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Rotational variation of predissociation linewidths for the Schumann-Runge bands of molecular oxygen
w22-6073/80/1
I. Quonr. Spectrosc. Rod& Transfer Vol. 24. PP. 365-369 @ Pergamon Press Ltd., 1960. Printed in Great Britain
l01-0365/%?.00/0
ROTATIONAL VARIATION OF PREDISSOCIATION LINEWIDTHS FOR THE SCHUMANN-RUNGE BANDS OF MOLECULAR OXYGEN B. R. LEWIS and J. H. CARVER Research School of Physical Sciences, The Australian National University, Canberra, Australia 2600
and T. I. HOBBS, D. G. MCCOY,and H. P. F. GIES Department of Physics, University of Adelaide, Australia 5001 (Receiwd 11 January 1980) Abstract-Predissociation linewidths are presented as a function of rotation for the (3-0)-(14-0) SchumannRunge bands of molecular oxygen. While there may be a slight tendency overall for the linewidths to increase with rotation, it is shown that previous assumptions of linewidths constant with rotation are generally valid within the experimental error for the range of rotation studied, N”~21. There is no evidence for the sudden increase in linewidth with rotation reported elsewhere. INTRODUCTION
The B3EU- state of molecular oxygen, which gives rise to the Schumann-Runge bands, is predissociated.’ Qualitative evidence of the line broadening was first seen by Wilkinson and Mulliken’ and Carroll3 and there have since been three distinct sets of linewidth measurements taken. Ackerman and Biaume4 made photographic estimates of linewidths for the (O-O)- (19-O) bands, while Hudson and Mahle’ applied a fitting procedure to the photoelectric results of Hudson and Carter6 to obtain linewidths for the (2-0)-(16-O) bands. Frederick and Hudson7 have recently reanalyzed the results of Hudson and Carter6 to provide new estimates of Iinewidths for the (2-0)-(13-O) bands. Lewis et al.gg have presented new measurements of linewidths for the (2494 14-O)bands using a photoelectric technique with a curve of growth type of analysis. Schaefer and Miller” and Murrell and Taylor” have discussed the predissociation theoretically, and Julienne and Krauss,” and more recently Julienne13 have calculated theoretical linewidths which may be compared with the experimental values. The Schumann-Runge bands of molecular oxygen play an important role in the atmospheric absorption of solar radiation from 1750 to 2000 A. Nicolet and Peetermansr4 review this problem and emphasize that exact knowledge of linewidths is required for precise calculations where the atmospheric optical depth is much above unity. The possible variation of linewidth with rotation was first investigated theoretically by JulienneI who performed a limited number of calculations giving the centrifugal distortion in the 3ZU--3C,‘F2 partial width for a particular model interaction. Because of the number of possible factors involved in width variation with rotation, the problem is complex and no overall theoretical predictions have been made. Frederick and Hudson’ have presented linewidths for selected rotational lines from the (8-O) to (13-O) Schumann-Runge bands in an attempt to analyze linewidth variation with rotation. Their results show a general tendency for larger widths with higher rotation, with no significant variation observed for some bands. Their (11-O) results show a sudden doubling of linewidth at J = 17, an unexpected effect. Lewis et al? have stated that it was not possible to detect statistically significant rotational variation of linewidth in their results. In this paper the individual linewidths measured by Lewis et ai.‘*9as a function of rotation are presented for the (3-0)-(14-O) bands and it is confirmed that, while there is a slight tendency overall for the linewidths to increase with rotation, the assumption of linewidths constant with rotation is generally valid within the experimental error. The measurements do not show the sudden increase in linewidth observed by Frederick and Hudson’ in the (11-O) band. 365
366
B.R.LEWIS
et a/.
