Total Absorptance of Carbon Dioxide in the Infrared

Darrell E. Burch, David A. Gryvnak, and Dudley Williams

- Total absorptance f A(v)dv has been determined as a function of absorber concentration w and equivalentpressure Pe for the major infrared absorption bands of carbon dioxide with centers at 3716, 3609, 2350,1064, and 961 cm-. The results in the 875-495 cm' region are expressed in terms of mean spectral ab-sorptance (vv ) - = fv~l A(v)dv/(v - 2) for five separate subregions. The effects of temperaturevariations on absorption in someregionsarediscussed. Estimates of band intensityf k(v)dv are given foreach band and are compared with the results of others.

This paper is the third in a series dealing with thetotal absorptance of atmospheric gases as a function ofpressure and absorber concentration. The generalexperimental techniques involved as well as the sym-bols and nomenclature employed were described inearlier papers.1-3 The work on carbon dioxide reportedin the present paper is intended as a supplement to arelated study conducted several years ago by Howard,Burch, and Williams (HBW)4 ; in addition to the em-ployment of improved experimental techniques, thepresent study extends the HBW work to new ranges ofpressure and absorber concentration, to lower spectralfrequencies, and to the effect of temperature on certain"hot bands" produced by transitions from initial excitedvibrational energy states. In addition to these exten-sions of the earlier study, attempts have been made toobtain values of total absorptance for narrower spectralregions and, in some cases, to measure "mean spectralabsorptance" A ( 1 - P2) over spectral ranges bounded byPi and P2, a quantity obtained from total absorptanceas follows:

zT(v - V2) - A(v)dv. (1)V ! 2 v,2

The present extension and elaboration of the earlier

The authors were at the Laboratory of Molecular Spectroscopyand Infrared Studies, Department of Physics and Astronomy,The Ohio State University. Darrell E. Burch was a GeneralMotors Postdoctoral Research Fellow. His present addressand that of David Gryvnak is Aeronutronic Division, Ford Mo-tor Company, Newport Beach, California. Dudley Williams ispresently a National Science Foundation Senior PostdoctoralFellow, University of Liege.

Received 16 January 1962.This work was supported in part by Geophysics Research Di-

rectorate, Air Force Cambridge Research Laboratories.

HBW work is desirable in view of recent proposals forspectroscopic investigations of the terrestrial atmos-phere by spectrographs in balloons, jet aircraft, andearth satellites and for infrared studies of the atmos-pheres of other planets by space probes.

In the present work, the double-beam Model 21Perkin-Elmer spectrograph equipped with dual multi-ple-traversal cells was employed for path lengths of400 to 3200 cm. Shorter cells 1.55 and 12.8 cm inlength were used with a Perkin-Elmer Model 99 spec-trometer mounted in a vacuum tank to eliminate theeffects of absorption by atmospheric carbon dioxide andwater vapor; the effects of absorbing atmosphericgases within a spectrometer and the associated opticalpaths are not completely eliminated even by double-beam techniques and can introduce serious errors in thecase of samples producing small values of total absorp-tance. These errors become negligible for sampleswith larger values of absorber concentration. Thechoice of absorption-path lengths mentioned above wasmade in order to supplement the HBW work, in whicha 22-meter multiple-traversal cell was employed.

The values for equivalent pressure Pe were computedfrom the relation Pe = P + 0.30p based on the valueB = 1.30 for the self-broadening coefficient of CO2 ascompared with N2 obtained in the recent study of linebroadening in the infrared.' This value is based on aset of careful measurements and is in agreement withtheoretical studies of Kaplan' and Benedict.6 TheHBW results were presented in terms of a "weightedpressure," for which the corresponding equivalentpressure would be P = P + p, with B = 2. An evenlarger value corresponding to B = 2 + 0.5p (with pin atmospheres) was recently used by Edwards.7

The major infrared absorption bands of CO2 occur

November 1962 / Vol. 1, No. 6 / APPLIED OPTICS 759

o40 1 38 20 -1.38 228 -o

(1) 138 4\39m 60 38 821

i- 3 8 16506.08 514

i80986.08

C' 103400 3500 3600 3700 3800

WAVENUMBER in cm-'

Fig. 1. Spectral absorptance in the vicinity of the 3716 and3609 cm-' bands of CO2. The spectral slit width is approxi-mately 8 cm-l.

