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Absorption cross section measurements of carbon dioxide in the wavelength region 118.7–175.5 nm and the temperature dependence
J. Quant.
Spectrosc.
0022-4073(!45)00135-2
Vol. 55, No. I. pp. 5340, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-4073/96 $I 5.00 + 0.00
Radar.
Transfer
ABSORPTION CROSS SECTION MEASUREMENTS OF CARBON DIOXIDE IN THE WAVELENGTH REGION 118.7-l 75.5 nm AND THE TEMPERATURE DEPENDENCE K. YOSHINO,”
J. R. ESMOND,” Y. SUN,” W. H. PARKINSON,’ K. ITO,b and T. MATSUIh
“Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A. and hPhoton Factory, National Laboratory of High Energy Physics, Tsukuba, Ibaraki 305, Japan (Received 17 January 1995)
Abstract-Laboratory measurements of the relative absorption cross sections of CO* at the temperatures 195 and 295 K have been made throughout the wavelength region 118.7-175.5 nm. Laboratory measurements of the absolute absorption cross sections of CO2 at the temperature 195 and 295 K have been made at 12 different wavelengths in the region
118.7-175.5 nm. These measurements of CO* 118.7-175.5 nm. Almost of the strong bands at regions.
absolute values have been used to put the relative cross section at 195 and 295 K on an absolute basis throughout the region no temperature dependence was observed in the peak cross sections 130 nm region, but large temperature effects are seen in the other
1. INTRODUCTION
In the Earth’s atmosphere, CO1 is present in the troposphere, stratosphere, and at higher altitties. In the stratosphere the CO* mixing ratio is - 350 ppmv where solar radiation causes its photolysis (CO, + hv -+ CO + 0) to CO. Typically, CO* is present in concentrations of lOI cmp3 at - 18 ikm, lOi cm-3 at - 50 km, and 10” cme3 at -80 km. In the lower thermosphere, from - 100 to 140ikm, the CO, concentration varies from 10” to lo6 cm- 3.’ Photolysis of CO, occurs mainly at relati;vely high altitudes where solar Lyman-a and continuum radiation at the Schumann-Runge (Q-R) wavelengths are present. The photolysis is due primarily to Lyman-M in the upper mesosphere,,and to the continuum radiation in the mesosphere, and stratosphere. Because of the strong temperdture dependence of the CO, absorption cross section in the region of the S-R bands, its photolysis~rate decreases by about one order-of-magnitude for a temperature change from 300 to 273 K.* Ni+let3 has calculated photodissociation coefficients of CO2 at the top of the atmosphere at the tiean solar-terrestrial distance for temperatures at 273, 250, and 220 K in the spectral region of the ‘S-R bands (2,0)-(19,0). Carbon dioxide is a major constituent of the atmospheres of Mars and Venus, and it occurs,also in the atmosphere of Titan. In the primitive prebiotic atmosphere of the Earth, the photoly&s of CO, and H,O constituted the only sources of oxygen; from a model calculation which incl/udes steady-state diffusion and photochemistry, Kasting et al4 have calculated that the predomi/nant source of oxygen at all altitudes was the photodissociation of CO*. In absorption, CO, is transparent down to at least 210 nm. Photoabsorption cross section measurements of CO, irh the u.v to v.u.v region have been studied by several investigators. Because of lack of resolution, pane of the studies in the period 1950-70 resolved the structures in the CO* absorption cross sec$ion. OgawaS measured absorption coefficients of CO, in the region 175-216 nm with a 3 m vaduum spectrometer (instrumental width 0.008 nm), and observed numerous absorption bands. Shepansky6 measured cross sections in the region 170-300 nm with a 2 m vacuum spectrometer but @ade most of the measurements with an effective resolution of only 0.4 nm. DeMore and Patbpof’ measured CO2 cross sections with an instrumental width of 0.06 nm in the range 185-220 nni and 53
K. Yoshino et al
54
examined the temperature dependence in the range 220-320 K. The most recent cross section measurements of CO* are those of Lewis and Carver.8 These were obtained at temperatures of 200, 300, and 370 K, and were measured throughout the region 120-197 nm at wavelength intervals of 0.05 nm with an instrumental width of 0.005 nm. Cossart-Magos et al9 photographed an absorption spectrum of CO, in the wavelength region 175-200 nm with the 10 m vacuum spectrograph at Meudon Observatory. They observed many discrete absorption bands of CO, and gave tentative analyses of nine bands. They kindly supplied, for our investigation, the entire CO, spectrum taken at a resolution of 0.0008 nm with a slit width of 30 pm. These spectra are sufficient to allow us to judge the resolution necessary for scanning the spectrum with a spectrometer. With the fine structured CO, bands in the 175-200 nm range, the cross sections obtained previously with low resolution are severely distorted by the instrumental widths. We photographed an absorption spectrum of CO, at 295 K below 170 nm, with the fourth order of a 6.65 m spectrograph at the Photon Factory, Japan. We confirmed from this spectrum that the structures in the CO, absorption spectrum are broad enough to allow the use of medium resolution instruments (a 3 m spectrometer). We report here the results of absolute cross section measurements of CO, at the temperatures 195 and 295 K in the wavelength region 118.7-175.5 nm. 2.
EXPERIMENTAL
PROCEDURE
The 3 m, normal incidence, vacuum spectrometer on the BL-20A beam line at the Photon Factory, KEK, Japan, was used in the first order of a 1200 I/mm grating to obtain absorption cross section measurements of COZ. The 3 m spectrometer is equipped with a gold-coated, holographic grating (Shimadzu, Japan), blazed at 40 nm, and it provided an estimated instrumental width of 0.0070 nm at 85 nm with entrance and exit slit widths of 0.02 mm. The absorption cell, which is directly connected to the exit slit assembly of the spectrometer, has two MgF, windows and provided a path length of 12.08 cm. The entire CO, column can be cooled to 195 K by immersing the cell in a slush of dry ice and alcohol. A SPACOM integrated detector system,” using a EMR 541 F photomultiplier, is mounted at the other end of the absorption cell without additional
5ooo
P B
3000
1000 1
I 120
I
I
I
I
I
I 150
I
I
I
I
I 130
Wavelength, nm Fig. I. The continuum background counts per second in the wavelength region 115-180 nm. The intensity decrease below 160 nm is due to the combination of spectral sensitivity of the detector, transmittance of MgF, windows, and reflectivity of the grating.