EXPERIMENTALMETHODANDDATAANALYSIS
The apparatus and experimental method have been fully discussed elsewhere.s~9Background radiation provided by a thyratron triggered hydrogen discharge was dispersed by the Adelaide 6m monochromator” and monitored by a gated photomultiplier detection system before entering and leaving the windowed absorption cell. Typical wavelength resolution was 0.06 A. Scans were performed across the absorption line of interest at two widely differing pressures and the resultant equivalent widths recorded. This technique allowed the simultaneous determination of both oscillator strength and predissociation linewidth by an iterative procedure.8 All factors considered in the iterative computer program enabling the linewidth determinations have been discussed in detail elsewhere. eP9Briefly, rotational absorption lines in the Schumann-Runge system exhibit both thermal and predissociation broadening, the relative importance of these two effects being expressed in terms of the mixing parameter u.~ The low pressure scan (approaching the linear region of the curve of growth) produced an equivalent width which was more sensitive to oscillator strength than to a. Assuming an initial approximation for a, the oscillator strength was then calculated for this scan. The high pressure scan’s equivalent width was more sensitive to a than to oscillator strength, and thus a refined value for a could be determined from the high pressure scan using the oscillator strength determined from the previous scan. The calculational loop was then repeated until a stable pair of results, (oscillator strength, a) was produced. Absorption spectra calculated using these results were then compared with the experimental scan spectra as a final check on the accuracy of the procedure. Linewidths obtained using this procedure for 119 individual lines or unresolved groups of lines from the (3-O) to (14-O) Schumann-Runge bands are presented in this paper. RESULTS
The predissociation linewidths measured in this work are shown in Table 1 as a function of N” together with estimated standard deviations. The unmarked values were taken from unresolved P (N”) R(N”+ 2) pairs, those marked (*) are the average of two separate P (IV”),R (N”) measurements, those marked (t) were taken from resolved R (N”) lines, while those marked (tt) refer to single measurements on P (N”) lines. The results are also displayed graphically in Figs. 1 and 2 together with the results of Frederick and Hudson.’ The continuous lines in Figs. 1 and 2 give the 3Cu--3E,,+F2 partial widths of JulienneI for the (7-O) and (9-O) bands, including centrifugal distortion. It is therefore apparent that these values are not strictly comparable with the experimentally determined total widths, but they have been displayed because the 3Cu--3E,,+partial width is the theoretically major contributor to the total predissociation widths for these two bands.13 The overall trends in the rotational variation of the present results were studied by dividing the widths into two groups for each band, those for N” c 11 and those for N” > 11. Using this technique, it has been found that, neglecting statistical significance at this stage, a decrease in linewidth with rotation is observed for three bands [(6-O), (10-O), (14-O)] and an increase for the others. If a student’s t statistical analysis is performed on the two groups of results, it is found that no significant variation of linewidth with rotation is observed at the 5% level for ten of the twelve bands studied. The remaining two bands [(9-O), (12-O)] exhibit a significant increase in linewidth with rotation (at the 5% level). Of the ten bands exhibiting no significant variation, the tendency towards an increase is largest for the (7-O) and (8-O) bands. The shape of the variation for the (140) band is not significant and could perhaps be explained by noting that the triplet splitting factors are not well known for the lower rotational lines in this band. Because of the low (14-O) predissociation widths, uncertainty in the triplet splitting may produce non-negligible errors in the determined linewidths. This factor is not important for the broader lines of the lower vibrational bands. In comparing the results of this work with those of Frederick and Hudson,’ it should be noted that we assume that the J given by these authors is in fact N”. It is seen from Figs. 1 and 2 that the agreement between the rotational behaviour observed by Frederick and Hudson’ and ourselves is quite good for the (8-0)-(10-O) and the (13-O) bands. Disagreement for the (12-O) band is quite marked, as is that for the higher rotational lines of the (11-O) band for which Frederick and Hudson’ observe a discontinuous increase of 100% at J = 17. Our results
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Table 1. Predissociation linewidths, W, (FWHM, cm-‘), measured in this work as a function of rotation for the (3-0)-(14-O)bands. Unmarked values refer to mean linewidths from unresolved P(W) R(N” + 2) doublets; those marked C*)refer to the mean linewidths from two distinct P(W). R(W) measurements, those marked (t) refer to sin& measurements on RCN”),lines, while those marked ittj iefkr tb single measurements on P(N”) lines.
368
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Fig. 1. Predissociation linewidths, W, (FWHM, cm-‘), measured in this work for the (3-0~@-O) Schumann-lunge bands as a function of rotation. Values obtained by Frederick and Hudson,’ and the theoretical centrifugal distortion predicted by Julienne” are also shown where applicable.
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Fig. 2. Predissociation linewidths, W, (FWHM, cm-‘), measured in this work for the (9-0)-(14-O) Schumann-Runge bands as a function of rotation. Values obtained by Frederick and Hudson,’ and the theoretical centrifugal distortion predicted by Julienne” are also shown where applicable.
show no increase at all. For the lower bands, Frederick and Hudson’ are unable to present rotational variation because of the narrow range of conditions enabling them to determine linewidths. The equivalent width analysis used in this work does not suffer from this limitation. Most of the low rotation linewidths presented by Frederick and Hudson’ in Table 3 of their paper have quite large random errors and this probably accounts for the poor agreement, in general, between their bandhead values and those of this work. Several points should be noted regarding the work of Frederick and Hudson.’ Firstly, the oscillator strengths which they present show greatest deviations from the smooth theoretical curve of Allison et ~1.‘~for the (11-O) and (12-O) bands. This is where the largest discrepancies exist between their linewidths and those of this work, their oscillator strengths being lower,
Rotational variation of predissociation linewidths
369
their linewidths higher. Perhaps the convergence of their program is more sensitive to statistical errors in this region. Secondly, no mention is made by Frederick and Hudson’ of the variation of band oscillator strength with rotation, a significant effect which has been extensively measured by Lewis et ~1.~”Their published values’ may be bandhead values only, but they should have been able to deduce values for the higher rotational lines for which they presented the linewidth variation. Comparing the results of this work with those of Julienne,13 it is seen that the increase in the (9-O) 3X,--3X,+ F2 partial width due to centrifugal distortion is reflected in the statistically significant increase in total width with rotation measured for the same band in this work. The theoreticali very slight decrease for the (7-O) band is not verified by the experimental results which show no statistically significant change from a constant value at the 5% level. However, if anything, there is a tendency towards higher widths for higher rotation in this band. The comparisons with the results of JulienneI are only included as a guide and must be interpreted with caution for the reasons mentioned earlier. The present results would tend to indicate that, for the (7-O) and (9-O) bands, the predissociation due to the 3EU+state is the sole significant contributor to the total predissociation width. CONCLUSION
An accurate knowledge of Schumann-Runge band predissociation linewidths is vital when considering the absorption of solar ultraviolet radiation in the atmosphere. Whereas it has previouslys” been demonstrated that the quite considerable decrease in band oscillator strength with rotation should be taken into account when dealing with this problem, this work shows that the variation with rotation of the predissociation linewidths is not nearly so significant. Of the twelve bands studied, nine show a tendency towards increased linewidths with rotation, but only two of these [(9-O), (12-O)] show statistically significant increases at the 5% level. For the remainder the assumption of constant linewidth with rotation is a valid one. It is clear that the present results and those of Frederick and Hudson’ are not sufficiently accurate statistically for an unambigous detection of the small rotational deviations in linewidth expected for N” G 21. It is planned to examine this problem further with precise measurements on a single Schumann-Runge band, with some attempt to study higher rotational lines with N”>21. Acknowledgements-The authors would like to thank the Australian Research Grants Committee for its support and F. A. Smith for his valuable assistance. One of us (B. R. L.) held a Queen Elizabeth II Fellowship during the course of this work. REFERENCES 1. P. Krupenie, J. Phys. Chem. Ref. Data 1,423 (1972). 2. P. Cl. Wilkinson and R. S. Mulliken, Astrophys. 1. 125,594 (1957). 3. P. K. Carroll, Astrophys. 1. 129,794 (1959). 4. M. Ackerman and F. Biaume, J. Molec. Spectrosc. 35,73 (1970). 5. R. D. Hudson and S. H. Mahle, J. Geophys. Res. 77,2902 (1972). 