High resolution absorption cross-sections and band oscillator strengths of the Schumann-Runge absorption bands of isotopic oxygen, 16O 18O, at 79 K

Plane% Space Sci., Vol. 37. No. 4, pp. 41%426, Printed in Gnat Britain.

1989 0

OD32-0633/89 $3.00+0.00 1989 Pergmon Press plc

HIGH RESOLUTION ABSORPTION CROSS-SECTIONS AND BAND OSCILLATOR STRENGTHS OF THE SCHUMANN-RUNGE ABSORPTION BANDS OF ISOTOPIC OXYGEN, 160180, AT 79 K K. YOSHINO, D. E. FREEMAN, J. R. KSMOND, R. S. FRIEDMAN* and W. H. PARKINSON

Harvard-Smithsonian

Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A. (Received 26 January 1989)

AhstractXross-sections of ‘60’80 at 79 K have been obtained from photoabsorption measurements on mixtures of 1602,180*and ‘60’*0 at various pressures throughout the wavelength region 180.5-195.3 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mn-’ grating and having an instrumental width (FWHM) of 0.0013 nm. The measured absorption cross-sections of the 1sO’80 Schumann-Runge bands (11, Ok(3, 0) are independent of the instrumental width. The measured crosssections are presented graphically and are available at wavenumber intervals of N 0.1 cm-’ as numerical compilations stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. Band oscillator strengths of those bands have been determined by direct numerical integration of the measured absolute cross-sections and are in agreement with our theoretically calculated valies.

1. INTRODUCTION

The Schumann-Runge (S-R) absorption bands of O2 at 79 K arise from the transition B ‘Z; -X(O) ‘Z; in the wavelength region 175205 nm. There has recently been interest in the role of the photodissociation and photoabsorption of isotopic oxygen through the Schumann-Runge bands. Besides the interest in the Schumann-Runge bands of ordinary oxygen, the effects from the Schumann-Runge bands of ‘601*0 cannot be ignored in the photochemistry in the atmosphere, since the abundance of ‘60180 is 0.41% of that of the ordinary oxygen molecule and it is the fifth most abundant molecule in the atmosphere excluding water. Cicerone and McCrumb (1980) did the first calculation on the isotopic effect to account for the excess ozone observed in the atmosphere. Blake et al. (1984) did a more detailed calculation of the same effect. Recently, Omidvar and Frederick (1987) did a line by line calculation of the Schumann-Runge bands of ordinary and isotopic oxygen to obtain the contribution to the photodissociation of these molecules at different altitudes. A basic aeronomic problem, viz., the providing of an explanation for the confirmed enhancement of heavy ozone “03 in the stratosphere (Mauersberger, 1987), has been addressed in a recent theoretical study by Bates (1988). In addition, a relevant laboratory study of the isotopic fractionation in ozone decomposition has recently appeared (Bhatta*Also, Department of Chemistry, Harvard University, Cambridge, MA 02138, U.S.A.

charya and Thiemens, 1988). Although selective absorption by ‘60’s0 has been negated by Kaye and Strobe1 (1983) as the mechanism responsible for heavy ozone enhancement, ‘a0’80 remains an important atmospheric trace species. The only recent laboratory results for the band oscillator strengths of the S-R bands of ‘aO’sO are those of Lewis et al. (1987), obtained with medium resolution (-0.005 nm), and not directly from absolute cross-section measurements. Isotopically pure ‘601s0 is unavailable for absolute cross-section measurements of ‘60180, which must be derived from measurements of available isotopic mixtures of 1602, 160’80 and “02, in which all three sets of the S-R absorption bands are simultaneously present. To obtain the cross-sections of ‘60’*0 bands from studies of such mixtures, we need detailed absorption wavelengths and cross-sections of 1602 and “OP For 1602, we previously published high resolution photographic wavelength measurements (Yoshino et al., 1984), their spectroscopic analysis (Cheung et al., 1986), and absorption cross-section measurements at 300 K (Yoshino et al., 1983 ; Cheung et al., 1984) and at 79 K (Yoshino et al., 1987). We have recently completed similar wavelength measurements and spectroscopic analyses for 1802(Cheung et al., 1988) and 160180 (Cheung et al., 1989), and absorption crosssection measurements of ‘*02 at 79 K (Yoshino et al.,

1988). In the present paper, we report absolute crosssection measurements of (3,0)-( 11,O) S-R bands of 419

K.