CO2 cross sections in the 118.7-175.5 nm
55
optical elements. The background continuum is provided by the synchrotron radiation source without any order separation, because any higher order radiation is cut off by the MgFz windows. 2.1. The scanned cross sections The shape of the observed intensity of the background source in the wavelength region 118.7-l 75.5 nm, results from several factors including the intensity of the synchrotron source, the reflectivity of the grating, the transmittance of windows, and the spectral sensitivity of the detector. Figure 1 shows the typical spectrum (counts per second) of the background intensity in the wavelength region 130-175 nm. We divided the spectral region into seven sections of 10 nm extent. We obtained data for every 0.005 nm step with three to five different pressures to get optical dep/ths between 0.3 and 2.5. The background continuum level was established by scanning the empty cell before and after each photoabsorption measurement, assuming linear interpolation of the background intensity levels. Any small shifts in the background continuum were corrected for with the aid of the absolute cross sections measurements, mentioned in the following section. 2.2. The absolute cross sections Most of the uncertainty in cross section measurements with a single beam spectrometer is in ithe estimation of the background intensity levels since they may vary with time and wavelength during the photoabsorption scans; this is specially true for continuum cross section measurements. ~To overcome this problem, we measured the absolute cross sections of CO, at twelve wavelenjgth positions in the range 118.7-175.5 nm with two wavelength positions in each scanning range.; At each wavelength, we measured output counts without CO* in the absorption cell, and with CO, in it, alternatively every two minutes. The background intensity was estimated from the averages of two measurements without CO*. We used six different pressures of CO>. 2.3. The wavelength calibration
The 3 m vacuum spectrometer was scanned under computer control but the wavelength given by the computer needed to be corrected. We measured wavelengths of the absorption spectrum of the Fourth Positive bands of CO at 78 K, and compared these with the known wavelengths of the band head positions from Tilford and Simmons. ” We found that corrections required to computer wavelengths ranged from 1.38 nm at 131 nm through 2.47 nm at 175 nm with an estimated uncertainty of 0.02 nm. 3. RESULTS
AND DISCUSSION
The absolute cross sections of CO* were measured at two temperatures, 195 and 295 K, at 12 wavelengths in the 12s175.5 nm region. The averaged cross sections, measured at six diffebent pressures, are listed in Table 1 along with the ratio of cross sections at 195 and 295 K.’ All wavelengths were chosen to be near minima in the cross sections. The uncertainty of 2% ins the measured cross sections arises from the statistical scatter of 0.3-0.5% observed in the optical depths, an uncertainty of 0.2% in the optical path lengths, uncertainties of O.lHl.4% in the temperatures, and an uncertainty of 0.5% in the pressure measurements. The uncertainty in the wavelength scale is 0.02 nm as mentioned in Sec. 2.3. The cross sections of CO* were obtained in the scan mode with 3-5 different column densities also at 195 and 295 K. The averaged cross sections were calibrated onto the absolute scale by using the absolute absorption cross sections. The final cross sections are presented in Fig. 2. Peak cross sections at 195 and 295 K are tabulated in Table 2 along with ratios of cross sections at 195~and 295 K. Almost no temperature dependence was observed in the peak cross sections in the wavenumber region 70,000-80,000 cm-‘, however, the ratio of the cross sections is down to 0.7 ar und 60,000 cm-‘. The ratios of the cross sections, a,,,/a 295,are plotted against wavenumbers in F’g. ! 3. Below 70,000 cm-‘, cross sections at 195 K decrease with increasing wavenumber. The peak cross sections of the strong structures around 75,000 cm-’ are temperature independent, but the cross sections at minima are down to 80% of those at 295 K. Plots of the cross sections at 195 K! and
K. Yoshino Table
1. Absolute
Wavelength mn 121.36 125.58 127.63 132.05 135.56 140.60 143.87 150.02 153.72 160.79 168.98 171.91 n Cross
absorption
Wavenumber cm-’ 82399. 79630. 78351. 75729. 73768. 71124. 69507. 66653. 65053. 62193. 59179. 58170.
sections
et al
cross sections
of carbon
Cross Section’ 295 K 195 K 4.37f0.06 5.08zkO.09 15.98f0.22 18.29f0.24 22.9f0.2 27.750.4 43.1hO.5 53.7fl.O 48.5f0.3 53.6f0.5 46.0f0.4 47.8f1.2 49.3k0.6 46.6f0.5 41.2f0.4 36.5f0.6 29.7f0.3 33.3zko.4 11.91f0.19 9.27f0.03 1.84f0.05 2.68fO.04 1.209rtO.084 0.659f0.007
are given in units of IO-”
dioxide. ~I%/%95 0.86 0.87 0.83 0.80 0.90 0.96 0.95 0.89 0.89 0.78 0.69 0.55
cm*.
ratios of cross sections, 0,9S/a295, are compared in Fig. 4 in the wavenumber range 74,000-82,000 cm-‘. The plots of ratios emphasize band structures more clearly than those of cross section plots. Our measured cross sections at 295 K are compared with those of Lewis and Carver* in the wavenumber region 72,00&80,000 cm-’ in Fig. 5. Very good agreement between cross sections values is seen but there are some shifts in the wavenumbers which can be noticed over the entire wavelength region. The shifts are not constant nor in a single direction. Our wavelength calibration is based on the Fourth Positive bands of CO; the Lewis and Carver measurements* were made with more or less the same resolution, but the interval of measurement, 0.05 nm, could not represent fine structures. -17 i -18
1
195 K
--._--. 60,ooO
80,000
Wavenumber Fig. 2. The absorption
cross sections
of CO, at 195 and 295 K.