6. R. D. Hudson and V. L. Carter, J. Opt. Sot. Am. 58, 1621(1968). 7. J. E. Frederick and R. D. Hudson, J. Molec. Spectrosc. 74,247 (1979). 8. B. R. Lewis, J. H. Carver, T. I. Hobbs, D. Cl. McCoy, and H. P. F. Gies, JQsRT 20, 191(1978). 9. B. R. Lewis, J. H. Carver, T. I. Hobbs, D. G. McCoy, and H. P. F. Gies, JQSRT 22,213 (1979). IO. H. F. Schaefer and W. H. Miller, J. Chem. Phys. 55,4107 (1971). Il. I. N. Murrell and J. M. Taylor, Molec. Phys. 16, 609 (1%9). 12. P. S. Julienne and M. Krauss, J. Molec. Spectrosc. 56,270 (1975). 13. P. S. Julienne, J. Molec. Spectrosc. 63, 60 (1976). 14. M. Nicolet and W. Peetermans, Planet. Space. Sci. 28,85 (1980). IS. J. H. Carver, G. N. Haddad, T. I. Hobbs, B. R. Lewis, and D. G. McCoy, Appl. Opt. 17,420 (1978). 16. A. C. Allison, A. Dalgamo, and N. W. Pasachoff, Planet. Space Sci. 19, 1463(1971).
I. Quonr. Spectrosc. Rod& Transfer Vol. 24. PP. 365-369 @ Pergamon Press Ltd., 1960. Printed in Great Britain
l01-0365/%?.00/0
ROTATIONAL VARIATION OF PREDISSOCIATION LINEWIDTHS FOR THE SCHUMANN-RUNGE BANDS OF MOLECULAR OXYGEN B. R. LEWIS and J. H. CARVER Research School of Physical Sciences, The Australian National University, Canberra, Australia 2600
and T. I. HOBBS, D. G. MCCOY,and H. P. F. GIES Department of Physics, University of Adelaide, Australia 5001 (Receiwd 11 January 1980) Abstract-Predissociation linewidths are presented as a function of rotation for the (3-0)-(14-0) SchumannRunge bands of molecular oxygen. While there may be a slight tendency overall for the linewidths to increase with rotation, it is shown that previous assumptions of linewidths constant with rotation are generally valid within the experimental error for the range of rotation studied, N”~21. There is no evidence for the sudden increase in linewidth with rotation reported elsewhere. INTRODUCTION
The B3EU- state of molecular oxygen, which gives rise to the Schumann-Runge bands, is predissociated.’ Qualitative evidence of the line broadening was first seen by Wilkinson and Mulliken’ and Carroll3 and there have since been three distinct sets of linewidth measurements taken. Ackerman and Biaume4 made photographic estimates of linewidths for the (O-O)- (19-O) bands, while Hudson and Mahle’ applied a fitting procedure to the photoelectric results of Hudson and Carter6 to obtain linewidths for the (2-0)-(16-O) bands. Frederick and Hudson7 have recently reanalyzed the results of Hudson and Carter6 to provide new estimates of Iinewidths for the (2-0)-(13-O) bands. Lewis et al.gg have presented new measurements of linewidths for the (2494 14-O)bands using a photoelectric technique with a curve of growth type of analysis. Schaefer and Miller” and Murrell and Taylor” have discussed the predissociation theoretically, and Julienne and Krauss,” and more recently Julienne13 have calculated theoretical linewidths which may be compared with the experimental values. The Schumann-Runge bands of molecular oxygen play an important role in the atmospheric absorption of solar radiation from 1750 to 2000 A. Nicolet and Peetermansr4 review this problem and emphasize that exact knowledge of linewidths is required for precise calculations where the atmospheric optical depth is much above unity. The possible variation of linewidth with rotation was first investigated theoretically by JulienneI who performed a limited number of calculations giving the centrifugal distortion in the 3ZU--3C,‘F2 partial width for a particular model interaction. Because of the number of possible factors involved in width variation with rotation, the problem is complex and no overall theoretical predictions have been made. Frederick and Hudson’ have presented linewidths for selected rotational lines from the (8-O) to (13-O) Schumann-Runge bands in an attempt to analyze linewidth variation with rotation. Their results show a general tendency for larger widths with higher rotation, with no significant variation observed for some bands. Their (11-O) results show a sudden doubling of linewidth at J = 17, an unexpected effect. Lewis et al? have stated that it was not possible to detect statistically significant rotational variation of linewidth in their results. In this paper the individual linewidths measured by Lewis et ai.‘*9as a function of rotation are presented for the (3-0)-(14-O) bands and it is confirmed that, while there is a slight tendency overall for the linewidths to increase with rotation, the assumption of linewidths constant with rotation is generally valid within the experimental error. The measurements do not show the sudden increase in linewidth observed by Frederick and Hudson’ in the (11-O) band. 365
366
B.R.LEWIS
et a/.