420

YOSHINO et al.

‘60’80 at 79 K throughout the wavelength region 180.5-195.3 nm. These results have been obtained at an instrumental band width (FWHM) of 0.0013 nm (N 0.4 cm-‘) which suffices for the measurement of these absolute cross-sections. We have also determined band oscillator strengths for the (3,0)-(11,O) bands of ‘60’80 by numerical integration of the crosssections obtained at 79 K, and we have compared these experimental band oscillator strengths with our theoretically calculated values. 2. EXPERIMENTAL

PROCEDURE

The apparatus and procedure for this work are the same as described in our previous paper (Yoshino et al., 1987). A 6.65 m vacuum spectrometer (Yoshino et al., 1980) is used in the first order of a 2400 lines mm-’ grating to provide a reciprocal linear dispersion of 0.06 nm mm-‘. The entrance and exit slit widths are lo-12 pm, and the resulting instrumental band width (FWHM) is 0.0013 nm. The continuous background for ‘the photoabsorption measurements of ‘60’80 is provided by a hydrogen continuum discharge source. The absorption cell is made of stainless steel tubing, of internal diameter 25 mm and length 81 cm, with Cajon fittings on both ends. Two Pyrex tubes, each 25 mm in diameter and 23 cm in length, and each with a silica (Suprasil-1) window on one end, are inserted, window first, through Cajon fittings into opposite ends of the stainless steel tubing. This arrangement provides an optical pathlength of 50.3 cm between the two silica windows. The cell is immersed in a liquid-nitrogen bath, 58 cm in internal length and 17 cm in internal depth. The isotopic oxygen (MSD No. 2639-B, 50% I80 atomic) is introduced into the absorption cell. The oxygen pressure is varied from 0.30 to 145 torr, providing column densities of 1.84 x 10’8-8.91 x 1020 cm-‘, for cross-section measurements of the (3,0)-(15,0) bands. The photoelectric scanning is continuous ; in this spectral region the counting period is set at 0.225 s which corresponds to accumulating counts at intervals of 4 pm in length or 0.00024 nm in wavelength (-0.1 cn-‘) in the focal surface of the spectrometer. The background continuum is obtained by scanning before and after the photoabsorption measurements ; the small decrease in the intensity of the background continuum during a scan is taken into account by interpolation. The Fourth Positive bands of CO (Yoshino et al., 1987), excited by dc discharge through carbon dioxide, are used to obtain a dispersion for each scan and the absolute wavenumber scale is established by the use of the known wavelengths of rotational lines of the S-R bands of ‘a0’80 (Cheung et al., 1989).

The ratio of the incident intensity Z,(v) to the intensity transmitted Z(v) through a medium of column density N(cm-*) is related to the absorption crosssection a(v) (cm*) by the formula : ln

Z,(v)= ~ Z(v)

NJ(v).

This expression applies strictly for monochromatic radiation, and effectively for radiation for which the instrumental width is negligible compared with the width of the spectral feature being measured. With respect to the present instrumental width (FWHM = 0.0013 nm), the spectral features of the SR bands with v’ = 3-15 prove broad enough for the measured cross-sections to be absolute, as discussed in the previous papers (Yoshino et ai., 1983 ; Cheung et al., 1984). In our previous cross-section measurements of 1602 or 1802, we could use essentially pure sample gases and the column density could be deduced from measurements of path lengths, temperature and gas pressure. However, the cross-section measurements for ‘60’80 involved all three isotopes in the approximate ratio of 1: 2 : 1 for 160,, ‘60’80 and 1802,respectively. A total pressure measurement does not lead to a column density of any single isotope. As shown in Figs 1 and 2 of Cheung et al. (1989), the three (v’, 0) isotope bands are overlapped even at 79 K. The procedure to obtain the column density of ‘60’80 is presented in the following section.

3. CROSS-SECTIONS BANDS

OF THE SCHUMANN-RUNGE OF “O’*O AT 79 K

Provisional cross-sections of the mixed isotopes have been obtained from photoabsorption measurements and column density as obtained previously from the total pressure. The (9,0) bands from such measurements are shown in Fig. 1D for the three overlapping bands of 1602, 160180 and 1802,from right to left in order. The absolute cross-sections of 1602 and 1802from our previous measurements are shown in (A) and (B). The cross-sections in (A) and (B) are reduced in (C) to match the corresponding crosssections in (D) and we now have the fractions and also the column densities of 1602 and 1802. The remaining balance should be the column density of ‘60’80. The cross-section in (C) can be subtracted from the crosssection (D) and the isolated cross-section of ‘60’80 is obtained as shown in (E). The cross-section in (F) is the final absolute cross-section of ‘60’80 after the correction of the provisional column density by a now known fraction.

