Total Absorptance of Carbon Monoxide and Methane in the Infrared Darrell E. Burch and Dudley Williams The total absorptance f A(v)dv of the major infrared bands of carbon monoxide and methane has been measured as functions of absorber concentration w and equivalent pressure P, over wide ranges of these variables. The experimental results are presented graphically, and empirical equations relatingf A(v)dv, w, and Pe are presented. By employing small values of w and large values of P,, it has been possible to determine the band strengths or intensities f k(v)dv for the fundamental band of carbon monoxide and for V2, P, and 4 fundamentals investigators. The present paper is the second in a series of papers dealing with the total absorptance of gases found in planetary atmospheres. The laboratory techniques involved in the study of synthetic atmospheres consist- ing of pure samples of absorbing gases and samples consisting of absorbing gases mixed with nitrogen have been described in the first paper in the series.' The symbols and nomenclature employed in the present paper are those introduced in the first paper and in a still earlier article dealing with absorption line broad- ening in the infrared. 2 Total absorptance of carbon monoxide and methane will be discussed in the present paper. Migeottel first identified CO as a permanent atmos- pheric constituent on the basis of solar spectra obtained at Columbus, Ohio, and later verified his findings at the high-altitude laboratory located at the Jungfraujoch. Several investigators including Shaw 4 have subse- quently made detailed spectroscopic studies of CO abundance and report a concentration of approximately 0.1 atm cm per air mass. Although it has long been known that CH 4 is present in large quantities in the atmospheres of the major outer planets of the solar system, not until Migeotte's Ohio work of 1948 was The authors are of the Laboratory of Molecular Spectroscopy and Infrared Studies, Department of Physics and Astronomy, The Ohio State University. The present address of D. E. Burch is Aeronutronic Division, Ford Motor Company, Newport Beach, California. D. Williams is a National Science Founda- tion Senior Postdoctoral Fellow at the University of Li6ge. Received 16 January 1962. This work has been supported in part by Geophysics Research Directorate, Air Force Cambridge Research Laboratories. of methane; the values obtained are compared with results of other CH 4 shown to be a permanent constituent of the telluric atmosphere.' The average abundance of CH 4 in the earth's atmosphere has subsequently been found to be 1.6 parts per million by volume. Although CO and CH 4 are relatively minor telluric atmospheric constituents and have relatively little influence on the earth's heat balance, their absorption bands in the infrared occur for the most part in "atmos- pheric windows" between the strong bands of carbon dioxide and water vapor. Therefore, CO and CH 4 bands are of considerable practical importance in limiting infrared transmission through the "windows" in which they occur. Carbon Monoxide Absorptance The fundamental band of CO is located at 2143 cm-' near the 3 fundamental of N20. The major absorption by CO in this region is due to C20 6 ; however, there is some absorption due to less abundant isotopic species and to some difference bands, which are very weak at room temperature. For the purposes of the present study, all these overlapping absorption bands will be referred to as the 2143 cm-' band. Approximately 180 records of spectral absorptance in this region were made with a double-beam spectrometer employing absorption cells of length 1.55, 6.35, and 400 cm. Dif- ferent combinations of CO and N2 were used to provide absorber concentrations w ranging from 9.6 X 10-4 to 45.6 atm cm and total pressures ranging from 3 to to 3000 mm Hg. In Fig. 1 are shown several records of spectral ab- sorptance in the vicinity of the CO fundamental; all records were obtained with the same spectrometer September 1962/ Vol. 1, No. 5 / APPLIED OPTICS 587
Transcript
Total Absorptance of Carbon Monoxide and
Methane in the Infrared
Darrell E. Burch and Dudley Williams
The total absorptance f A(v)dv of the major infrared bands of carbon monoxide and methane has beenmeasured as functions of absorber concentration w and equivalent pressure P, over wide ranges of thesevariables. The experimental results are presented graphically, and empirical equations relatingf A(v)dv,w, and Pe are presented. By employing small values of w and large values of P,, it has been possibleto determine the band strengths or intensities f k(v)dv for the fundamental band of carbon monoxideand for V2, P, and 4 fundamentalsinvestigators.