EXPERIMENTALMETHODANDDATAANALYSIS
The apparatus and experimental method have been fully discussed elsewhere.s~9Background radiation provided by a thyratron triggered hydrogen discharge was dispersed by the Adelaide 6m monochromator” and monitored by a gated photomultiplier detection system before entering and leaving the windowed absorption cell. Typical wavelength resolution was 0.06 A. Scans were performed across the absorption line of interest at two widely differing pressures and the resultant equivalent widths recorded. This technique allowed the simultaneous determination of both oscillator strength and predissociation linewidth by an iterative procedure.8 All factors considered in the iterative computer program enabling the linewidth determinations have been discussed in detail elsewhere. eP9Briefly, rotational absorption lines in the Schumann-Runge system exhibit both thermal and predissociation broadening, the relative importance of these two effects being expressed in terms of the mixing parameter u.~ The low pressure scan (approaching the linear region of the curve of growth) produced an equivalent width which was more sensitive to oscillator strength than to a. Assuming an initial approximation for a, the oscillator strength was then calculated for this scan. The high pressure scan’s equivalent width was more sensitive to a than to oscillator strength, and thus a refined value for a could be determined from the high pressure scan using the oscillator strength determined from the previous scan. The calculational loop was then repeated until a stable pair of results, (oscillator strength, a) was produced. Absorption spectra calculated using these results were then compared with the experimental scan spectra as a final check on the accuracy of the procedure. Linewidths obtained using this procedure for 119 individual lines or unresolved groups of lines from the (3-O) to (14-O) Schumann-Runge bands are presented in this paper. RESULTS
The predissociation linewidths measured in this work are shown in Table 1 as a function of N” together with estimated standard deviations. The unmarked values were taken from unresolved P (N”) R(N”+ 2) pairs, those marked (*) are the average of two separate P (IV”),R (N”) measurements, those marked (t) were taken from resolved R (N”) lines, while those marked (tt) refer to single measurements on P (N”) lines. The results are also displayed graphically in Figs. 1 and 2 together with the results of Frederick and Hudson.’ The continuous lines in Figs. 1 and 2 give the 3Cu--3E,,+F2 partial widths of JulienneI for the (7-O) and (9-O) bands, including centrifugal distortion. It is therefore apparent that these values are not strictly comparable with the experimentally determined total widths, but they have been displayed because the 3Cu--3E,,+partial width is the theoretically major contributor to the total predissociation widths for these two bands.13 The overall trends in the rotational variation of the present results were studied by dividing the widths into two groups for each band, those for N” c 11 and those for N” > 11. Using this technique, it has been found that, neglecting statistical significance at this stage, a decrease in linewidth with rotation is observed for three bands [(6-O), (10-O), (14-O)] and an increase for the others. If a student’s t statistical analysis is performed on the two groups of results, it is found that no