iii four separate spectral regions: (1) the "2.7 p region,"'in which the v, + v, combination band at 3716 cm-' andthe V3 + 2

V2 band at 3609 cm-' overlap the strong bandsof atmospheric water vapor; (2) the strong V3 funda-mental at 2350 cm-'; (3) the "hot bands" at 1064 and961 cm-'; and (4) the 875-495 cm-' region in whichthe fundamental vP band is overlapped by several"hot bands" with prominent Q-branches. More than369 records of spectral absorptance were made in thepresent study and results were obtained for wide rangesof w and P,. On the basis of the records, total absorp-tance was measured for the 3716, 3609, 2350, 1064,and 961 cm-' bands, separately. The measured valuesof total absorptance are estimated to be accurate to±5% for jfA(v)dv > 10 cm-'; to -t10% for 3 <f A(v)dv < 10 cm-'; and to 415% or 420% forf A(v)dp < 3 cm-'. In the case of the complex ofoverlapping bands in the 875-495 cm-' region, meanspectral absorptance was determined for five subregions.The estimated uncertainty in mean spectral absorptanceis ±45% for values greater than 0.10 and increases to±415% to ±420% for the smallest values measured.

"The 2.7 Region"

Fifty-four records of spectral absorptance were madein this region; typical records of spectral absorptancein the vicinity of the 3716 and 3609 cm-' bands areshown in Fig. 1. The vertical dashed line at 3660 cm-'represents the boundary employed in determining totalabsorptance of the separate bands. The choice of suchboundaries must be made with care, since the conditionfor ' A (v)dv for a band to be independent of slit widthholds only if the limits of integration include all absorp-tion due to the band. This condition is not fulfilledfor overlapping bands like those shown in Fig. 1, sincesome of the absorptance to the left of the dashed line isdue to the band on the right and some of the absorp-tance to the right of the dashed line is due to the bandon the left. However, if the spectral absorptance curveis "symmetrical" with respect to the boundary, errors

in proper assignment of total absorptance to the twobands are minimized. Values of the 3716 and 3609cm-' bands were determined separately, and values oftotal absorptance for the entire 2.7 region were alsomeasured; no difficulties were encountered in selectingintegration limits for the entire region.

Values of total absorptance f A (v)dv for variousvalues of P, are plotted on a logarithmic scale as afunction of absorber concentration for the 3716 and3609 cm-' bands in Fig. 2, parts (a) and (b), respec-tively. The curves in Fig. 2 give data for low valuesof w not employed by HBW. Similar plots of f A (P)dvfor the entire 2.7 p region are shown in Fig. 3; the solidcurves in this figure are based on the present data, whilethe dashed curves represent earlier HBW data.

On the basis of the plots in Figs. 2 and 3 and relatedplots showing f A (v)dv for various values of w as a func-tion of Pa, it is possible to develop empirical equationsgiving f A (v)dv in terms of P. and w. The empiricaequation applicable to the combined total absorptanceof the 3716 and 3609 cm-' bands is

fA(v)dv = 3.5 (Pc.65)0.58 (2)

for 20< P< 250mm Hg and 10< f A(v)dv< 50cm-'.Equation (2) can also be used for 20 < P < 760 mm Hgprovided 30 < f A()dv < 50 cm-'. In (2) and similar

E

.9

(a)100

lo.G

>

w in atmos cm

(b)

Fig. 2. Total absorptance as a function of absorber concen-tration for various values of equivalent pressure: (a) the 3716cm-' band; (b) the 3609 cm- band.

760 APPLIED OPTICS / Vol. 1, No. 6 / November 1962

Fig. 3. Total absorptance for the entire "2.7 1u region" versusabsorber concentration for various values of equivalent pressure.

equations shown later w is expressed in atm cm at STP;in calculating w the CO2 partial pressure was correctedto standard temperature. Values of fA (v)dv forvarious values of P0 and w can also be obtained by inter-polation between the curves of Fig. 3; this can be doneoutside the ranges of validity of (2) but within theranges of variables covered in the figure. Extrapola-tions should be avoided. It is believed that (2) to-gether with Fig. 3 can lead to more satisfactory predic-tions of fA (v)dv than the earlier HBW equations,which were based on relatively narrow ranges of w andP0 . Figure 2 provides additional information regard-ing the distribution of total absorptance within the2.7 region; this additional information was not pro-vided by HBW.