57
COz cross sections in the 118.7-175.5 nm
Chan et alI2 observed the CO2 electronic spectrum by using their dipole (e,e) method of electron-impact excitation” which is free of optical instrument function and saturation errors. Their measurements with a high resolution dipole (e,e) spectrometer below 10eV (about 124nm) consisted of two broad peaks which were assigned to transitions of final states, ‘A, and ‘II,. Our results are compared with the results of Chan et al” in Fig. 6 where the smooth curve is their fit to their data. Their values fit well at the peak of bands at 130 nm, but show higher values for the bands at 145 and 120 nm. The higher values at 120 nm region could be wing effects of the strong bands at 11 eV (not shown in our work). The higher values of the bands at 145 nm are also wing effects from the stronger bands at 130 nm. The absorption spectra of CO, consist of two distinctive bands: the stronger bands below 1401nm and the weaker bands above 140 nm (see Fig. 6). Based on theoretical calculations, Knowles et alI4 assigned the stronger bands to the ‘l$ c ‘Xi transition, and weaker bands to the ‘C: , ‘A,,t ‘Ci transitions. The spacing of the absorption peaks is irregular and the structure can not be described by just the symmetric stretch motion of C02. The cross section at the Lyman-a line of hydrogen is important because Ly-a is a dominant source of solar radiation in this wavelength region. For CO*, many measurements have been done at this wavelength: 7.5 x -20 cm2 by Preston,” 7.3 x -20 cm2 by Inn et aLI6 8.2 x --20cm2 by Nakata et al 2” 7.6 x p20cm* by Lewis et al.* We obtained a cross section at the Ly-cr line (82,259 cm’) of 6.54 x -20cm* at 295 K and 6.11 x -‘“cm2 at 195 K. They are lower than previous measurements; however, cross sections vary rapidly in this wavelength region, changing from 5.29 to 7.37 x -2acm2 in a 0.2 nm range.
Table 2. Cross sections of carbon dioxide at peak absorption. WN 59086. 59262. 59602. 59907. 60167. 60253. 60348. 60537. 60738. 60841. 61029. 61371. 61500. 61660. 62102. 62284. 62465. 62626. 62809. 63037. 63108. 63229. 63441. 63604. 63668. 63802. 63881. 63970.
295 K Cross Section WL 169.25 3.44 168.74 3.35 4.90 167.78 4.81 166.92 166.20 6.27 6.58 165.97 165.71 6.34 6.41 165.19 8.37 164.64 164.36 9.04 8.68 163.86 162.94 12.15 162.60 11.85 162.18 11.79 161.03 15.72 160.56 16.54 160.09 19.13 159.68 20.7 159.21 16.51 158.64 22.8 158.46 24.8 158.16 22.7 157.63 22.8 157.22 27.9 157.06 30.7 156.73 26.8 156.54 27.0 156.32 27.3
64179. 64348. 64502. 64659. 64659. 64963. 65135. 65309. 65390. 65441. 65561. 65683. 65837. 65934. 66179. 66478. 66725. 66881. 67022.
155.81 155.41 155.03 154.66 154.18 153.93 153.53 153.12 152.93 152.81 152.53 152.25 151.89 151.67 151.11 150.43 149.87 149.52 149.20
32:7 35.9 30.3 36.9 40.0 38.0 35.9 42.8 43.3 42.3 41.3 40.2 48.8 49.5 42.6 56.2 46.0 55.5 56.6
WN 59086. 59248. 59596. 59914.
195 K WL Cross Section 169.25 2.60 168.78 2.60 3.68 167.80 3.55 166.91
0195/~m
60245.
165.99
5.00
0.760
60537. 60735. 60853. 61027. 61368. 61503. 61655. 62101. 62283. 62468. 62625. 62816. 62909. 63111. 63239. 63431. 63599. 63670. 63791. 63887. 63978. 64067. 64175. 64357. 64520. 64664. 64860.
165.19 164.65 164.33 163.86 162.95 162.60 162.19 161.03 160.56 160.08 159.68 159.19 158.96 158.45 158.13 157.65 157.24 157.06 156.76 156.53 156.30 156.09 155.82 155.38 154.99 154.65 154.18
5.05 6.60 6.69 6.79 9.82 9.48 10.20 13.03 13.78 15.96 18.41 13.41 16.60 22.4 19.69 19.02 24.8 27.5 23.5 23.6 23.2 24.2 28.7 32.5 26.4 32.2 36.3
0.788 0.789 0.740 0.782
65116. 65313. 65411. 65463. 65574. 65669. 65841. 65953. 66174. 66476.
153.57 31.6 153.11 38.7 152.88 39.7 152.76 38.6 152.50 37.7 152.28 35.7 151.88 44.9 151.62 45.3 151.12 38.8 150.43 52.4
0.880 0.904 0.917 0.912 0.913 0.888 0.920 0.915 0.911 0.932
66904. 67017.
149.47 51.9 149.22 52.3
0.935 0.924
0.755 0.776 0.751 0.738
0.808 0.800
0.865 0.829 0.833 0.839 0.889 0.812 0.728 0.903 0.867 0.834 0.889 0.896 0.877 0.874 0.850 0.878 0.905 0.871 0.873 0.908
continued otierleaf
K. Yoshino et al
58
Table 2-continued. 295 K
55000
WN
wl.
67230. 67379. 67527. 67720. 67812.
148.74 148.41 148.09 147.67 147.47
51.5 53.0 59.5 54.2 52.7
68132. 68623. 68941. 69128.
146.77 145.72 145.05 144.66
64:3 64.6 55.5 62.5
69678. 69805. 70216. 70537. 70630. 70838. 71292. 71766. 71914. 72198. 72475. 73032. 73307. 73562. 74205. 74681. 74917. 75265. 75358. 75517. 75990. 76157. 76793. 77460. 77933. 78121. 78601. 78783. 79240. 79385. 79829. 80069. 80444. 80500. 80723. 81118. 81643. 81956. 82186.