The present paper is the second in a series of papersdealing with the total absorptance of gases found inplanetary atmospheres. The laboratory techniquesinvolved in the study of synthetic atmospheres consist-ing of pure samples of absorbing gases and samplesconsisting of absorbing gases mixed with nitrogen havebeen described in the first paper in the series.' Thesymbols and nomenclature employed in the presentpaper are those introduced in the first paper and in astill earlier article dealing with absorption line broad-ening in the infrared.2 Total absorptance of carbonmonoxide and methane will be discussed in the presentpaper.
Migeottel first identified CO as a permanent atmos-pheric constituent on the basis of solar spectra obtainedat Columbus, Ohio, and later verified his findings at thehigh-altitude laboratory located at the Jungfraujoch.Several investigators including Shaw4 have subse-quently made detailed spectroscopic studies of COabundance and report a concentration of approximately0.1 atm cm per air mass. Although it has long beenknown that CH4 is present in large quantities in theatmospheres of the major outer planets of the solarsystem, not until Migeotte's Ohio work of 1948 was
The authors are of the Laboratory of Molecular Spectroscopyand Infrared Studies, Department of Physics and Astronomy,The Ohio State University. The present address of D. E. Burchis Aeronutronic Division, Ford Motor Company, NewportBeach, California. D. Williams is a National Science Founda-tion Senior Postdoctoral Fellow at the University of Li6ge.
Received 16 January 1962.This work has been supported in part by Geophysics Research
Directorate, Air Force Cambridge Research Laboratories.
of methane; the values obtained are compared with results of other
CH4 shown to be a permanent constituent of the telluricatmosphere.' The average abundance of CH4 in theearth's atmosphere has subsequently been found to be1.6 parts per million by volume.
Although CO and CH4 are relatively minor telluricatmospheric constituents and have relatively littleinfluence on the earth's heat balance, their absorptionbands in the infrared occur for the most part in "atmos-pheric windows" between the strong bands of carbondioxide and water vapor. Therefore, CO and CH4 bandsare of considerable practical importance in limitinginfrared transmission through the "windows" in whichthey occur.
Carbon Monoxide Absorptance
The fundamental band of CO is located at 2143 cm-'near the 3 fundamental of N20. The major absorptionby CO in this region is due to C20 6 ; however, thereis some absorption due to less abundant isotopic speciesand to some difference bands, which are very weak atroom temperature. For the purposes of the presentstudy, all these overlapping absorption bands will bereferred to as the 2143 cm-' band. Approximately180 records of spectral absorptance in this region weremade with a double-beam spectrometer employingabsorption cells of length 1.55, 6.35, and 400 cm. Dif-ferent combinations of CO and N2 were used to provideabsorber concentrations w ranging from 9.6 X 10-4to 45.6 atm cm and total pressures ranging from 3 toto 3000 mm Hg.
In Fig. 1 are shown several records of spectral ab-sorptance in the vicinity of the CO fundamental; allrecords were obtained with the same spectrometer
A(v), expressed as "Percent Absorption," in the vicinity of the CO fundamental at 2143 cm-l.
slitwidths, which corresponded to approximately 25cm-'. The records in the left-hand panels of the figurewere obtained with a single value of absorber concentra-tion w and show the effect of increasing the total pres-sure P by the addition of N2. The records in the right-hand panel show results obtained by simultaneous in-crease of w and P. Values of equivalent pressure P,were obtained from the partial pressure p of CO and thetotal pressure P by the expression P, = P + 0.02p,which is based on the self-broadening coefficient B
Fig. 2. Total absorptance jA(v)dv of the 2143 cm-' band forvarious values of absorber concentration w as a function ofequivalent pressure P-.