The 2350 cm-' Band

The major portion of CO2 absorption in the 2330cm-' region is due to the V3 fundamental of the C026molecule. However, absorption due to heavier isotopicspecies of CO2 leads to measurable spectral absorptanceand makes significant contributions to total absorp-tance, particularly in the region where the P-branch ofV3 for C120216 appears; the P3 band of C02" 6 is centeredat 2284 cm-'. Typical plots of spectral absorptanceare shown in Fig. 4 and were obtained with the effectiveslit widths equal to approximately 3.5 cm-'. Growthof the C 130216 band with increasing absorber concen-tration is evident. Ninety-four records of spectralabsorptance for different values of w and P were madein the 2350 cm-' region.

A plot showing f A ()dv for various values of P0as a function of w is given in Fig. 5. The solid curvesin the figure are based on the present data, while thedashed curves represent HBW data. Over intermedi-ate values of w some of the curves are nearly paralleland linear with a slope of 0.54, which is close to theslope of 0.5 that might be expected for strong lines.Corresponding plots of f A ()dv for various values ofw as a function of P exhibit curves having a slope of0.4. These results are in agreement with the previousstudies" 2 in this series, which show that fA (v)dv de-

pends less strongly on P0 than on w even in cases forwhich a square-root dependence might be expected forboth parameters if one did not take into account thecontribution of the weak lines in the band.

It is found that total absorptance of the 2350 cm-'band can be expressed in terms of the empirical equation

f A(v)dv = 15.0 (WP.0.75)0.54 (3)

for 6 < f A()dv < 50 cm-' and 10 < Pe < 250 mmHg. This equation is also valid for 10 < P < 760 mmHg provided 15 < f A(v)dv < 50 cm-'. For values ofP0 outside the above ranges, values of f A (v)dv forvarious values of w can be determined by interpolationbetween the curves in Fig. 5.

The 1064 and 961 cm-' "Hot Bands"The absorptance of the 1064 and 961 cm-' bands is of

negligible importance in practical studies of the role ofthe atmosphere in maintaining the earth's heat balance.However, these bands could be of some importance inthe atmospheres of other planets, where the totalamount of CO2 is greater than on the earth. Some ideaof the strength of these two bands can be obtained from

0_C mcs CCt Hg20 0.08 16967.8

0.008 / ~~~~28.64 0 0,0108 10 3'

0.0108 42 3

1.29 \9\ I 195 801.29 335-

8 1.7 2 I (2 693

too s P20

1.2008 \ J1 15620

100

2754 0

60 006 4360 4 99

499

100

-004 3.4 0 0.043 4. 90.0 43 04

0043 403

O2 .5 7 I 18 25 2t\ 'li 7672 5 5 '' y 14 70C)

0 _ .1 _ _ _ _ L _ j . ._ L _ _14.606f 24.2

0.169 \ \ \Il / 441 -

0.169 1 70

0.169 35~~~~~~~6 7

11.2 1~~~~~~78011.2~~~~~~~~~~9 965I

2200 2300 2400 2200

WAVENUMBER i cm-

Fig. 4. Spectral absorptance in the vicinity of the 2350 cm'band. The spectral slit width is approximately 3.5 cm'

30

100

2 10

co,2350 c

Pe in mm Hg1500760250'004020I0

.UI U.1 Iw in tmos cm

10 IOU 1000

Fig. 5. Total absorptance of the 2350 cm-' band for variousequivalent pressures as a function of absorber concentration.

November 1962 / Vol. 1, No. 6 / APPLIED OPTICS 761

) ( l ,

I

IIIz

iI

EI

0 - T

° 20 ot. os ,n

40 625 21 ICD 825 38< 60 625 I 798I-80 - 573011 I /11 l2WI 3932° \ t\ 50

u I 3 3 2100 11200 3800

Q 1100 1000 900 800 700WAVENUMBER in cm-'

Fig. 6. Spectral absorptance of CO2 in the frequency range1150-700 cm-'. The spectral slit widths are indicated and areapproximately 5-6 cm-'.

Fig. 6, which shows spectral absorptance in the regionbetween 1150 and 700 cm-'; it will be noted that forthe smallest sample represented in the figure, spectralabsorptance in the vicinity of the hot bands is veryslight whereas spectral absorptance is essentially com-plete in the vicinity of the v2 fundamental. The dashedvertical lines in Fig. 6 give the arbitrary boundariesadopted for measuring fA(v)dv for the two bands;1000 cm-' is the boundary between the two hot bandsand 875 cm-' is the boundar.y between the 961 cm-'

band and the region of strong absorption in the vicinityof V2. Forty-two records of spectral absorptance weremade.