143.52 143.26 142.42 141.77 141.58 141.17 140.27 139.34 139.06 138.51 137.98 136.93 136.41 135.94 134.76 133.90 133.48 132.86 132.70 132.42 131.60 131.31 130.22 129.10 128.32 128.01 127.22 126.93 126.20 125.97 125.27 124.89 124.31 124.22 123.88 123.28 122.48 122.02 121.68
60:4 58.1 57.8 52.8 52.6 55.7 55.6 52.8 50.1 52.1 68.5 67.0 56.7 96.0 124.5 86.6 105.1 85.0 92.2 112.7 90.9 119.5 115.3 89.5 60.1 63.8 50.4 44.3 34.7 33.2 23.9 23.8 18.07 17.76 15.07 12.34 9.98 7.62 7.45
60000
Cross
Section
65000
67244. 67365. 67522. 67722. 67810. 68027. 68128. 68621. 68934. 69152. 69302. 69677. 69820. 70234. 70539. 70632. 70836. 71284. 71757. 71914. 72192. 72480. 73042. 73284. 73567. 74204. 74677. 74924. 75253. 75363. 75520. 76015. 76162. 76789. 77468. 77931. 78138. 78599. 78796. 79232. 79382. 79838. 80081.
195 K WL Cross Section 148.71 48.4 148.45 49.5 148.10 56.4 147.66 51.7 147.47 50.0 147.20 53.6 146.78 62.0 145.73 61.5 145.07 52.2 144.61 61.0 144.30 56.5 143.52 59.6 56.7 143.23 142.38 57.3 141.77 52.5 141.58 51.4 56.2 141.17 54.7 140.28 139.36 52.5 139.05 49.3 138.52 50.8 137.97 68.5 66.4 136.91 136.45 52.1 135.93 95.1 134.76 122.3 81.1 133.91 133.47 102.7 74.4 132.89 132.69 87.7 132.42 108.0 131.55 86.6 131.30 117.5 130.23 112.0 129.09 89.2 128.32 55.2 62.2 127.98 127.23 48.6 126.91 42.7 126.21 31.9 125.97 30.5 125.25 21.7 124.87 21.9
80507. 80730. 81130. 81650. 81998. 82216.
124.21 123.87 123.26 122.47 121.95 121.63
WN
15.81 13.00 11.23 9.24 6.53 6.51
70000
75000
~195 a295 0.940 0.934 0.948 0.954 0.949
0.964 0.952 0.941 0.976 0.987 0.976 0.991 0.994 0.977 1.009 0.984 0.994 0.984 0.975 1.000 0.991 0.919 0.991 0.982 0.936 0.977 0.875 0.951 0.958 0.953 0.983 0.971 0.997 0.918 0.975 0.964 0.964 0.919 0.919 0.908 0.920 0.890 0.863 0.910 0.926 0.857 0.874
BOO00
85000
Wavenumber, cm-’ Fig. 3. The ratio of absorption cross sections of CO, at 195 and 295 K. The peak cross sections of the bands around 75,000 cm-’ are the same at 195 and 295 K.
CO, cross sections
in the 118.7-175.5
ROOOO -__.
7snnn --__
59
nm
0
Wavenumber Fig. 4. The absorption
cross sections of CO, at 195 K and the ratio of cross sections The ratio emphasizes band structure.
at 195 and 295 K
Our cross sections above 160 nm region show some structures which suggest that the measurements here may not be accurately representing the cross sections. High resolution cross section measurements of CO2 will be obtained with 6.65 m vacuum spectrometers at the Photon Factory and at the CfA, depending on wavelength region to be studied. The cross sections reported here are available on the World Wide Web at wavelength interval of 0.005 nm. The URL is http://cfa-www.harvard.edu/amp/data/amdata.html.
1.5,
7
80000
Wavenumber Fig. 5. The absorption cross sections of CO, at 195 K between 72,000 and 80,000 cm-‘. The solid curve represents our results and the open circles are from Lewis and Carver.* There is good agreement in cross sections, but there are some shifts in wavenumbers.
K. Yoshino
60
et al
b
Wavenumber Fig. 6. The absorption cross sections of CO, at 295 K compared with those of Chan et al.” The smooth curve is from Chan et al. Their values fit well at the peak of bands at 130nm, but show higher values for the bands at 145 nm.
Acknowledgements-We thank C. E. Brion for providing their cross section results in digital form. for providing their high resolution spectrogram of CO,. The work was supported by the NASA Research Program under Grant No. NAGS-484 to the Smithsonian Astrophysical Observatory and Atmospheric Science, under grant No. ATM-9320175 to Harvard University. The measurements approval of the Photon Factory Advisory Committee (proposal 91-149). K.Y., J.R.E. and W.H.P. Photon Factory for their hospitality.