Schumann-Runge hands of ‘sO’8Oat 79 K

b:i,d, 54250

I 54600

421

measured cross-sections of the S-R bands of ‘6O’8O at 79 K are shown as separate linear plots, one per band, for the (3,0)-(11,O) bands. The rotational assignments of lines belonging to the main branches can be deduced from the few explicitly given as R(N) or P(N). The complete assignment of the observed lines and their wavenumbers measured from high resolution photographic spectra are presented in a separate publication (Cheung et al., 1989). The experimental error in an absolute cross-section a(v) derived from equation (1) depends on the error in the column density N and in the optical depth In [Z,,(v)/Z(v)]. An uncertainty of -3% in total N results from an estimated error of 1% in the pressure measurements and of +2 K in the temperature estimation ; the error in the optical path length, f 1 mm in 50.3 cm, is negligible. Uncertainties of 4% for the cross-sections of r602 and 1802 have to be taken into account since they are used in the subtraction process described above for the determination of their column densities. Errors in the final values of the absolute absorption cross-section of ‘Q’*O are estimated to be -7% in regions of the principal absorption peaks. Errors in the range below 1 x lo-” cm* are somewhat greater as a result of the limited column densities available at the highest pressures we have used.

Wavenumber

FIG.

1. -bE

CROSS-SECTIONS

OF THE

(9,o) BANDS

OF THB MIXED

JSOTOPFS.

(A) and (B) are our previously measured absolute crosssections of (9,0) bands of 160, and ‘*02,respectively. They are reduced in (C) to match the observed cross-sections of the mixed isotopes in (D). (C) can be subtracted from the observed cross-sections (D) and the isolated cross-sections of ‘60’*0appear in (E). With its now known column density, the absolute cross-section of ‘sO’8Ois deduced in (FL See Section 3 of the text. In this way, absolute absorption cross-sections of i60’*0 at 79 K have been obtained, for the first time, for the S-R bands (l l, O)-(3,0) which occur within the wavelength region 180.5-195.3 nm. For the (D’,0) bands with v’ = 12-15, one of the other overlapping isotopic bands is too sharp for its absolute crosssection to have been measured by us ; therefore we could not deduce the (12,0)-( l&O) cross-sections of 160’*0, even though the measured optical depths of these 1601*0 bands are independent of our instrumental width. The cross-sections of the (3,0)-( 11,O) bands are available at wavenumber intervals of N 0.1 cm-’ as numerical compilations stored on magnetic tape, from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. In this paper, we present the crosssections in graphical format. In Figs 2a-i, the

4.

BAND OSCILLATOR STRENGTHS OF THE SCHUMANN-RUNGE BANDS OF “0%

The band oscillator strength is given by (Yoshino et al., 1983) 2 1 j-(0’, ,“) = “?c _ a(v) dv ne2 N(o”) s

(2)

in which fl(((u”)is the fractional Boltzmann population of the absorbing vibrational level, and the integration of the cross-section a(v) is performed over all of the rotational lines belonging to the @‘,a”) band. The constants m, e, and c are the electronic mass, electronic charge, and velocity of light, respectively: mc*/ne* = 1.130x lOi* cm-‘. For iaO’*O, AG’i’,*= 1513.1 cm-’ (Huber and Herzberg, 1979), so that N(0) = 1.000 at 79 K, and equation (2) yields for the band oscillator strength of the (D’,0) band, j(u’,O) = 1.130 x lo’*

u(v) dv. s

(3)

Band oscillator strengths flu’, 0) with t)’ = 3-l 1 have been determined from equation (3) by direct numerical integration of the measured cross-sections and are shown in the third column of Table 1. The uncertainty

K. YOSHINO et al.

422

b

I,

0 51050

5

1

51400

Wavenumber (b)

4.0

Wavenumber

b

L

0 52200

Wavenumber FIG.2 (a-c).

52550

Schumann-Runge

bands of ‘60’80 at 79 K

423

(d)

6.0

Wavenumber (0) 7.0

Wavenumber (f)

6.0

54150

Wavenumber FIG.