Many of the experimental data are plotted in Fig. 2,which gives total absorptance fA (v)dv as a function ofequivalent pressure P. Each curve shown in thefigure corresponds to the fixed value of absorber con-centration listed and shows the effect of increasingthe equivalent pressure by the addition of N2. CurvesF, G, H, and I represent data obtained with the samerange of absorber concentrations as the data representedby curves B, C, D, and E; in spite of the large differ-ences in the absorption paths employed, the two setsof curves agree to 43%.
The general features of the curves in Fig. 2 are similarto those discussed in some detail for the 2224 cm-'
2 0 band in the earlier article.I Some of the curveshave portions that are nearly straight and parallelto one another; the slope of these curves is 0.44 andindicates that f A (v)dv oc PF2-4 4 for samples with P,and w within the ranges covered by the linear portionsof the curves. The value of the exponent 0.44 is greaterthan the corresponding value of 0.38 obtained from thenearby N20 band but is somewhat less than the valueof 0.5 to be expected on the basis of simple theory.It will be noted that there is a clustering of curves inthe upper right-hand corner of Fig. 2 in the regionof large w and large P, due to the fact that k(v)u > 1for most frequencies within the band. There is agradual decrease in the slopes of the upper curves withincreasing PI but the "saturation effects" of the typenoted for the 2224 cm-' N20 band 1 are not nearly sopronounced in the case of CO. This is presumablydue to the smaller line half-width at a given pressureand to the greater separation of rotational lines in the
spectrum of CO, resulting in the persistence of consid-erable rotational fine structure of the band even atPe = 3000 mm Hg. On the basis of Shaw's work,4 thehalf-widths of the lines would be expected to be equalto the line spacing for values of Pe equal to approx-imately 36,000 mm Hg; in this pressure range rota-tional fine structure would be nearly eliminated andcurves such as those in Fig. 2 would converge and havenear zero slope. The saturation effects for the lowestcurves of Fig. 2 corresponding to low values of w areof the type to be expected for "completely broadened"spectral lines and for lk(v)w < 1 for all frequencies inthe band.
It should be noted that the curves in Fig. 2 are plottedfor samples having constant absorber concentration.Related plots for samples of pure CO, in which isproportional to Pe, give curves with linear portionshaving slopes of 0.98. Therefore, in these linear por-tions f A(v)dv wz'PF, where m + n = 0.98. In viewof the value 0.44 for the slope of the curves in Fig. 2,it would appear that m = 0.54 and n = 0.44. Thisstronger dependence of f A ()dv on w than on Pe issimilar to the results obtained for N20 in the earlierstudy.
Figure 3 shows curves of f A (v)dv as a function of wfor various values of Pe. These curves are useful inmaking estimates of total absorptance for samples hav-ing any values of w and Pe within the entire rangecovered in the present investigation, since interpolations
I1 I I I I I I I 1 1 I I I I I I- i 7 - - - -I I
100- CO
2143 cm-'
A B
I0
0 Z
between the various curves can easily be made. Thelower portion of the uppermost curve, which cor-responds to Pe = 3000 mm Hg, is linear and has a slopeof unity, indicating that f A (v)dv is directly propor-tional to w. From this portion of the curve it is pos-sible to determine the value of band intensity f k(v)dpin terms of measured values of f A (v)dv and w. Thevalue of band intensity of the CO fundamental is
f k(v)dv = 260 (atm cm)-' cm-'. (1)
This value, which is believed to be accurate to :i 10%,can be compared with the value 237 reported by Pennerand Weber,6 who apparently did not reduce their valuefor w to STP. If the values of w in Fig. 3 had notbeen corrected to standard temperature, the valuewould have been 235, a result in nearly perfect agree-ment with the Penner-Weber value.
Although the curves in Fig. 3 tend to converge forlarge and small values of w, the five lower curves arelinear and nearly parallel over a wide range of w nearthe centers of the curves. The slope of the parallelportions of these curves is approximately 0.55; thus,for the range of w involved, f A (v)dv w0 -55 . This re-sult is in excellent agreement with the relation f A (v)dv
w0W54PO0 44proposed above. In view of this agree-ment, an attempt was made to obtain empirical equa-tions relating f A(v)dv to w (wP,08)o55 byplotting f A (v)dv versus wPe8. It was found thatf A(v)dv can be expressed to an accuracy of 5%
w in atmos cm
Fig. 3. Total absorptance of the 2143 cm- 'band for various values of equivalent pressure as a function of absorber concentration.