We thank F. Launay Upper Atmospheric by NSF, Division of were made with the thank the staff of the
REFERENCES 1. H. Trinks and K. H. Fricke, J. geophys. Res. A83, 3883 (1978). 2. G. Brasseur and S. Solomon, Aeronomy of the Middle Atmosphere, D. Reidel, Boston (1984). 3. M. Nicolet, Etude des RPactions Chimiques de I’Ozone dans la StratosphPre, p. 161, Institut Meteologique de Belgique, Bruxelles (1978). 4. J. F. Kasting, S. C. Liu, and T. M. Donahue, J. geophys. Res. 84, 3097 (1979). 5. M. Ogawa, J. them. Phys. 54, 2550 (1971). 6. D. E. Shemansky, J. them. Phys. 56, 1582 (1972). 7. W. B. DeMore and M. Patapoff, J. geophys. Res. 77, 6291 (1972). 8. B. R. Lewis and J. H. Carver, JQSRT 30, 297 (1983). 9. C. Cossart-Magos, F. Launay, and J. E. Parkin, Molec. Phys. 75, 835 (1992). 10. K. Yoshino, D. E. Freeman, and W. H. Parkinson, Appl. Opt. 19, 66 (1980). 11. S. G. Tilford and J. D. Simmons, J. Phys. Chem. Ref Data 1, 147 (1972). 12. W. F. Chan, G. Cooper, and C. E. Brion, Chem. Phys. 178, 401 (1993). 13. W. F. Chan, G. Cooper, and C. E. Brion, Phys. Rev. A 44, 186 (1991). 14. P. J. Knowles, P. Rosmus, and H.-J. Werner, Chem. Phys. Lett. 146, 230 (1988). 15. W. M. Preston, Phys. Ret?. 57, 887 (1940). 16. E. C. Y. Inn, K. Watanabe, and M. Zalikoff, J. them. Phys. 21, 1648 (1953). 17. P. S. Nakata, K. Watanabe, and F. M. Matsunaga, Sci. Light 14, 54 (1965).
Royal
Spectrosc.
0022-4073(!45)00135-2
Vol. 55, No. I. pp. 5340, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-4073/96 $I 5.00 + 0.00
Radar.
Transfer
ABSORPTION CROSS SECTION MEASUREMENTS OF CARBON DIOXIDE IN THE WAVELENGTH REGION 118.7-l 75.5 nm AND THE TEMPERATURE DEPENDENCE K. YOSHINO,”
J. R. ESMOND,” Y. SUN,” W. H. PARKINSON,’ K. ITO,b and T. MATSUIh
“Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A. and hPhoton Factory, National Laboratory of High Energy Physics, Tsukuba, Ibaraki 305, Japan (Received 17 January 1995)
Abstract-Laboratory measurements of the relative absorption cross sections of CO* at the temperatures 195 and 295 K have been made throughout the wavelength region 118.7-175.5 nm. Laboratory measurements of the absolute absorption cross sections of CO2 at the temperature 195 and 295 K have been made at 12 different wavelengths in the region
118.7-175.5 nm. These measurements of CO* 118.7-175.5 nm. Almost of the strong bands at regions.
absolute values have been used to put the relative cross section at 195 and 295 K on an absolute basis throughout the region no temperature dependence was observed in the peak cross sections 130 nm region, but large temperature effects are seen in the other
1. INTRODUCTION
In the Earth’s atmosphere, CO1 is present in the troposphere, stratosphere, and at higher altitties. In the stratosphere the CO* mixing ratio is - 350 ppmv where solar radiation causes its photolysis (CO, + hv -+ CO + 0) to CO. Typically, CO* is present in concentrations of lOI cmp3 at - 18 ikm, lOi cm-3 at - 50 km, and 10” cme3 at -80 km. In the lower thermosphere, from - 100 to 140ikm, the CO, concentration varies from 10” to lo6 cm- 3.’ Photolysis of CO, occurs mainly at relati;vely high altitudes where solar Lyman-a and continuum radiation at the Schumann-Runge (Q-R) wavelengths are present. The photolysis is due primarily to Lyman-M in the upper mesosphere,,and to the continuum radiation in the mesosphere, and stratosphere. Because of the strong temperdture dependence of the CO, absorption cross section in the region of the S-R bands, its photolysis~rate decreases by about one order-of-magnitude for a temperature change from 300 to 273 K.* Ni+let3 has calculated photodissociation coefficients of CO2 at the top of the atmosphere at the tiean solar-terrestrial distance for temperatures at 273, 250, and 220 K in the spectral region of the ‘S-R bands (2,0)-(19,0). Carbon dioxide is a major constituent of the atmospheres of Mars and Venus, and it occurs,also in the atmosphere of Titan. In the primitive prebiotic atmosphere of the Earth, the photoly&s of CO, and H,O constituted the only sources of oxygen; from a model calculation which incl/udes steady-state diffusion and photochemistry, Kasting et al4 have calculated that the predomi/nant source of oxygen at all altitudes was the photodissociation of CO*. In absorption, CO, is transparent down to at least 210 nm. Photoabsorption cross section measurements of CO, irh the u.v to v.u.v region have been studied by several investigators. Because of lack of resolution, pane of the studies in the period 1950-70 resolved the structures in the CO* absorption cross sec$ion. OgawaS measured absorption coefficients of CO, in the region 175-216 nm with a 3 m vaduum spectrometer (instrumental width 0.008 nm), and observed numerous absorption bands. Shepansky6 measured cross sections in the region 170-300 nm with a 2 m vacuum spectrometer but @ade most of the measurements with an effective resolution of only 0.4 nm. DeMore and Patbpof’ measured CO2 cross sections with an instrumental width of 0.06 nm in the range 185-220 nni and 53
K. Yoshino et al
54
examined the temperature dependence in the range 220-320 K. The most recent cross section measurements of CO* are those of Lewis and Carver.8 These were obtained at temperatures of 200, 300, and 370 K, and were measured throughout the region 120-197 nm at wavelength intervals of 0.05 nm with an instrumental width of 0.005 nm. Cossart-Magos et al9 photographed an absorption spectrum of CO, in the wavelength region 175-200 nm with the 10 m vacuum spectrograph at Meudon Observatory. They observed many discrete absorption bands of CO, and gave tentative analyses of nine bands. They kindly supplied, for our investigation, the entire CO, spectrum taken at a resolution of 0.0008 nm with a slit width of 30 pm. These spectra are sufficient to allow us to judge the resolution necessary for scanning the spectrum with a spectrometer. With the fine structured CO, bands in the 175-200 nm range, the cross sections obtained previously with low resolution are severely distorted by the instrumental widths. We photographed an absorption spectrum of CO, at 295 K below 170 nm, with the fourth order of a 6.65 m spectrograph at the Photon Factory, Japan. We confirmed from this spectrum that the structures in the CO, absorption spectrum are broad enough to allow the use of medium resolution instruments (a 3 m spectrometer). We report here the results of absolute cross section measurements of CO, at the temperatures 195 and 295 K in the wavelength region 118.7-175.5 nm. 2.