2 (d-f).

424

K. YOSIUNO et al.

O54250

4 -

Wavenumber (h)

I

54650

Wavenumber

Wavenumber BANDS (u’, 0) WITHV’ FIG. 2. ABSOLUTE CROSS-SECTIONS OF THE SC HUMANN-RUNGE 79 K ARESHOWN IN FIGS 2a-i, RESPECTIVELY.

=

3-11

OF ‘60’80

AT

425

Schumann-Runge bands of ‘So’*0at 79 K TABLE1. BAND~SCILLATUR lo-“,

STRENGTHS,

OF THE s CHUMANN--RUNGE

Present (CfA) v’

x

0 1 2 3 4 5 6 7 8 9 10 11 12

10 9 8 8 I 7 6 6 6 6 5 5 5

Exp. 79 K

5.67 1.79 5.09 1.19 2.55 4.69 7.56 1.17 1.63

Theory 79K 300K 1.44 1.79 1.15 5.02 1.69 4.66 1.09 2.24 4.11 6.82 1.04 1.45 1.87

1.40 1.75 1.12 4.89 1.64 4.53 1.06 2.18 3.98 6.60 0.999 1.39 1.79

flu’,

0) IN UNITS OF

BANDS OF ‘60’*0

Lewis et al. (1987) Exp: f0

1.25 5.28 1.82 4.87 1.20 2.51 4.76 7.79 1.22 1.63 2.00

Exp.t h

1.11 5.33 1.70 4.67 1.15 2.34 4.47 7.37 1.13 1.61 1.95

are isotopically invariant, are as described in Yoshino et al. (1988). For the ‘6O’*Oisotopic molecule, due to the distinguishability of the nuclei, the summation of line oscillator strengths must include initial rotational levels with both even and odd values of N; in the homonuclear isotopic molecules 160x and ‘*02, only initial rotational levels with N odd are populated. 5. DISCUSSION

The cross-sections at 79 K for any particular band of ‘60180 are free. from contributions from lines of other bands of ‘60’*0. Summation of the cross-sections at 79 K over any particular band belongs solely to the band considered. The present band oscillator strengths for ‘601*0 obtained by such summations, are compared with the recent results of Lewis et al. (1987) in Table 1. The results of Lewis et al. (1987) * Rotationless (N = 0) values of Lewis et al. (1987). were obtained, not from absolute cross-section t Boltzmann weighted average values for 300 K, Lewis et measurements, but from measurements of equivalent al. (1987). widths which are then fitted by a detailed modelling procedure in which the band oscillator strength and introduced into the band oscillator strength by the predissociation line width are adjustable parameters. integration procedure itself is negligible, so that our The band oscillator strengths in our work are results estimated uncertainty of 7% in the measured crossat 79 K, but the results of Lewis et al. are for N = 0 cfO) and for a weighted average at room temperature section represents also the uncertainty in the band oscillator strength. At 79 K, there are no hot bands cf.“). (v” > 0) overlapped with the main bands (u” = 0), In Table 1, our band oscillator strengths for ‘60180, obtained from cross-sections measured at 79 K, are and rotational lines with N larger than 17 are not compared with values from theoretical calculations. observed. The agreement between our reported theoretical and Also presented in Table 1 are theoretical band oscillator strengthsflu’, 0) determined at 79 and 300 K by experimental oscillator strengths for 160180 is not summing rotational line oscillator strengths weighted quite as good as the agreement for the other two by the Boltzmann population of the initial rotational isotopic molecules of O2 for which, on average, the theoretical values are O-l % smaller and 2-3% larger level. The oscillator strengths are calculated for the than the experimental values at 79 K for 1602 and lines in the Schumann-Runge bands arising from the ‘*02, respectively (Yoshino et al., 1988). The i60”0 F2 component of the X 32; state, but the dependence theoretical band oscillator strengths are on average of the oscillator strength on the fine-structure component is expected to be weak (Lewis et al., 1987). smaller by 10% than the experimental values. However, an increase in the B 32; -2’ 3X; dipole The Ho&London factors were taken from Tatum moment by 1.5% (which is within the uncertainty of and Watson (1971) for a coupling case intermediate the calculated transition moment) would bring our between Hund’s cases (a) and (b). They were calculated using the accurate r601*0 molecular patheoretical and experimental flu’, 0) of ‘60180 into satisfactory agreement ; the r602 theoretical values rameters of the X3X; state (Steinbach and Gordy, 1975) and the B3C; state (Cheung et al., 1989). would remain within the 4% estimated uncertainty of the experimental oscillator strengths while the ‘*02 Because accurate parameters for the lowest two theoretical values would fall, on average, only slightly vibrational levels of the B ‘E; state were unavailable, case (b) Hiinl-London factors were used for thefl0, 0) outside the stated experimental uncertainty. An increase in the dipole moment is consistent with andf( 1,O) calculations. However, even for the higher vibrational levels o’, there is an insignificant difference. the work of Gibson et al. (1983) whose dipole moment in the critical region of 2.5-2.9a, is larger than the between oscillator strengths calculated using case (b) and intermediate case rotational line strengths. The X transition moment values that we have adopted (Yoshino et al., 1988) wh? their dipole moment is and B state potentials as well as the dipole moment, corrected by a factor of J3 to be consistent with the which within the Born-Oppenheimer approximation