Fig. 4. Total absorptance of the 2143 cm-' band for various values of equivalent pressure as a function of the product of absorberconcentration and equivalent pressure.
by the relation
fA(v)dv = 2.75(wPO8).055 (2)
for 4 < f A(v)dv < 40 cm-' if 20 < PF, < 250 mm Hgand for 10 < fA(v)dp < 40 cm-' if 20 < PF < 760 mmHg. In (2) and in similar subsequent equations w isexpressed in atm cm.
From the standpoint of existing theory, it is desirableto attempt to find relationships between f A (v)dv andwP,. With this in view, the plots of f A (v)dv versuswP, shown in Fig. 4 were prepared. Over the majorportion of the figure, the curves corresponding to highvalues of P, lie to the right of the curves for low valuesof PF, as one might expect in view of (2). However,in the upper right-hand corner the curves tend to con-verge. Complete convergence would indicate thevalidity of the "strong-line approximation"'; on the
100
C)
._0-10
10 100
w in atmos cmFig. 5. Total absorptance of the 4260 cm-
CO for various values of equivalent pressureabsorber concentration.
basis of the experimental results plotted in Fig. 4,it can be concluded that the strong-line approximation isaccurate to less than 5% error only for values of w solarge that absorption within the band limits is nearlycomplete, and the variation of f A (v)dv with w and P,is slight. However, for lower values of P, the approx-imation introduces an error less than 20% over a ratherwide range of variable wP,. The range over which theapproximation is valid to a given accuracy can be madelarger by limiting its use to lower pressures.
On the basis of the plots in Fig. 4, the followingemperical equations were developed:
f A(v)dv = 3.20(wP,)G-43 (3)
for 40 < f A(v)d < 120 cm-' and 20 < P, < 760 mmHg and
f A(7)dv = -106 + 61 log (wP,) (4)
for 120 < fA(v)dv < 200 cm-' and 20 < P, < 3000mm Hg. The empirical equations (3) and (4) are
0 ~ believed to be accurate to -t 10%, except near the endsof their ranges of validity where errors as large as
0°/ zz =15% may be encountered. Extrapolations of (2),
(3), and (4) should be avoided.The CO overtone band at 4260 cm-' was investigated
by means of a double-pass, single-beam Model 99Perkin-Elmer instrument with spectral slit widthsbetween 15 and 20 cm-'. A multiple-traversal cellset to give absorption paths of 625, 1232, 2445, and
1Q00 4875 cm was employed. On the basis of earlier studiesof CO self-broadening in the overtone band, equivalent
overtone band of pressures were obtained from the expression P, =
as a function of P + 0.08p. This relation employs the value of B= 1.08 for the overtone, which is slightly larger than
the value B = 1.02 for the fundamental; these meas-ured values of self-broadening coefficients are believedto be accurate to 4 6%.
In view of the fact that the overtone is much weakerthan the fundamental, curves for the overtone cor-responding to those in Fig. 2 for the fundamental showno saturation effects for the ranges of w between 37and 1140 atm cm and Pe between 50 and 760 mm Hgcovered in the present work. Plots of f A (v)dv versusw for various values of P, are shown in Fig. 5. In com-paring Fig. 5 with Fig. 3, which gives correspondingplots for the fundamental, it is interesting to note that,at a given value of Pe, f A (v)dv is the same for the twobands provided w for the overtone is 150 times thatfor the fundamental. In fact, the total absorption forthe overtone can be determined from the curves in Fig. 3by using the same value of P, and a value of w that is1/150 that actually used for the overtone; Eqs. (1),(2), (3), and (4) are probably applicable to the over-tone provided w/150 is substituted for w in the equa-tions.