EXPERIMENTAL
PROCEDURE
The 3 m, normal incidence, vacuum spectrometer on the BL-20A beam line at the Photon Factory, KEK, Japan, was used in the first order of a 1200 I/mm grating to obtain absorption cross section measurements of COZ. The 3 m spectrometer is equipped with a gold-coated, holographic grating (Shimadzu, Japan), blazed at 40 nm, and it provided an estimated instrumental width of 0.0070 nm at 85 nm with entrance and exit slit widths of 0.02 mm. The absorption cell, which is directly connected to the exit slit assembly of the spectrometer, has two MgF, windows and provided a path length of 12.08 cm. The entire CO, column can be cooled to 195 K by immersing the cell in a slush of dry ice and alcohol. A SPACOM integrated detector system,” using a EMR 541 F photomultiplier, is mounted at the other end of the absorption cell without additional
5ooo
P B
3000
1000 1
I 120
I
I
I
I
I
I 150
I
I
I
I
I 130
Wavelength, nm Fig. I. The continuum background counts per second in the wavelength region 115-180 nm. The intensity decrease below 160 nm is due to the combination of spectral sensitivity of the detector, transmittance of MgF, windows, and reflectivity of the grating.
CO2 cross sections in the 118.7-175.5 nm
55
optical elements. The background continuum is provided by the synchrotron radiation source without any order separation, because any higher order radiation is cut off by the MgFz windows. 2.1. The scanned cross sections The shape of the observed intensity of the background source in the wavelength region 118.7-l 75.5 nm, results from several factors including the intensity of the synchrotron source, the reflectivity of the grating, the transmittance of windows, and the spectral sensitivity of the detector. Figure 1 shows the typical spectrum (counts per second) of the background intensity in the wavelength region 130-175 nm. We divided the spectral region into seven sections of 10 nm extent. We obtained data for every 0.005 nm step with three to five different pressures to get optical dep/ths between 0.3 and 2.5. The background continuum level was established by scanning the empty cell before and after each photoabsorption measurement, assuming linear interpolation of the background intensity levels. Any small shifts in the background continuum were corrected for with the aid of the absolute cross sections measurements, mentioned in the following section. 2.2. The absolute cross sections Most of the uncertainty in cross section measurements with a single beam spectrometer is in ithe estimation of the background intensity levels since they may vary with time and wavelength during the photoabsorption scans; this is specially true for continuum cross section measurements. ~To overcome this problem, we measured the absolute cross sections of CO, at twelve wavelenjgth positions in the range 118.7-175.5 nm with two wavelength positions in each scanning range.; At each wavelength, we measured output counts without CO* in the absorption cell, and with CO, in it, alternatively every two minutes. The background intensity was estimated from the averages of two measurements without CO*. We used six different pressures of CO>. 2.3. The wavelength calibration
The 3 m vacuum spectrometer was scanned under computer control but the wavelength given by the computer needed to be corrected. We measured wavelengths of the absorption spectrum of the Fourth Positive bands of CO at 78 K, and compared these with the known wavelengths of the band head positions from Tilford and Simmons. ” We found that corrections required to computer wavelengths ranged from 1.38 nm at 131 nm through 2.47 nm at 175 nm with an estimated uncertainty of 0.02 nm. 3. RESULTS
AND DISCUSSION
The absolute cross sections of CO* were measured at two temperatures, 195 and 295 K, at 12 wavelengths in the 12s175.5 nm region. The averaged cross sections, measured at six diffebent pressures, are listed in Table 1 along with the ratio of cross sections at 195 and 295 K.’ All wavelengths were chosen to be near minima in the cross sections. The uncertainty of 2% ins the measured cross sections arises from the statistical scatter of 0.3-0.5% observed in the optical depths, an uncertainty of 0.2% in the optical path lengths, uncertainties of O.lHl.4% in the temperatures, and an uncertainty of 0.5% in the pressure measurements. The uncertainty in the wavelength scale is 0.02 nm as mentioned in Sec. 2.3. The cross sections of CO* were obtained in the scan mode with 3-5 different column densities also at 195 and 295 K. The averaged cross sections were calibrated onto the absolute scale by using the absolute absorption cross sections. The final cross sections are presented in Fig. 2. Peak cross sections at 195 and 295 K are tabulated in Table 2 along with ratios of cross sections at 195~and 295 K. Almost no temperature dependence was observed in the peak cross sections in the wavenumber region 70,000-80,000 cm-‘, however, the ratio of the cross sections is down to 0.7 ar und 60,000 cm-‘. The ratios of the cross sections, a,,,/a 295,are plotted against wavenumbers in F’g. ! 3. Below 70,000 cm-‘, cross sections at 195 K decrease with increasing wavenumber. The peak cross sections of the strong structures around 75,000 cm-’ are temperature independent, but the cross sections at minima are down to 80% of those at 295 K. Plots of the cross sections at 195 K! and
K. Yoshino Table
1. Absolute
Wavelength mn 121.36 125.58 127.63 132.05 135.56 140.60 143.87 150.02 153.72 160.79 168.98 171.91 n Cross
absorption
Wavenumber cm-’ 82399. 79630. 78351. 75729. 73768. 71124. 69507. 66653. 65053. 62193. 59179. 58170.
sections
et al
cross sections
of carbon
Cross Section’ 295 K 195 K 4.37f0.06 5.08zkO.09 15.98f0.22 18.29f0.24 22.9f0.2 27.750.4 43.1hO.5 53.7fl.O 48.5f0.3 53.6f0.5 46.0f0.4 47.8f1.2 49.3k0.6 46.6f0.5 41.2f0.4 36.5f0.6 29.7f0.3 33.3zko.4 11.91f0.19 9.27f0.03 1.84f0.05 2.68fO.04 1.209rtO.084 0.659f0.007
are given in units of IO-”
dioxide. ~I%/%95 0.86 0.87 0.83 0.80 0.90 0.96 0.95 0.89 0.89 0.78 0.69 0.55
cm*.