Measured values of fA (v)dv for various values of P0

are plotted on a logarithmic scale as a function of w

_0

(a)

.

a0

-'3

'40 Oo 1000 IOOOw in otmos cm

(b)

Fig. 7. Total absorptance for various equivalent pressures as afunction of absorber concentration: (a) the 1064 cm ' band; (b)the 961 cm -I band.

in Fig. 7 (a) and (b); the large values of absorberconcentration should be noted. The data shown in thefigure are those corresponding to a sample temperatureof 260 C. It is found that the data represented in Fig. 7can be expressed to a good approximation by two empir-ical equations. The equation for the 1064 cm-' band is

f A(t)dv = 0.023 (wP3D 8 75 (4)

and applies for 1 < f A(v)dv < 40 cm-' and 100 < P.760 mm Hg. The empirical equation for the 961 cm-'band is

fA(v)dv = 0.016 (w P0 0-25)0.78

z0

0(I)

I-zLdi0LdC0

WAVENUMBER

(5)

in cm-'

Fig. 8. Spectral absorptance of the 1064 and 961 cm-' bands atvarious temperatures.

and is valid for 1 < K A(P)dv < 35 cm-' and 100 < Pe< 760 mm Hg. Both equations show a strong depend-ence of fA(v)dv on w which is intermediate betweenlinear and square-root, and a rather weak dependenceon Pe.

Since the 1064 and 961 cm-' absorption bands areproduced by transitions from initial excited vibrationallevels,8 their total absorptances are strongly dependenton temperature. This is evident from the spectralabsorptance curves shown in Fig. 8, which show thespectral absorptance of a given sample at various tem-peratures between 23°C and 77°C. Several effects,such as increased absorption line width due to increasedcollision frequency at elevated temperatures, are prob-ably involved, but since increasing the sample tempera-ture by 50°C was found to produce comparativelysmall changes in absorption bands involving transitionsfrom the ground state, it was concluded that increasesin the populations of their initial levels was the domi-nant cause of the observed variations of total absorptancefor the 1064 and 961 cm-' bands.

762 APPLIED OPTICS / Vol. 1, No. 6 / November 1962

z0

oc

0

z0cc

800 750 700 650

WAVENUMBER in cm-l600

Fig. 9. Spectral absorptance in a portion of the 800-495 cm-'region. The spectral slit widths are indicated.

Quantitative tests indicate that the observed increasein total absorptance with increase in temperature canbe accounted for on the basis of an increase in effectiveabsorber concentration we as more molecules are "pro-moted" to the initial excited state as the temperatureincreases. If w is the absorber concentration at 26'C= 299'K, the effective absorber concentration at tem-peratures T between 299 and 350'K can be approxi-mated by

W = U7 exp ok (29 T- (6)

where E is the energy of the lower levels involved intransitions producing the hot bands. The values of Efor the 1064 and 961 cm-' bands calculated on thebasis of the observed variation of f A(v)dv with Tand the additional information given in (4), (5), andFig. 7 fall within 6% of the known values of these energylevels. It should be noted that the present work coversa relatively small range of temperatures; however,within the range covered, fA ()dv can be calculatedfrom (4) and (5) provided w is replaced by we obtainedfrom (6).

20- A 'P8:329 65 i0 w [n\%>,mm Hg meH' 3

40 ntmos t 32 3at 32.5TEMP cm for t for t

60- 32.5 C 23.5 060 A68- 57.5 23.5 0

80-32.0 11 9

100.

2 , - I 20-

TEMP w 576 'P 1560'40 F soICos ' 'C m Hg | /

- 6. 0 a tcm | 26.060- 4 5.5 ' C '

65.580-

100O * 1 - E , |,.

20 TEP w2470 Pe88451

40 -mos cm jmm Hg

60 o95 C0 dt 29.5

80 65.0 , \100 I

875 800 700

WAVENUMBER in cm-'600 540

iFig. 10. Spectral absorptance in the spectral range 875-540 cm'at various temperatures.