Although the band intensity of the overtone was notdetermined directly in the present study, the resultsjust cited would appear to be in close agreement withthe earlier work of Penner and Weber,6 who foundthat f k(v)dv for the fundamental was 145 times thatof the overtone.
Methane Absorptance
The portion of the CH4 spectrum investigated in thepresent study consists of the strong V3 fundamental at3020 cm-', the strong 4 fundamental at 1306 cm-',and the much weaker 2 fundamental first definitelyobserved at 1534 cm-' by Burgess, Bell, and Nielsen7
and discussed more recently by Armstrong and Welsh.'The methane spectrum is more complicated than thoseof N20 and CO, since each band has a Q-branch and acomplex line structure. 9
z2 20
A Pin mm Hg078Fig 40 26i
mental0V3 at 3020 cm020
3400 3200 3000 2800 2600 2400 2200
WAVENUMBER in cm-t
Fig. 6. Spectral absorptance of C 4 in the vicinity of its funda-mental v at 3020 cm '
Z20 w= -23 8 atmos cm120 0 1
30 iinmm Hg
0 17.3mOn40O 55
CO 309A r t 10 r748
H 0 3040
LU
a0 80
90100
1800 1600 1400 1200WAVENUMBER in cm-'
Fig. 7. Spectral absorptance of CH4 in the vicinity of itsstrong V fundamental at 1306 cm' and its weak 2 fundamentalnear 1534 cm-'.
Approximately 150 records of the spectral absorp-tance of CH4 were made in the spectral region between3400 and 700 cm'. Absorption path lengths of 6.35,400, and 1600 cm were used to obtain data on sampleshaving absorber concentrations in the range 0.015 to188 atm cm and total pressures in the range 3 to 3000mm Hg. Virtually all the observable absorption occursin two spectral intervals, one between 3400 and 2200cm-' called the 3020 cm-' band and the second between1750 and 1100 cm-'. Typical records of spectral ab-sorptance in the 3020 cm-' region are given in Fig. 6,which shows the effect of increasing the total pressureby adding N2 to a cell in which the absorber concentra-tion is kept constant; the spectral slit width was ap-proximately 25 cm-'. Figure 7 presents similar datafor the second region of absorption obtained with aneffective slit width of approximately 10 cm-'. Someof the spectral absorptance curves shown in Figs. 6and 7 have been corrected for spurious effects due toCO2 and H20 vapor present in the spectrograph andas slight impurities in the CH4 samples.
It will be noted from Fig. 7 that the weak 2 funda-mental overlaps the much stronger 4 fundamental.For the purposes of the present study, the 1750-- 100cm-' region of absorption was arbitrarily dividedat 1535 cm-' into what is referred to as the 1306 cm-'band and the 1550 cm-' band; this procedure adds theweak absorption due to the P-branch of 2 to the muchstronger overlapping V4 band.
Earlier studies2 gave a value B = 1.30 for the self-broadening coefficient of CH4 in the 3020 cm-' bandand B = 1.38 for the 1306 cm-' band. Therefore,equivalent pressures were calculated from P = P+ 0.30p for the 3020 cm-' band and from P, = P± 0.38p for the 1306 cm-' band. The second expres-sion was also used to obtain P, for the neighboring1550 cm-' band, since no independent value could beobtained for this very weak band.
Fig. 8. Total absorptance of the 3020 cm-l band of CH4 for various values of equivalent pressure as a function of absorber concentration.
f A (v)dv for various values of w as a function of pres-sure on a logarithmic scale are nearly parallel and linearfor intermediate values of P. The slopes of the linearportions of these curves have the value 0.22, which isconsiderably smaller than the corresponding slopes forN2 0 and CO bands; this value indicates that f A (v)dv
p,,o. 22 for intermediate values of pressure. Thereis little indication of "saturation" at high pressures forlarge values of absorber concentration. An unusualfeature of these logarithmic plots for low pressures isthat the slopes of some of the curves tend to increaseital increasing pressure for values of P as high as 75
mm Hg, which is considerably greater than pressuresat which Doppler broadening is important. Thisunusual dependence of f A (v)dv on P,, may result fromthe fact that the 3020 cm-' band consists of strong linesclustered in widely separated groups and other muchweaker lines occurring throughout the band. If the"clusters" of strong lines can be considered as singlestrong Elsasser lines,' their total absorptance mightshow a square-root dependence on pressure for lowpressures at which the total absorptance due to themuch weaker lines is nearly independent of pressure.With increasing pressure, the relative contributionof the pressure-dependent strong lines to the total bandabsorptance might increase; this phenomenon couldaccount for the observed effects. This possible ex-planation could be checked by studies made with aspectrograph having high resolving power.