significant variation of linewidth with rotation is observed at the 5% level for ten of the twelve bands studied. The remaining two bands [(9-O), (12-O)] exhibit a significant increase in linewidth with rotation (at the 5% level). Of the ten bands exhibiting no significant variation, the tendency towards an increase is largest for the (7-O) and (8-O) bands. The shape of the variation for the (140) band is not significant and could perhaps be explained by noting that the triplet splitting factors are not well known for the lower rotational lines in this band. Because of the low (14-O) predissociation widths, uncertainty in the triplet splitting may produce non-negligible errors in the determined linewidths. This factor is not important for the broader lines of the lower vibrational bands. In comparing the results of this work with those of Frederick and Hudson,’ it should be noted that we assume that the J given by these authors is in fact N”. It is seen from Figs. 1 and 2 that the agreement between the rotational behaviour observed by Frederick and Hudson’ and ourselves is quite good for the (8-0)-(10-O) and the (13-O) bands. Disagreement for the (12-O) band is quite marked, as is that for the higher rotational lines of the (11-O) band for which Frederick and Hudson’ observe a discontinuous increase of 100% at J = 17. Our results
3
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1
1.85*0.20 1.43t0.20 0.749to.11 0.664fO.li l.oito.i4 0.64tO.O; 0.171fO.O~ 0.070*0:02 t 1.57+0.20 1.61iO.20 0.627kO.li 0.606+O.li l.olfo.i4 0.64tO.O; 0.173~0.03
, 4.14t0.90 2.61t0.60 1.66-+0.16 1.45tO.20 1.25f0.20 0.516f0.15 0.766f0.15 0.90~0.20 0.60+0.0; 0.154tn.01 0.090*0:02 1.33tO.20
0.125iO.03 0.063~0.03 . . 1.4OtO.20 3.59tO.90 1.32tO.60 1.45+0.16 1.29t0.20 1.45+0.20 0.459+0.15 0.752+0.15 0.67kO.20 0.54+0.07 0.127+0.02 0.076iO.02
0.90~0.20
'
9
7
5
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Table 1. Predissociation linewidths, W, (FWHM, cm-‘), measured in this work as a function of rotation for the (3-0)-(14-O)bands. Unmarked values refer to mean linewidths from unresolved P(W) R(N” + 2) doublets; those marked C*)refer to the mean linewidths from two distinct P(W). R(W) measurements, those marked (t) refer to sin& measurements on RCN”),lines, while those marked ittj iefkr tb single measurements on P(N”) lines.
368
B. R. LEWIS et al.
f
THIS
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FREDERICK
WORK AND
-JULIENNE (5-
HUDSON
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(1976)
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1
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5
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1
2’
Fig. 1. Predissociation linewidths, W, (FWHM, cm-‘), measured in this work for the (3-0~@-O) Schumann-lunge bands as a function of rotation. Values obtained by Frederick and Hudson,’ and the theoretical centrifugal distortion predicted by Julienne” are also shown where applicable.
f
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-
JULIENNE
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(1976)
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N”
Fig. 2. Predissociation linewidths, W, (FWHM, cm-‘), measured in this work for the (9-0)-(14-O) Schumann-Runge bands as a function of rotation. Values obtained by Frederick and Hudson,’ and the theoretical centrifugal distortion predicted by Julienne” are also shown where applicable.
