Improved absorption cross-sections of oxygen in the wavelength region 205–240 nm of the Herzberg continuum

Planer. Space Sci., Vol. 36, No. 12, pp. 1469-1475, Printed in Great Britain.

1988 0

0032-0633/88 S3.00+0.00 1988 Pergamon Press plc

IMPROVED ABSORPTION CROSS-SECTIONS OF OXYGEN IN THE WAVELENGTH REGION 205-240 nm OF THE HERZBERG CONTINUUM K. YOSHINO, A. S.-C. CHEUNG,*

J. R. FSMOND, W. H. PARKINSON, D. E. FREEMAN and S.

L. GUBERMANt Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A.

A. JENOUVRIER, B. COQUART and M. F. MERIENNE U.A. 776, Spectrometrie Moleculaire et Atmosphkique, UER Sciences, Moulin de la Housse, B.P. 347,51062 Reims Cedex, France (Received 3 August 1988) Abstract-The laboratory values of the Herzberg continuum absorption cross-section of oxygen at room temperature from Cheung et al. (1986, Planet. Space Sci. 34,1007), Jenouvrier ef al. (1986a, Planer. Space Sci. 34,253) and Jenouvrier et al. (1986, J. quant. Spectrosc. radial. Transfer 36,349) have been compared and re-analyzed. There is no discrepancy between the absolute values of these two sets of independent measurements. These values have been combined together in a linear least-squares fit to obtain improved values of the Herzberg continuum cross-section of oxygen at room temperature throughout the wavelength region 205-240 nm. Agreement with in situ and other laboratory measurements is discussed.

1. INTRODUCTION

In the wavelength region 205-240 nm of the Herzberg continuum, the absorption of solar radiation by atmospheric 0, causes dissociation into OCP) atoms that subsequently produce 03. In addition, in this region the absorption by 0, and 0, controls the depth of penetration of solar radiation into the atmosphere, where it is available to dissociate important trace species. References to the need for accurate laboratory determination of the Herzberg continuum absorption cross-sections of Oz. cr&), are given in Section 1 of a previous paper (Cheung et al., 1986). The two most recent laboratory determinations of the Her&erg absorption cross-sections by Cheung et al. (1986) (herein called CfA) and Jenouvrier et al. (1986a,b) (herein called Reims) were performed independently and published almost simultaneously, so that neither takes account of the other. In the Reims experiment, optical path lengths up to 1400 m and pressures of oxygen in the range Z-100 torr were used, whereas in the CfA work, path lengths up to 133 m

* Present address : Chemistry Department, University of Hong Kong, Hong Kong. t Presentaddress: Institute for Scientific Research, 33 Bedford Street, Lexington, MA 02173, U.S.A.

and pressures in the range 30-760 torr were used. Comparison of the values of a,(1) of each study reveals that the ratios of the Reims to the CfA values in the region 205-235 nm, range from 1.039 at 206 nm to 1.149 at 222 nm. The mean value of the 3 1 ratios is 1.107 kO.028, i.e. the Reims values are, on the average, w 11% higher than CfA values. Similarly, comparison of the coefficients c@), which specify the pressure dependence, reveals that the ratios of the Reims to the CfA values range from 0.84 at 217 mn to 1.27 at 231 nm. The mean value of the 31 ratios is 1.06& 0.12. These differences are very small in comparison with those from previous measurements which are demonstrated in Fig. 2 of Reims (Jenouvrier et al., 1986b) and Fig. 6 of CfA (Cheung et al., 1986). However, it is necessary to know the cause of discrepancy between the two measurements, since the transmittance of the solar radiation in this wavelength region is so important for atmospheric photochemical modeling calculations. In this paper, we report the results of combining the Reims and CfA cross-section data to obtain improved values of the Herzberg continuum of O2 at room temperature throughout the wavelength region 205-240 nm and the pressure dependence of the continuous absorption. We find no systematic differences between the values of Reims and CfA.