The 875-495 cm-' Region

As indicated earlier, the region between 875 and 495cm-' in the CO2 spectrum is complex. In addition tothe dominant band, v2for C' 202 1

6 , there are overlapping V2bands due to isotropic molecules and hot bands involv-ing transitions from initial excited states. In view of thebroad spectral range involved and the complex of over-lapping bands, it was decided to divide the region intofive subregions and to measure mean spectral absorp-tance for each subregion as a function of w and P.This procedure offers some advantages in possible useof the present results in studies of the earth's atmos-phere, since the pure rotational band of water vaporoverlaps the entire region and provides additional com-plications.

Spectral absorptance curves for a large portion of theregion, as observed with the Perkin-Elmer Model 21equipped with a KBr prism, are given in Fig. 9. Thevertical dashed lines show three of the boundariesselected for subregions; these boundaries were selectedat Q-branches occurring near 720, 667, and 617 cm-'.For large values of w there is additional absorptionat lower frequencies, and an additional boundary wasestablished at 545 cm-' as the upper frequency limitof the subregion of lowest frequency. A Perkin-ElmerModel 99 equipped with a KBr prism was used toinvestigate samples which produce small values oftotal absorptance. One-hundred and seventy-ninerecords of spectral absorptance were made in the 875-495 cm-' region.

Because of the presence of overlapping hot bands,the appearance of the spectral absorptance curves aswell as the total absorptance for the entire region varieswith temperature. The dependence of spectral absorp-tance on temperature is illustrated qualitatively by thecurves in Fig. 10. Variation of average spectralabsorptance A - v2) for each subregion was studiedquantitatively over the temperature range between26'C and 70'C. The results of this study shown inFig. 11 have been used to correct all values of A (v -v)

to a temperature of 26'C; these corrected values areused in the subsequent figures in the present paper.

Values of A (720-825 cm-') for various values of Peare plotted as a function of absorber concentration w inFig. 12. The curves have many of the properties of thef A ()dp versus w plots for the bands discussed earlier.Corresponding plots of A(667-720 cm-'), A(617-667cm-'), A(545-617 cm-'), and A(495-545 cm-') asfunctions of absorber concentration are given in Figs.13, 14, 15, and 16, respectively. Saturation effects forlarge values of w will be noted in the plots correspondingto the subregions of intense absorption in the centralparts of the region. It is recommended that Figs. 12-16be used directly in determining the mean spectralabsorptance or the total absorptance to be expected for

November 1962 Vol. 1, No. 6 / APPLIED OPTICS 763

20 - 31~= H

0.48. 0.5

40 074

60 8.1 596.1578

303 ,,, so

z01o.-11011

I

"IZI

II

samples of known composition and pressure. Interpola-tions between curves can readily be made and lead tomore reliable results than those based on a set ofempirical equations.

Comparison shows that the present results for totalabsorptance in the 875-495 cm-' region are in fairagreement with HBW results for samples in which theranges of w and P0 are the same although these are theranges in which the HBW resultsOn the average, the HBW valuessamples were 2.8% greater than

0.4

0.4

0. ,

r,~ -,+ I ,-T +-

W. 119 amoscm

- 23.5

,,,,,,,I I. .. . . . . .~~~~~~~~~~~~~

10

.4

2630 40 50 60 70

are least accurate.of f A(v)dv for 48those "predicted"

26 30 40 50 60 70

TEMPERATURE .S

Fig. 11. Mean spectral absorptance of pure CO2 samples withthe indicated absorber concentrations as a function of tempera-ture. Pressure increased in accordance with the general gaslaw. The results shown in the figure along with tabulatedvalues of A(vi - v2) w, P 0, and T were used in reducing all datapresented in later figures to a temperature of 26 0C. Note: Novalid data were obtained in the 545-495 cm-' subregion; A(545-495 cm- 1 ) was rather small, and water vapor evolved fromthe heated cell walls may have introduced serious errors.

w in atmos cm

Fig. 12. Mean spectral absorptance in the 720-875 cm-'subregion, denoted by A(720-875), as a function of absorber con-centration at the indicated values of equivalent pressure.

n

CDM

(0

I<

from the curves in the present paper; in the worst casethere was a difference of 9%. The recent study ofEdwards7 included 16 samples at an ambient tem-perature slightly different from the temperature em-ployed in the present work. The values of w, P, and pfor Edwards' samples were used in conjunction withthe curves of the present paper to predict total absorp-tance; the difference between the "predicted values"and Edwards' observed values was less than 5% exceptfor the sample showing least total absorptance, forwhich the difference was 9.7%. The "predicted values"averaged 3% greater than Edwards' observed values;correction to a common temperature reduces the differ-ences still further. The HBW and Edwards data thusprovided a satisfactory check of the present results forintermediate values of w, but no previous data wereavailable for comparison with the largest and smallestvalues of w employed in the present work.