show no increase at all. For the lower bands, Frederick and Hudson’ are unable to present rotational variation because of the narrow range of conditions enabling them to determine linewidths. The equivalent width analysis used in this work does not suffer from this limitation. Most of the low rotation linewidths presented by Frederick and Hudson’ in Table 3 of their paper have quite large random errors and this probably accounts for the poor agreement, in general, between their bandhead values and those of this work. Several points should be noted regarding the work of Frederick and Hudson.’ Firstly, the oscillator strengths which they present show greatest deviations from the smooth theoretical curve of Allison et ~1.‘~for the (11-O) and (12-O) bands. This is where the largest discrepancies exist between their linewidths and those of this work, their oscillator strengths being lower,
Rotational variation of predissociation linewidths
369
their linewidths higher. Perhaps the convergence of their program is more sensitive to statistical errors in this region. Secondly, no mention is made by Frederick and Hudson’ of the variation of band oscillator strength with rotation, a significant effect which has been extensively measured by Lewis et ~1.~”Their published values’ may be bandhead values only, but they should have been able to deduce values for the higher rotational lines for which they presented the linewidth variation. Comparing the results of this work with those of Julienne,13 it is seen that the increase in the (9-O) 3X,--3X,+ F2 partial width due to centrifugal distortion is reflected in the statistically significant increase in total width with rotation measured for the same band in this work. The theoreticali very slight decrease for the (7-O) band is not verified by the experimental results which show no statistically significant change from a constant value at the 5% level. However, if anything, there is a tendency towards higher widths for higher rotation in this band. The comparisons with the results of JulienneI are only included as a guide and must be interpreted with caution for the reasons mentioned earlier. The present results would tend to indicate that, for the (7-O) and (9-O) bands, the predissociation due to the 3EU+state is the sole significant contributor to the total predissociation width. CONCLUSION
An accurate knowledge of Schumann-Runge band predissociation linewidths is vital when considering the absorption of solar ultraviolet radiation in the atmosphere. Whereas it has previouslys” been demonstrated that the quite considerable decrease in band oscillator strength with rotation should be taken into account when dealing with this problem, this work shows that the variation with rotation of the predissociation linewidths is not nearly so significant. Of the twelve bands studied, nine show a tendency towards increased linewidths with rotation, but only two of these [(9-O), (12-O)] show statistically significant increases at the 5% level. For the remainder the assumption of constant linewidth with rotation is a valid one. It is clear that the present results and those of Frederick and Hudson’ are not sufficiently accurate statistically for an unambigous detection of the small rotational deviations in linewidth expected for N” G 21. It is planned to examine this problem further with precise measurements on a single Schumann-Runge band, with some attempt to study higher rotational lines with N”>21. Acknowledgements-The authors would like to thank the Australian Research Grants Committee for its support and F. A. Smith for his valuable assistance. One of us (B. R. L.) held a Queen Elizabeth II Fellowship during the course of this work. REFERENCES 1. P. Krupenie, J. Phys. Chem. Ref. Data 1,423 (1972). 2. P. Cl. Wilkinson and R. S. Mulliken, Astrophys. 1. 125,594 (1957). 3. P. K. Carroll, Astrophys. 1. 129,794 (1959). 4. M. Ackerman and F. Biaume, J. Molec. Spectrosc. 35,73 (1970). 5. R. D. Hudson and S. H. Mahle, J. Geophys. Res. 77,2902 (1972). 6. R. D. Hudson and V. L. Carter, J. Opt. Sot. Am. 58, 1621(1968). 7. J. E. Frederick and R. D. Hudson, J. Molec. Spectrosc. 74,247 (1979). 8. B. R. Lewis, J. H. Carver, T. I. Hobbs, D. Cl. McCoy, and H. P. F. Gies, JQsRT 20, 191(1978). 9. B. R. Lewis, J. H. Carver, T. I. Hobbs, D. G. McCoy, and H. P. F. Gies, JQSRT 22,213 (1979). IO. H. F. Schaefer and W. H. Miller, J. Chem. Phys. 55,4107 (1971). Il. I. N. Murrell and J. M. Taylor, Molec. Phys. 16, 609 (1%9). 12. P. S. Julienne and M. Krauss, J. Molec. Spectrosc. 56,270 (1975). 13. P. S. Julienne, J. Molec. Spectrosc. 63, 60 (1976). 14. M. Nicolet and W. Peetermans, Planet. Space. Sci. 28,85 (1980). IS. J. H. Carver, G. N. Haddad, T. I. Hobbs, B. R. Lewis, and D. G. McCoy, Appl. Opt. 17,420 (1978). 16. A. C. Allison, A. Dalgamo, and N. W. Pasachoff, Planet. Space Sci. 19, 1463(1971).