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20

5

a

a

I

L 00

200

,I00

400

600

6

Pressure(torr)

FIG. 1. CONTINUUMCROSS-SECTION, up(l), OF 0, AT 220 nm AS A FUNCTIONOF 0, PRESSURE. (a) Including all data points from Reims and CfA. (b) Excluding two groups of CfA data points, viz. the diamonds and stars in (a).

2. PROCEDURE AND ANALYSIS

The measured continuous absorption cross-section of oxygen ~~(1) in the region 205-240 nm is observed to be linearly dependent on the pressure P : up(a) = o,(l) + a(l)P.

(1)

In equation (l), o,(l) is obtained by linear extrapolation of the cross-section to zero pressure, and is the sum of the Herzberg continuum cross-section of OZ, a,(n), and the Rayleigh scattering cross-section (Bates, 1984). The term a(A)P is a cross-section involving two molecules of OZ. In order to search for systematic differences between the two sets of experimental data of Reims and CfA, we used the basic “raw” data from each experiment. Figure 1(a) shows preliminary values of up@) results where the Reims data are presented by dots and the CfA data by the other symbols. Within the experimental uncertainties no systematic difference was noted. However, the line obtained by a linear least-squares fit to all the data

OO

200

00°

200

400

600

400

600

Pressure(torr)

800

FIG. 2. CONTINUUMCROSS-SECTION, a&), OF O2 AT 240 nm AS A FUNCTIONOF 0, PRESSURE. (a) Including all data points from Reims and CfA. (b) Excluding CfA data points with optical depths less than 0.3.

goes through the bottom of the Reims data. We found that two groups of CfA data points (diamonds and stars) show relatively smaller values because of a small error in the estimation of the background intensities. After removing the two sets of CfA data, agreement is excellent as shown in Fig. 1(b). The small discrepancy observed between a,,(J) values of Reims and CfA are known to result from a small error of extrapolation to zero pressure in the CfA work, partly caused by the effects of two groups of data points mentioned above. When all the observed data points from Reims and CfA (except for the two CfA groups of data discussed above) are combined together, for every 1 nm interval, in most cases the results are excellent. But some of the CfA data points, for low pressure at longer wavelengths have larger scattering, as shown in Fig. 2(a) for 240 nm. Due to the shorter optical path lengths (133 m at CfA compared with 1400 m at Reims) some of the CfA data had to be obtained at very small optical depths in order to cover the wide range of pressure. Since the Reims data combines low pressure

Cross-section of the Henberg continuum of O2

1471

TABLE1. LABORATORY

VALUESOF THE CROSS-SECTIONOF &,U&),AND ITSPRF%WR@DEPENDENCE,(1(~), IN THEWAVELENGTHRRGION 205-240nrno~~m HERZBERGCONTINUUM

Pressure dependence, a(l)?

Cross-sections, u&)* Wavelength (nm)

Combined

Reimsj

CfA

Combined

Reimsl

CfA

205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240

7.71 7.48 7.39 7.19 7.00 6.82 6.54 6.35 6.18 5.99 5.86 5.62 5.39 5.13 4.89 4.70 4.50 4.32 4.11 3.89 3.66 3.42 3.18 2.97 2.82 2.62 2.44 2.28 2.12 1.95 1.80 1.65 1.51 1.38 1.26 1.16

7.61 7.36 7.32 7.09 6.92 6.71 6.50 6.36 6.21 6.04 5.90 5.67 5.47 5.16 4.89 4.72 4.50 4.30 4.09 3.89 3.62 3.35 3.11 2.99 2.73 2.54 2.34 2.19 2.04 1.89 1.75 1.60 1.44 1.33 1.21 1.11

7.21 7.08 6.93 6.65 6.44 6.20 5.98 5.82 5.69 5.52 5.31 5.04 4.78 4.60 4.36 4.18 3.96 3.74 3.59 3.45 3.21 3.01 2.82 2.62 2.44 2.26 2.13 1.99 1.85 1.67 1.57 1.50 1.34 1.21 1.19 1.11