K. YOSIGINO et al.

426

recommendations of Whiting ei al. (1980). Using the dipole moment of Gibson et al. (1983), Blake et al. (1984) calculated ‘60180 band oscillator strengths which are in very good agreement with our experimental oscillator strengths. However, contrary to our theoretical values, the calculated 1602 band oscillator strengths of Blake et al. (1984), although consistent with the experimental work of Gies et al. (1981), do not agree well with the high resolution results of Yoshino et al. (1988). In either case, whether our transition moment is increased or not, the ratios of our theoretical band oscillator strengths of 1602 to those of 160180 for the (3,0)-(11,0) bands differ by less than 3% from the corresponding ratios of FranckCondon factors calculated by Halmann and Laulicht (1966) for the two isotopic molecules. Due to the decrease in rotational oscillator strengths within a given band with increasing N, the calculated band oscillator strengths at 300 K are about 3% smaller than those at 79 K. These theoretical band oscillator strengths at 300 K for the (2,0)-(12,0) bands are on average 7.6% smaller than the measured mean band oscillator strengths of Lewis et al. (1987).

Cheung, A. S-C., Yoshino, K., Parkinson, W. H. and Freeman, D. E. (1986) Molecular spectroscopic constants of O,(B’X;): the upper state of the Schumann-Runge bands. J. molec. Spectrosc. 119, 1. Cicerone, R. J. and McCrumb, J. L. (1980) Photodissociation of isotopically heavy 0, as a source of atmospheric Oj. Geophys. Res. Lett. 7,25 1. Gibson, S. T., Gies, H. P. F., Blake, A. J., McCoy, D. G. and Rogers, P. J. (1983) Temperature dependence in the Schumann-Runge photoabsorption continuum of oxygen. J. quant. Spectrosc. radiat. Transfer 30, 385. Gies, H. P. F., Gibson, S. T., McCoy, D. G., Blake, A. J. and Lewis, B. R. (1981) Experimentally determined oscillator strengths and linewidths for the SchumannRunge band system of molecular oxygen. III. The (7-O) to (19-O) bands. J. quant. Spectrosc. radiat. Transfer 26, 469.

Halmann, M. and Laulicht, I. (1966) Isotope effect on FranckCondon factors. V. Electronic transitions of isotopic 02, N2, C2 and H2 molecules. J. them. Phys. 44,2398. Huber, K. P. and Herzberg, G. (1979) Molecular Spectra and Molecular Structure. Iv

: Constants of Diatomic Molecules.

Van Nostrand Reinhold, New York. Kaye, J. A. and Strobel, D. F. (1983) Enhancement of heavy ozone in the Earth’s atmosphere. J. geophys. Res. 88,8447. Lewis, B. R., Berzins, L. and Carver, J. H. (1987) Oscillator strengths for the Schumann-Runge bands of ‘60’80. J. quant. Spectrosc.

radiat. Transfer 37,219.

Mauersberger, K. (1987) Ozone isotope measurements in the

stratosphere. Geophys. Res. Lett. 14,80. are pleased to acknowledge valuable discussions with Prof. A. Dalgamo. The work reported was supported by the NASA Upper Atmospheric Research Program under Grant NAG 5-484 to the Smithsonian Astrophysical Observatory and by the National Science Foundation under Grant ATM-87-13204 to Harvard University. Acknowledgement-We

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