ratios of cross sections, 0,9S/a295, are compared in Fig. 4 in the wavenumber range 74,000-82,000 cm-‘. The plots of ratios emphasize band structures more clearly than those of cross section plots. Our measured cross sections at 295 K are compared with those of Lewis and Carver* in the wavenumber region 72,00&80,000 cm-’ in Fig. 5. Very good agreement between cross sections values is seen but there are some shifts in the wavenumbers which can be noticed over the entire wavelength region. The shifts are not constant nor in a single direction. Our wavelength calibration is based on the Fourth Positive bands of CO; the Lewis and Carver measurements* were made with more or less the same resolution, but the interval of measurement, 0.05 nm, could not represent fine structures. -17 i -18
1
195 K
--._--. 60,ooO
80,000
Wavenumber Fig. 2. The absorption
cross sections
of CO, at 195 and 295 K.
57
COz cross sections in the 118.7-175.5 nm
Chan et alI2 observed the CO2 electronic spectrum by using their dipole (e,e) method of electron-impact excitation” which is free of optical instrument function and saturation errors. Their measurements with a high resolution dipole (e,e) spectrometer below 10eV (about 124nm) consisted of two broad peaks which were assigned to transitions of final states, ‘A, and ‘II,. Our results are compared with the results of Chan et al” in Fig. 6 where the smooth curve is their fit to their data. Their values fit well at the peak of bands at 130 nm, but show higher values for the bands at 145 and 120 nm. The higher values at 120 nm region could be wing effects of the strong bands at 11 eV (not shown in our work). The higher values of the bands at 145 nm are also wing effects from the stronger bands at 130 nm. The absorption spectra of CO, consist of two distinctive bands: the stronger bands below 1401nm and the weaker bands above 140 nm (see Fig. 6). Based on theoretical calculations, Knowles et alI4 assigned the stronger bands to the ‘l$ c ‘Xi transition, and weaker bands to the ‘C: , ‘A,,t ‘Ci transitions. The spacing of the absorption peaks is irregular and the structure can not be described by just the symmetric stretch motion of C02. The cross section at the Lyman-a line of hydrogen is important because Ly-a is a dominant source of solar radiation in this wavelength region. For CO*, many measurements have been done at this wavelength: 7.5 x -20 cm2 by Preston,” 7.3 x -20 cm2 by Inn et aLI6 8.2 x --20cm2 by Nakata et al 2” 7.6 x p20cm* by Lewis et al.* We obtained a cross section at the Ly-cr line (82,259 cm’) of 6.54 x -20cm* at 295 K and 6.11 x -‘“cm2 at 195 K. They are lower than previous measurements; however, cross sections vary rapidly in this wavelength region, changing from 5.29 to 7.37 x -2acm2 in a 0.2 nm range.
Table 2. Cross sections of carbon dioxide at peak absorption. WN 59086. 59262. 59602. 59907. 60167. 60253. 60348. 60537. 60738. 60841. 61029. 61371. 61500. 61660. 62102. 62284. 62465. 62626. 62809. 63037. 63108. 63229. 63441. 63604. 63668. 63802. 63881. 63970.
295 K Cross Section WL 169.25 3.44 168.74 3.35 4.90 167.78 4.81 166.92 166.20 6.27 6.58 165.97 165.71 6.34 6.41 165.19 8.37 164.64 164.36 9.04 8.68 163.86 162.94 12.15 162.60 11.85 162.18 11.79 161.03 15.72 160.56 16.54 160.09 19.13 159.68 20.7 159.21 16.51 158.64 22.8 158.46 24.8 158.16 22.7 157.63 22.8 157.22 27.9 157.06 30.7 156.73 26.8 156.54 27.0 156.32 27.3
64179. 64348. 64502. 64659. 64659. 64963. 65135. 65309. 65390. 65441. 65561. 65683. 65837. 65934. 66179. 66478. 66725. 66881. 67022.
155.81 155.41 155.03 154.66 154.18 153.93 153.53 153.12 152.93 152.81 152.53 152.25 151.89 151.67 151.11 150.43 149.87 149.52 149.20
32:7 35.9 30.3 36.9 40.0 38.0 35.9 42.8 43.3 42.3 41.3 40.2 48.8 49.5 42.6 56.2 46.0 55.5 56.6
WN 59086. 59248. 59596. 59914.
195 K WL Cross Section 169.25 2.60 168.78 2.60 3.68 167.80 3.55 166.91
0195/~m
60245.
165.99
5.00
0.760
60537. 60735. 60853. 61027. 61368. 61503. 61655. 62101. 62283. 62468. 62625. 62816. 62909. 63111. 63239. 63431. 63599. 63670. 63791. 63887. 63978. 64067. 64175. 64357. 64520. 64664. 64860.
165.19 164.65 164.33 163.86 162.95 162.60 162.19 161.03 160.56 160.08 159.68 159.19 158.96 158.45 158.13 157.65 157.24 157.06 156.76 156.53 156.30 156.09 155.82 155.38 154.99 154.65 154.18
5.05 6.60 6.69 6.79 9.82 9.48 10.20 13.03 13.78 15.96 18.41 13.41 16.60 22.4 19.69 19.02 24.8 27.5 23.5 23.6 23.2 24.2 28.7 32.5 26.4 32.2 36.3
0.788 0.789 0.740 0.782
65116. 65313. 65411. 65463. 65574. 65669. 65841. 65953. 66174. 66476.
153.57 31.6 153.11 38.7 152.88 39.7 152.76 38.6 152.50 37.7 152.28 35.7 151.88 44.9 151.62 45.3 151.12 38.8 150.43 52.4
0.880 0.904 0.917 0.912 0.913 0.888 0.920 0.915 0.911 0.932
66904. 67017.