13.7 13.4 12.9 12.5 12.1 11.7 11.5 11.1 10.7 10.30 9.76 9.47 9.11 8.89 8.53 8.10 7.70 7.29 6.96 6.61 6.29 6.01 5.82 5.51 5.16 4.89 4.63 4.38 4.13 3.91 3.68 3.50 3.30 3.07 2.90 2.69

16.6 16.5 15.1 15.2 14.7 14.9 13.4 11.9 11.1 10.08 9.50 9.21 8.54 8.70 9.35 8.45 8.38 8.16 7.63 7.04 7.17 7.56 7.30 7.19 6.80 6.50 6.48 5.91 5.34 4.88 4.21 4.05 4.19 3.80 3.52 3.28

14.5 14.0 13.6 13.3 13.0 12.7 12.4 12.0 11.5 11.0 10.6 10.4 10.1 9.7 9.4 8.9 8.6 8.2 7.8 7.3 7.0 6.7 6.3 6.0 5.7 5.4 5.1 4.8 4.5 4.3 4.4 3.6 3.5 3.3 2.9 2.7

*In units of 1O-24 cn-‘.

The uncertainties in the combined CfA/Reims

values of u&)

are

in the

range (0.014.03) x 10mz4cm*. t In units of 10mz7cm’ torr-‘. The uncertainties in the combined CfA/Reims values of a(I) are in the range (0.040.10) x 10mz7cm* torr-‘. $ The present values differ slightly from those published by Jenouvrier et al. (1986b). The small differences result from new measurements made after that publication.

with longer path lengths we need not use the very low optical depth CfA data where larger errors could be expected. Therefore, we eliminated all data points of optical depth, In Z,&I)/Z(I), less than 0.3 from the CfA data. The results for 240 nm are shown in Fig. 2(b). 3. RESULTS AND DISCUSSION The final values for the cross-sections o,,(n) and pressure dependence coefficients cr(l) are listed in Table 1 in columns two and five, respectively, where only data with optical depth larger than 0.3 were used.

These results are also shown in Figs 3 and 4 for crosssections and pressure dependence, respectively. As expected, the values of combined cross-section, a,#.), are very close to those of Reims since the Reims data were obtained at lower pressures. Similarly, the values of the pressure dependent part, a(A), are close to those from the CfA because those data were obtained over a wider range of pressure. At shorter wavelengths where the cross-sections are larger, the Reims data have a tendency to yield a larger slope, as shown in Fig. 5 for 205 mn. This can be seen between 205 and 211 nm to yield larger values of u(n)

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et

al.

200 240

210 Wavelength.

nm

FIG.

CONnNUUMCROSS-SECTION, OF 9 AGAINST WAVELENGTHTHEREGION 205-240 nm. Cross-section of CfA are given by stars, Reims by open circles, and combined results by the solid curve.

400

Pressure(torr)

600

tIO 0

FIG. 5.CONTINUUM CROSS-SECTION, up(A),OF O2 AT 205 nm

ASAFUNCTION OF0, PRESSURE. Small dots represent values from Reims and their slope is slightly higher than that of the solid line obtained from the combined data.