Plots of f A (v)dv versus absorber concentration wfor various values of P, are shown in Fig. 8, from whichf A (v)dp can be obtained by interpolating between thecurves for any value of w and P, in the ranges coveredin the investigation. The linear portions of the curves
have slopes of 0.55, indicating that f A (v)dv cc 55
for the ranges of w and P,, involved. It was found thattotal absorptance for the 3020 cm-' band can be ex-pressed by means of the following equations, in whichw is given in atm cm:
fA(P)dp = 15.5(wP0-4)0.55 (5)
for 15 < f A(v)dv < 250 cm-' and 10 < P, < 760 mmHg, and
f A(v)dp = -375 + 272 log (wPe0-4) (6)
for 250 < f A(P)d < 500 cm-' and 10 < P < 3000mm Hg. Values of f A(v)dp given by (5) and (6)are accurate to 10%, except for values of f A (v)dv< 40 cm-', for which errors as great as i+15% are en-countered.
It will be noted from Fig. 8 that even for the smallestvalue of w and the highest value of P, the total absorp-tance increases with increasing P,,. Thus, the require-ment that k(v)w < 1 is not fulfilled even for P =3000 mm Hg and the smallest values of w; hence, avalue of band intensity f k(v)dv cannot be obtaineddirectly from the plot. However, by extrapolating theuppermost curve to smaller values of w with the require-ment that its maximum slope be unity, an estimate ofband intensity can be made. With w reduced to STP,the result is
fA(v)dv = 320 (atm cm)-' cm-' (7)
for the 3020 cm-' band. This value, which is believedto be valid to i 15%, is approximately 12% less thanthe value of 360 cm-' reported by Welsh, Pashler,and Dunn" and 2.5% less than the value of 328 re-ported more recently by Armstrong and Welsh.8
w in atmos cmFig. 9. Total absorptance of the 1306 cm-I band of CH4 for various values of equivalent pressure as a function of absorber concentration.
When values of f A(v)dv for the 1306 cm-' band forvarious values of w are plotted as a function of Pe,some of the curves on the logarithmic plot have linearportions with a slope of 0.30 for intermediate values ofP, indicating that f A (v)dv Pe0 0 ; this pressure de-pendence is greater than that for the 3020 cm-' bandbut is considerably less than the square-root dependencepredicted by simple theory. At high pressures, there issome evidence of the onset of saturation for large valuesof w. At low pressures there is no evidence of theunusual features exhibited by the corresponding plotsfor the 3020 cm-' band. High-resolution studies9
indicate different structures for the two bands and henceit is not surprising that their pressure dependence isquite different.
Values of f A (v)dv for various values of P, are plottedas a function of w in Fig. 9 for the 1306 cm-' band.Considerable portions of some of the curves have a slopeof 0.48, indicating that f A (v)dv varies essentially asthe square root of the absorber concentration for theranges of P,, and w involved. In view of the observeddependence of total absorptance on P, and w, attemptswere made to derive empirical equations in terms ofwP 06 for intermediate values of f A (v)dv. The follow-ing equation gives correct values of total absorptanceto +4 15% of the experimental values:
fA(v)dv = 7.3(wP,,o.6)o.45
mm Hg. As usual, values of f A (v)dv for small valuesof wP, cannot be easily expressed in terms of an equa-tion, and direct use of Fig. 9 is necessary.