149.47 51.9 149.22 52.3
0.935 0.924
0.755 0.776 0.751 0.738
0.808 0.800
0.865 0.829 0.833 0.839 0.889 0.812 0.728 0.903 0.867 0.834 0.889 0.896 0.877 0.874 0.850 0.878 0.905 0.871 0.873 0.908
continued otierleaf
K. Yoshino et al
58
Table 2-continued. 295 K
55000
WN
wl.
67230. 67379. 67527. 67720. 67812.
148.74 148.41 148.09 147.67 147.47
51.5 53.0 59.5 54.2 52.7
68132. 68623. 68941. 69128.
146.77 145.72 145.05 144.66
64:3 64.6 55.5 62.5
69678. 69805. 70216. 70537. 70630. 70838. 71292. 71766. 71914. 72198. 72475. 73032. 73307. 73562. 74205. 74681. 74917. 75265. 75358. 75517. 75990. 76157. 76793. 77460. 77933. 78121. 78601. 78783. 79240. 79385. 79829. 80069. 80444. 80500. 80723. 81118. 81643. 81956. 82186.
143.52 143.26 142.42 141.77 141.58 141.17 140.27 139.34 139.06 138.51 137.98 136.93 136.41 135.94 134.76 133.90 133.48 132.86 132.70 132.42 131.60 131.31 130.22 129.10 128.32 128.01 127.22 126.93 126.20 125.97 125.27 124.89 124.31 124.22 123.88 123.28 122.48 122.02 121.68
60:4 58.1 57.8 52.8 52.6 55.7 55.6 52.8 50.1 52.1 68.5 67.0 56.7 96.0 124.5 86.6 105.1 85.0 92.2 112.7 90.9 119.5 115.3 89.5 60.1 63.8 50.4 44.3 34.7 33.2 23.9 23.8 18.07 17.76 15.07 12.34 9.98 7.62 7.45
60000
Cross
Section
65000
67244. 67365. 67522. 67722. 67810. 68027. 68128. 68621. 68934. 69152. 69302. 69677. 69820. 70234. 70539. 70632. 70836. 71284. 71757. 71914. 72192. 72480. 73042. 73284. 73567. 74204. 74677. 74924. 75253. 75363. 75520. 76015. 76162. 76789. 77468. 77931. 78138. 78599. 78796. 79232. 79382. 79838. 80081.
195 K WL Cross Section 148.71 48.4 148.45 49.5 148.10 56.4 147.66 51.7 147.47 50.0 147.20 53.6 146.78 62.0 145.73 61.5 145.07 52.2 144.61 61.0 144.30 56.5 143.52 59.6 56.7 143.23 142.38 57.3 141.77 52.5 141.58 51.4 56.2 141.17 54.7 140.28 139.36 52.5 139.05 49.3 138.52 50.8 137.97 68.5 66.4 136.91 136.45 52.1 135.93 95.1 134.76 122.3 81.1 133.91 133.47 102.7 74.4 132.89 132.69 87.7 132.42 108.0 131.55 86.6 131.30 117.5 130.23 112.0 129.09 89.2 128.32 55.2 62.2 127.98 127.23 48.6 126.91 42.7 126.21 31.9 125.97 30.5 125.25 21.7 124.87 21.9
80507. 80730. 81130. 81650. 81998. 82216.
124.21 123.87 123.26 122.47 121.95 121.63
WN
15.81 13.00 11.23 9.24 6.53 6.51
70000
75000
~195 a295 0.940 0.934 0.948 0.954 0.949
0.964 0.952 0.941 0.976 0.987 0.976 0.991 0.994 0.977 1.009 0.984 0.994 0.984 0.975 1.000 0.991 0.919 0.991 0.982 0.936 0.977 0.875 0.951 0.958 0.953 0.983 0.971 0.997 0.918 0.975 0.964 0.964 0.919 0.919 0.908 0.920 0.890 0.863 0.910 0.926 0.857 0.874
BOO00
85000
Wavenumber, cm-’ Fig. 3. The ratio of absorption cross sections of CO, at 195 and 295 K. The peak cross sections of the bands around 75,000 cm-’ are the same at 195 and 295 K.
CO, cross sections
in the 118.7-175.5
ROOOO -__.
7snnn --__
59
nm
0
Wavenumber Fig. 4. The absorption
cross sections of CO, at 195 K and the ratio of cross sections The ratio emphasizes band structure.
at 195 and 295 K
Our cross sections above 160 nm region show some structures which suggest that the measurements here may not be accurately representing the cross sections. High resolution cross section measurements of CO2 will be obtained with 6.65 m vacuum spectrometers at the Photon Factory and at the CfA, depending on wavelength region to be studied. The cross sections reported here are available on the World Wide Web at wavelength interval of 0.005 nm. The URL is http://cfa-www.harvard.edu/amp/data/amdata.html.
1.5,
7
80000
Wavenumber Fig. 5. The absorption cross sections of CO, at 195 K between 72,000 and 80,000 cm-‘. The solid curve represents our results and the open circles are from Lewis and Carver.* There is good agreement in cross sections, but there are some shifts in wavenumbers.
K. Yoshino
60
et al
b
Wavenumber Fig. 6. The absorption cross sections of CO, at 295 K compared with those of Chan et al.” The smooth curve is from Chan et al. Their values fit well at the peak of bands at 130nm, but show higher values for the bands at 145 nm.
Acknowledgements-We thank C. E. Brion for providing their cross section results in digital form. for providing their high resolution spectrogram of CO,. The work was supported by the NASA Research Program under Grant No. NAGS-484 to the Smithsonian Astrophysical Observatory and Atmospheric Science, under grant No. ATM-9320175 to Harvard University. The measurements approval of the Photon Factory Advisory Committee (proposal 91-149). K.Y., J.R.E. and W.H.P. Photon Factory for their hospitality.
We thank F. Launay Upper Atmospheric by NSF, Division of were made with the thank the staff of the
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Royal