laboratory values of a,(n) and those of Johnston et al. (1984) are lower than all other previous laboratory values throughout the region 205-240 nm of our studies. In the in situ stratospheric determination of a&), Frederick and Mental1 (1982) presented their results for the region 200-210 nm in the form of two sets of reduction factors by which the high laboratory crosssection values of Hasson and Nicholls (1971) or the lower values of Shardanand and Prasad Rao (1977) should be multiplied to produce consistency with the in situ measurements. In Table 2 we have translated these reduction factors to Herzberg continuum crosssections and they are given in the sixth and seventh column of Table 2. The stratospheric values found by Herman and Mental1 (1982) for the region 200-220 nm are consistent with our laboratory values and are I I presented in column eight of Table 2. Anderson and Hall (1983) have calculated, from stratospheric measurements, an average value of 7.0 x 10ez4 cm’ for the region 203-207 nm which agrees well with ours; the more recent stratospheric results of Anderson and Hall (1986) for an(ls) are essentially identical with those of Herman and Mental1 (1982). The only stratospheric values in serious conflict with our laboratory values are those of Pirre et al. (1984). Their crosssections for the region 202-220 nm are listed in the last column of Table 2. Saxon and Slanger (1986) report that the fractional I I contribution of the Herzberg I continuum to the total 210 240 Wavelength. nm continuum arising from the Herzberg I, II and III FIG.4. TIE COEFFICIENT, c@), SPECIFYING THECROSS-SECTION, transitions varies only a little, from 0.8-0.7, as the a(n)P, INVOLVING TWO MOLECULES OF OXYGEN, PLO'ITED wavelength increases from 195 to 240 nm. ConseAGAINST WAVELENGTHINTHE REGION 205-240 nm. Results of CfA are given by stars, Reims by open circles, and quently, the shape of the cross-section is expected to combined results by the solid curve. be not much different in this region from that of for the Reims data in Fig. 4. We have not yet explained this discrepancy and have ignored it for the final calculations herein. As discussed in the previous papers from Reims and CfA, the cross-sections o,,(n) are derived from measurements of the total optical depth due to absorption and Rayleigh scattering. To calculate the Rayleigh scattering cross-section of oxygen in the region 205-240 nm, we have used the formulae of Bates (1984), which are based on accurately measured refractivities and which take account of the wavelength dependence of the King correction factor (King, 1923). In Table 2, our combined values for the Herzberg continuum cross-section of oxygen an(n) are given along with some of the previous values. Our I

I

I

14.I3

Cross-section of the He&erg continuum of O2 TABLE2. HERZBERG CONTINUUM PI-IOTOALSORPTION CROss-sECnON,U,(D, (UNITS OF 10-u CItl*)OF o2 IN THEWAVELENGTHREGION

205-240tlm

Laboratory values Stratospheric values Wavelength (nm)

Combined

Reims

CfA

et al.

205 206 201 208 209 210 211 212 213 214 215 216 211 218 219 220 221 222 223 224 225 226 221 228 229 230 231 232 233 234 235 236 231 238 239 240

1.35 1.13 1.05 6.86 6.68 6.51 6.24 6.05 5.89 5.12 5.59 5.35 5.13 4.88 4.64 4.46 4.26 4.09 3.89 3.61 3.45 3.21 2.98 2.11 2.63 2.43 2.25 2.10 1.94 1.18 1.63 1.48 1.34 1.22 1.10 1.01

1.19 1.08 1.04 6.18 6.65 6.43 6.24 6.09 5.93 5.83 5.61 5.38 5.20 4.92 4.10 4.52 4.21 4.06 3.83 3.62 3.40 3.15 2.96 2.83 2.51 2.31 2.11 2.00 1.85 1.61

6.85 6.13 6.59 6.32 6.12 5.89 5.68 5.53 5.40 5.24 5.04 4.18 4.52 4.35 4.11 3.94 3.13

1.0

Johnston

1.51 1.43 1.29 1.18 1.06 0.91

6.6

4.1

4.5

FM

HM

1.3 1.8

6.5

1.1

7.4

7.0 7.8 7.6 1.4

7.2 7.3 7.1 6.9

6.3

6.3 6.3 6.5 6.7 6.7 6.4 6.2 6.0 5.8 5.5

PRH 9.8 8.8 8.5 8.0 8.0 1.1

5.1 5.1

6.1

4.9 4.4

4.8

3.51 3.31 3.23 3.00 2.80 2.62 2.42 2.25 2.01

2.8

1.95 1.81 1.61

1.50 1.40 1.33 1.18 1.05 1.03 0.96

FM : Frederick and Mentall (1982). The left and right columns above correspond, respectively, to the column headed cI.(O&’ and cJ.(OJ2 in Table 2 of their paper. HM : Herman and Mentall (1982). Their quoted errors are 0.8-0.6 x lo-% cm2 from 200-215 nm and 0.9 x lo-= cm* at 220 nm. PRH : Pirre et al. (1984).Their quoted errors increase from 0.9 x lOeN cm* at 202 nm to 4.1 x lo-% cm* at 220 nm.