Band Intensities

It was pointed out in the first paper' in this seriesthat band intensity f k(v)dv can be obtained from therelation

f k(P)dv = - A(v)dvf ~wf (7)

provided k(v)w <K 1 for all frequencies within the band.

w in otmos cm

Fig. 13. Mean spectral absorptance in the 667-720 cm-' sub-region for various equivalent pressures as a function of absorberconcentration.

N-(10(0(0

0.01 0.1 Iw in ctmos cm

10 OUU

Fig. 14. Mean spectral absorptance in the 617-667 cm-'subregion for various equivalent pressures as a function of ab-sorber concentration.

764 APPLIED OPTICS / Vol. 1, No. 6 / November 1962

W= 119

23.5

. I . I

L +

23.5

E

- w. 2 470

=-V TV - VV

5762

- 23.5

- --

I . . .. I -'I- ..

02 - -40P8 0n .mm Hg[1500 . ~

760 4

0.1 -2 0..

0.01~~~~-..

Table I. Band Intensitiesa

Observer f k(v)dv

3716 cm-' bandPresent study 54 ± 10Eggers and Crawford9 39

3609 cm-' bandPresent study 37 ± 8

9350 cm-' bandPresent study 2500 -t 400Eggers and Crawford 8 2100

1064 cm-' bandPresent study 0.045 -4- 0.010

961 cm-' bandPresent study 0.023 i 0.006

667 cm-' bandPresent study 330 ±4- 90Eggers and Crawford 9 161Kaplan and Eggersl° 240Thorndikell 187Weber et al.12 170Yamamoto and Sasamori 1'3 212

a The values of f k()dv are in units of (atm cm) -I cm -1 , whereatm cm are stated for STP. Values given for the present studycorrespond to a temperature of 260 C, and the estimated uncertain-ties are indicated.

The criterion for k(v)w < 1 is that f A (v)dv is directlyproportional to w but independent of Pe. Althoughthe determination of band intensity was not of primaryinterest in the present study, it was possible to obtainestimates of f k(v)dv for the CO2 bands discussed above.Estimated values of band intensity listed in Table Iare given in units of (atm cm)-' cm-', where atm cmapplies for STP; the values given in the table arebased on samples at 260 C. The values of band in-tensity reported by several other investigators arestated for comparison in Table I.

It will be noted that the estimates made in thepresent study appear to be consistently greater thanthose reported by Crawford and Eggers.' The spreadof values reported for the 667 cm-' band' 0 - 3 is ratherlarge; the present estimate is the largest value reportedbut its estimated uncertainty overlaps that reported byKaplan and Eggers. The difficulties of making accu-rate determinations of small values of w and of ensuringthat kI(v)w << 1 for all frequencies provide formidablelimitations to the precise determination of band in-tensities.

One of the authors (D. W.) wishes to thank P. Swingsand M. Migeotte for the use of the library and otherfacilities at the Institut d'Astrophysique, Universit deLiege, during the preparation of the present paper.

E

II8

I-

,<

Pe in mm HgC02 1500 -

760 100

2 50 40-

01 20

0.002 | 1_ 1 S l 1 , , ,,,,,, 1

0.002p

0.1 10

w in almos cm

100 1000

Fig. 15. Mean spectral absorptance in the 545-617 cm-'subregion for various values of equivalent pressure as a functionof absorber concentration.

0.2t-

E 0.1

Ina004

1<

0.01 ICIo 400 1000

w in atmos cm4000

Fig. 16. Mean spectral absorptance in the 495-545 cm-' sub-region for various equivalent pressures as a function of absorberconcentration.

References1. D. E. Burch and D. Williams, Appl. Opt. 1, 473 (1962).2. D. E. Burch and D. Williams, Appl. Opt. 1, 587 (1962).3. D. E. Burch, E. B. Singleton, and D. Williams, Appl. Opt.

1, 359 (1962).4. J. N. Howard, D. E. Burch, and D. Williams, J. Opt. Soc.

Am. 46, 186, 237, 242, 334, 452 (1956).5. L. D. Kaplan (private communication).6. William Benedict (private communication).7. D. K. Edwards, J. Opt. Soc. