a loss of rotational freedom of COz as density
0.4
0.5
0.6
07
08
0.9
0
Denslty (g/cm3)
Figure 4. Plot of ( 3 u 4 versus density for C 0 2for (0) pure supercritical C02, (m) 28 mM water in supercritical C02,and (0) 140 mM water in supercritical C 0 2 at 50 "C.
increases from the low interaction gas like region (10.5 g/cm3) to the higher interaction liquidlike region (10.50 g/cm3). Similar trends for supercritical COS using a spectroscopic probe have shown that a change in solvation environment about the solute molecule occurs over roughly the same density regime.'Vz1 This higher local density of solvent about the solute molecule has been correlated with a change in the partial molar volumeF1 The FTIR data presented in Figures 2 and 3 are a different manifestation of the same phenomenon. As density increases, the potential energy between solvent molecules decreases, with a minimum potential energy reached at the specific intermolecular distance for a pair of COz molecules at approximately O S g/cm3. The binary supercritical fluid system water/carbon dioxide provides interesting contrasts to pure supercritical COz. Figure 4 shows the results obtained by monitoring the 3v3band of carbon dioxide as a function of density (f0.75 cm-I). The frequency shifts for the two binary fluid solutions (28 and 140 mM water) and (21) Yonker, C . R.;Smith,
R.D.J . Phys. Chem. 1988, 92, 235.
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The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
Blitz et al.
5330
I
U
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U
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m
0
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r
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0
z
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37301 5310
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3710 53w C
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004
008
008
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~
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I
( E B - 1 ) / ( 2 E ~ +1 )
Figure 5. Plot of v2 C 0 2 at 50 OC.
+ v, for 28 mM H20versus [(eB
- 1)/(2cg + l)] for
for pure COz are similar. The concentration of water was kept below the two-phase solubility limit of water in carbon dioxide.2z These results are not unexpected since the solutions are very dilute and contributions resulting from CO2-H20 interactions will be overwhelmed by COz-COz interactions. The solute-solvent interactions giving rise to the frequency shifts in the u2 + u3 combination band of waterla in supercritical carbon dioxide can be discussed in terms of the Onsager electrostatic theory of the reaction field that influences the solute molecule.16 This reaction field theory was first applied to the interpretation of experimental spectroscopic results by K i r k w o ~ d . In ~ ~Kirkwood's model the spectroscopic shift of the solute molecule would be proportional to the reaction field determined by the dipole moment of the solute molecule and the solvent molecules in the local solvation environment about the solute molecule. The reaction field will be proportional to the solute dipole moment and will be collinear with the electrostatic field produced by the solute dipole moment.24 In systems of polar solute molecules and nonpolar solvent molecules, we assume that only nonspecific solute-solvent interactions occur and expect the behavior to be controlled by the dielectric effect of the reaction field. This behavior would be manifested as a linear shift in the absorbance maxima with a change in the reaction field (i.e., fluid dielectric). Any deviations from linear behavior could then be explained by assuming specific or localized solute-solvent interactions in the solvation shell about the solute molecule.z5 For Onsager electrostatic theory the induced solvent shifts in the absorbance spectra will depend upon the dipole-induced-dipole and instantaneous dipole-induced-dipole interactions. The reaction field is equal to Er = (2/4/a3)[(cB - 1 ) / ( 2 t B + 111 (1) where h, is the dipole moment of the solute, a is the radius of the cavity that the solute molecule inhabits; and tB is the dielectric constant of the bulk solvent. The refractivity virial coefficients for carbon dioxide have been used to determine the dielectric constant for C02 because tB and n2 are equal since C 0 2 has no permanent dipole ~ n o m e n t . ~ ~ - ~ ~ (22) Smith, R. D.; Udseth, H.R.; Wright, B. W. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A,, Krukonis, V. J., Eds.; Elsevier Science Publishers: Amsterdam, 1985; p 192. (23) Kirkwood, J. G.J . Chem. Phys. 1936, 4, 592. (24) Kamlet. M. J.; Abboud, J. L. M.; Taft, R. W. Prog. Phys. Org. Chem.
1980, 13, 485. (25) Horak, M.; Pliva, J. Spectrochim. Acta 1965, 21, 911. (26) Kholodov, E. P.; Timoshenko, N . I.; Yamnov, A. L. Therm. Eng. (Engl. Transl.) 1972, 19, 126. (27) Bose, T. K.; Cole, R. H. J . Chem. Phys. 1970, 52, 140. (28) Michels, A.; Kleerekoper, L. Physica (Amsterdam) 1939, 6, 586. (29) Keyes, F. G.; Kirkwood, J. G.Phys. Rev. 1930, 36, 754.
3700 0 1
00
3
02
(EB.1)1(2E B + 1 )
Figure 6. Plot of [(tg - 1)/(2eB+ l)] versus the asymmetric stretch of water in supercritical argon ( O ) , supercritical krypton (O), and supercritical xenon ).( at 25 OC.
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Figure 7. Plots of the infrared asorbance spectra of the asymmetric stretch of water (a) in the vapor phase (artificially broadened to 50 cm-I) and (b) in supercritical xenon at 965 bar at 25 O C .
Figure 5 is a plot of the term in brackets in eq 1 , which is proportional to the reaction field, and the frequency of maximum absorbance of the water combination band for the binary fluid mixture. If only nonspecific interactions occur between COzand water, then one would expect a linear relationship between fre1). As seen in Figure 5, there is a quency and (eB - 1 ) / ( 2 c B ichange in slope as one traverses the abscissa, going from low to high dielectric constant for carbon dioxide. This break is similar to that seen in the UV-visible solvatochromic work described by Yonker et al.l*z'and Kagimoto et aL30 The nonlinearity seen in Figure 5 is most likely due to changes in the solute-solute or solute-solvent intermolecular interactions as density is increased. Further experiments are planned to resolve questions concerning these observations. To investigate the validity of the reaction field model as applied to solvent shifts, binary fluid mixtures of water in argon, krypton, and xenon were studied. These systems should be limited to dipole-induced-dipole intermolecular interactions between water and the noble gas atoms. The interactions should be adequately described by the reaction field model. Figure 6 is a plot of the (30) Kagimoto, 0.; Futakami, M.; Kobayashi, T.; Yamasaki, K. J . Phys. Chem. 1988, 92, 1347.
J . Phys. Chem. 1989, 93, 6665-6671
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asymmetric stretch of water versus (eB - 1)/(2tB 1) for the binary fluid sdutions of argon/water, krypton/water, and xenon/water. The dielectric constant for the noble gases as a function of density was determined by using the dielectric virial coefficients available in the l i t e r a t ~ r e . ~ ' , ~ 'Within - ~ ~ the error of the estimation of the dielectric constant for the noble gases (*15%) and the experimental error of the frequency of maximum absorbance for the asymmetric stretch, a nearly linear relationship is Seen in Figure 6 over a wide range of the bulk dielectric constant. Therefore, the simple model of the dielectric continuum inducing the solvent shifts appears valid under these conditions as compared to the behavior of the C02/water system. This behavior is further illustrated in Figure 7a, which shows a spectrum of water vapor with the bands artificially broadened to 50 cm-' full width at half-height. The spectrum in Figure 7b of water at high dilution in xenon at 25 O C and 965 bar shows the collisional broadening of water and the solvent shift induced by the change in bulk dielectric of xenon. Otherwise, the two spectra are comparable, demonstrating that water at this high pressure is still freely rotating in the dense xenon phase. Conclusions The study of frequency shifts in supercritical fluids using FTIR spectroscopy can yield a better understanding of solvation phenomena in fluids as a function of density. Studies of supercritical (31) Paysia, D. Y.; Smith, B. L. J . Phys. C 1971, 4, 2254. (32) Chapman, J. A.; Finnimore, P. C.; Smith, B. L. Phys. Rev. Lett. 1968, 21, 1306. (33) Sinnock, A. C. J . Phys. C 1980, 13, 2375. (34) Marcoux, J. cah. J . Phys. 1970, 48, 244.