the Herzberg I continuum alone. As discussed in a previous paper (Cheung et al., 1986), the transition moment, D(B), evaluated at the r-centroid is a function of 1, and we have also used the approximation D(B) = A+rZB to determine the best least-squares values of constants A and B to fit values of D(R). The new values found from the combined results are A = 1.3 x lop3 and B = -2.8 x 10e6. A short extrapolation of this effective transition moment to shorter

wavelength then permits the calculation of the crosssection in the region 19%205 mn. The calculated cross-section for the region 192-238 nm is shown as the solid curve in Fig. 6. The solid curve can also be represented empirically as a function of wavelength by the following polynomial : UH= [4676.04-96.66661+0.73917041* -O.OO2478O7513+O.3O7613x 1O-514] x 1O-24cm* (2)

K.

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WAVELENGTH

( NM )

YOSHINO

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240

200 24C FIG. 6. THE COMBINEDEXPERIMENTAL AND CALCULATED Wavdmgth, nm HERZBERGCONTINUUMCROSS-SECTION,CT&), OF Oz. FIG. 7. THEEMPWCAL,culsnn FORMREPRESENTATION FORTHE The asterisks in the region 205-239 nm represent the comHERZBERGCONTINUUM CROSS-SECTIONS IN THEWAVELENGTH bined a,(1) values measured by us. The solid curve represents REGION194240 nm. the calculated Her&erg I continuum designed to fit our The solid curve is from equation (3), the asterisks are expermeasured (total) a,(n) in the region 205-237 nm, and imental points and the open circles are theoretical values extrapolated to the region 190-205 nm. The solid circles from equation (2). represent u,(n) measured by Johnston et al. (1984) in the region 205225 nm, after subtraction of the contribution from Rayleigh scattering, us(n). theoretical basis for believing the R- I2 extrapolation

where 192 Q 1 d 238 mn. However, equation (2) is inappropriate for 1> 238 nm. Hall (1988, personal communication) has fitted the cross-sections calculated from our equation (2) in the region 194-210 nm and the combined CfA/Reims experimental cross-sections in the region 21 l-240 nm (from our Table 2) to an equation of the form given by Johnston et al. (1984). The result is

is as good as the CfA calculated repulsive potential, the former can be used to fit the experimental crosssection in the 205-207 nm region better than the latter does. Further improvement in the calculated absorption cross-section below N 205 nm would require an elaborated and large-scale calculation designed to provide a more definitive characterization of the repulsive part of the A-state potential curve. At the present time, the representation of the Herzberg continuum crosssection in the region 194-240 nm in the closed form given by equation (3) is probably the best available. are grateful to Dr L. A. Hall for his personal communication containing equation (3) given in the present paper. K.Y., J.R.E., W.H.P. and D.E.F. thank Drs L. A. Hall, G. P. Anderson and R. E. Huffman of AFGL for their encouragement of this study and for the AFGL support through a MIPR to NASA. We are pleased to acknowledge support by the NASA Upper Atmospheric Research Program under Grant NAG5-484 to the Smithsonian Institution. A.S.C.C. thanks the Directorate of International Activities of the Smithsonian Institution for support. We also thank Miss S. Chiu for technical assistance. S.L.G. gratefully acknowledges support from NSF grant ATM-8616776, from the National Center for Atmospheric Research which is supported by the National Science Foundation, and from NASA Ames Cooperative Agreement NCC 2490. A.J., B.C. and M.F.M. are pleased to acknowledge support by the CNRS (ATP Physique de l’Atmosphere) and the “Minis&e de 1’Environnement” and they would like to thank J. P. Lux for technical assistance.. Acknowledgements-We

x exp [ -69.7374

(In (2F)r].