By extrapolating the curves of Fig. 9 to smallervalues of w with the requirement of unity as the max-imum slope for the top curve, the band intensity of the1306 cm-' CH4 was estimated to be
f k(v)dv = 185 (atm cm)-' cm-. (10)
This value, which is believed to be in error by less than+ 15%, is approximately 18% greater than the value of157 reported by Welsh and Sandiford1 ' and the valueof 158 reported more recently by Armst rong and Welsh.8
Data for the 1550 cm-' band are plotted in Fig. 10.This weak band is observable only for large values of
E
C
_0
(8)
for 20 < f A(v)dv < 130 cm-, and 10 < P < 760 mmHg and with w in atm cm at STP. For greater valuesof total absorptance, the following equation applies:
f A(v)dv = -190 + 115 log (wP,-6) (9)
for 130 < f A(v)dv < 300 cm-' and 10 < P, < 760
w in atmos cm
Fig. 10. Total absorptance of the 1550 cm-' band of CH4for various values of equivalent pressure as a function of absorberconcentration.
w, and the proximity of the strong 1306 cm-' bandtogether with the overlapping 2 band of atmosphericwater vapor makes determination of f A (v)dv for theweak band rather difficult. Values of f A (v)dv plottedin Fig. 10 are probably accurate to 410% for valuesgreater than 30 or 40 cm-' but may be in error by asmuch as 20%o for smaller samples. There wouldappear to be sufficient data to justify the relationf A(v)dv :Pc0- ,27 for certain ranges of w and P, butnone of the 'curves in Fig. 10 has a slope of 0.5 as do
corresponding plots for the other C 4 bands. Ap-parently the values of w employed were not sufficientlylarge for the exhibition of any "strong-line properties"by this band. No attempt to establish empirical ex-pressions for f A(v)dv for the 1550 cm-' band seemedjustified.
However, it was possible to obtain a rough estimateof band intensity for the V2 band of CH4. On the basisof values obtained for the 1550 cm-' band and on theassumption that the contribution of the P-branch, whichin the present study was included in the stronger ad-jacent 1306 cm-' band, is equal to the contribution ofthe R-branch to the total band intensity, it is estimatedthat f c(v)dv for the v2 band is between 2 and 3 (atmcm)-' cm-'. To the knowledge of the authors, the
only other published value is the value of 2.4 reportedrecently by Armstrong and Welsh.8
The authors wish to express their appreciation toWilbur France and E. B. Singleton, who assisted incertain parts of the work. One of the authors, (D.W.) wishes to express his appreciation to P. Swingsand M. Migeotte for the use of the library and otherfacilities of the Institut d'Astrophysique of the Uni-versite de Liege during preparation of the paper.
References1. D. E. Burch and D. Williams, Appl. Opt. 1, 473 (1962).2. D. E. Burch, E. B. Singleton, and D. Williams, Appl. Opt.
1, 359 (1962).3. M. V. Migeotte, Phys. Rev. 75, 1108 (1949).4. J. H. Shaw, Report 6, Contract AF 19(604)-1003, The Ohio
State University Research Foundation (1957).5. M. V. Migeotte, Phys. Rev. 73, 519 (1948).6. S. S. Penner and D. Weber, J. Chem. Phys. 19, 807 (1951).7. J. S. Burgess, E. E. Bell, and H. H. Nielsen, J. Opt. Soc.
Am. 43, 1058 (1953).8. R. L. Armstrong and H. L. Welsh, Spectrochim. Acta 16,
840 (1960).9. A. H. Nielsen and H. H. Nielsen, Phys. Rev. 48, 864 (1935).
10. H. L. Welsh, P. E. Pashler, and A. F. Dunn, J. Chem. Phys.19, 340 (1951).
11. H. L. Welsh and P. J. Sandiford, J. Chem. Phys. 20, 1646(1952).
Three atmospheric infraredders: R. M. Goody Director, BlueHill Observatory of Harvard University, K. Ya. Kondratiev Dept.of Atmospheric Physics, Leningrad State University, and J. N.Howard AFCRL, Bedford, Mass.