6665
carbon dioxide show an increase in intermolecular interaction between the COz molecules as the distance between molecules decreases (Le., increasing density). This is similar to solvent compression about a solute molecule as reported in the UV-visible solvatochromic studies of a supercritical fluid system as a function of density.121*MThe use of infrared spectroscopy allows the added benefit of studying solventsolvent interactions without the perturbations introduced by a spectroscopic probe. The application of the Onsager electrostatic theory of the reaction fluid created by the dielectric continuum of the fluid solvent has been successfully applied to the supercritical noble gases. The intermolecular interactions involved dipole-induced-dipole interactions between the water and noble gas and were accurately described by this theory. The solvent shifts seen for water as a function of the dielectric continuum were proportional to the reaction field. For the water/carbon dioxide binary fluid, a change in behavior is suggested at lower densities when applying the dielectric continuum model to the frequency shift in the v2 v3 combination band of water. This can be related to density-dependent changes of specific intermolecular interactions. Loss of rotational freedom can be studied in terms of collisional broadening and increased specific intermolecular interactions as a function of pressure. Such effects can be substantially decoupled by pressurizing the solvent with an inert diluant gas such as argon, potentially allowing study of the role molecular distance has upon solvent-solvent intermolecular interactions as density increases.
+
Acknowledgment. Work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.
Schumann-Runge Resonance Raman Scattering of 0,: A Rotationally Resolved Excitation Profile Study Y. P. Zhang and L. D. Ziegler* Department of Chemistry, Northeastern University, Boston, Massachusetts 021 15 (Received: March 22, 1989)
Rotationally resolved resonance Raman spectra and excitation profiles of O2excited with narrow-band radiation tunable throughout the u' = 5 absorption band of the Schumann-Runge (SR) region (190-192 nm) are reported. The pressure dependence and scattering polarization unambiguouslyidentify all the observed resonant emission intensity as Raman scattering (both resonant and off-resonant), not resonance fluorescence. This characterization is in contrast to the description of the resonant emission of the SR absorption bands offered in recent laser-excited studies. Excitation profile analysis determines rotationally specific lifetimes of the u' = 5 level. A homogeneous line width of 2.05 f 0.10 cm-' is determined for the rotational levels of this vibronic band. Within experimental uncertainty, this line width/lifetime is independent of the rotational angular momentum of the resonant predissociative rovibronic levels of the u' = 5 band. This value is in excellent agreement with the results of the most recent SR absorption contour analysis but is not in quantitative agreement with the most recent theoretical modeling of the rovibronic dynamics of the SR absorption bands.
Introduction The Schumann-Runge (SR) absorption bands of molecular oxygen (B 32; X 32;) extend from 205 to 175 nm and thus play a crucial role in the photochemistry of the upper atmosphere. Quantitative measurements of S R oscillator strengths and line widths are necessary for accurate determinations of optical depths and the subsequent modeling of photodissociation processes in the terrestial atmosphere.' The dipole allowed B state absorption spectrum consists of a series of weak discrete vibronic bands (to u' = 21) in this spectral region which exhibit diffuse partially resolved rotational structure.24 The diffuseness of these discrete
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( 1 ) For example see (and references within): Nicolet, M. J Ceophys. Res. 1984.89, 2573. (2) Krupenie, P. H. J . Phys. Chem. Ref.Data 1972, 1, 423.
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rovibronic features is due to predissociation. Most of the oscillator strength of the X B transition is found at energies to the blue of these weak features and appears as an absorption continuum (175-125 nm)above the O2 O(3P) O(lD) dissociation limit. High-resolution studies of the SR bands show that absorption line widths vary as a function of the excited-state vibrational level, u', in a highly nonmonotonic pattern corresponding to lifetimes in the range from 1 to -50 ~ S . ~ ' OTheoretical treatments have
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(3) Yoshino, K.; Freeman, D. E.; Parkinson, W. H. J . Phys. Chem. Ref. Data 1984, 13, 207. (4) Yoshino, K.; Freeman, D. E.; Esmond, J. R.; Parkinson, W. H. Planet. Space Sci. 1983, 31, 339. (5) Ackerman, M.; Biaume, F. J . Mol. Spectrosc. 1970, 35, 73. ( 6 ) Hudson, R. D.; Mahle, S. H. J . Geophys. Res. 1972, 77, 2902. (7) Lewis, B. R.; Carver, J. H.; Hobbs, T. I.; McCoy, D. G.; Gies, H. P. F. J . Quant. Spectrosc. Radiat. Transfer 1978, 20, 191; Ibid. 1979, 22, 213.
0 1989 American Chemical Society