(3)

In Fig. 7, the solid curve is from equation (3), the asterisks are experimental points and the open circles are theoretical values from equation (2). The agreement is good, so that equation (3) is an appropriate closed form for the representation of the Herzberg continuum cross-section in the wavelength region 194-240 nm. The Franck-Condon densities for absorption into the A-state continuum are sensitive to the shape of the repulsive part of the A-state potential curve. For example, we have shown that the R- I2 extrapolation of the known bound part of the A-state potential is steeper than the CfA curve determined from a CI calculation ; the former leads to a maximum of _ 8.0 x 1O-24cm’ in the absorption cross-section near 196 nm, whereas the CfA results yield a maximum of -6.9 x 1O-24 cm’ near 205 nm. Whilst there is no

REFERENCES Anderson, G. P. and Hall, L. A. (1986) Stratospheric determination of OZ cross-sections and photodissociation rate

Cross-section of the Her&erg continuum of O2 coefficients: 191-215 nm. .I. geophys. Res. 91, 14509. Anderson, G. P. and Hall, L. A. (1983) Attenuation of solar irradiance in the stratosphere: spectrometer results between 191 and 207 mn. .I. geophys. Res. 87,923. Bates, D. R. (1984) Rayleigh scattering by air. Planer. Space Sci. 32,785. Cheung, A. S.-C., Yoshino, K., Parkinson, W. H., Guberman, S. L. and Freeman, D. E. (1986) Absorption cross-section measurements of oxygen in the wavelength region 195241 nm of the Herzberg continuum. Planet. Space Sci. 34, 1007. Frederick, J. E. and Mentall, J. E. (1982) Solar irradiance in the stratosphere : implications for the Her&erg continuum absorption of OZ. Geophys. Res. Lett. 9,461. Hasson, V. and Nicholls, R. W. (1971) Absolute spectral absorption measurements on molecular oxy en 264&1920 8, : continuum measurements 2430-1920 1. J. Phys. B: Atomic Molec. Phys. 4, 1789.

Herman, J. R. and Mentall, J. E. (1982) O2 absorption crosssections (178-225 mn) from stratospheric solar flux measurements. J. geophys. Res. 87,8967. Jenouvrier, A., Coquart, B. and Merienne-Lafore, M. F. (1986a) New measurements of the absorption cross-

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sections in the Herzberg continuum of molecular oxygen in the region between 205 and 240 nm. Planet. Space Sci. 34,253.

Jenouvrier, A., Coquart, B. and Merienne, M. F. (1986b) Long path length measurements of oxygen absorption cross-sections in the wavelength region 205-240 nm. J. quant. Spectrosc. radiat. Transfer 36, 349.

Johnston, H. S., Paige, M. and Yao, F. (1984) Oxygen absorption cross-sections in the Herzberg continuum and between 206 and 327 K. J. ueophvs. Res. 89. 11661. King, L. V. (1923) On the complex anisotropic molecule in relation to the dispersion and scattering of light. Proc. R. Sot.

A104,333.

Pirre, M., Rigaud, P. and Huguenin, D. (1984) New in situ measurements of the absorption cross-sections of 0, in the Herzberg continuum. Geophys. Res. Leff. 11, 1199. Saxon, R. P. and Slanger, T. G. (1986) Molecular oxygen absorption continuua at 20&300 nm and 0, radiative lifetimes. J. geophys. Res. 91,9877. Shardanand and Prasad Rao, A. D. (1977) Collision-induced absorption of O2 in the Herzberg continuum. J. quant. Spectrosc. radiat. Transfer 17,433.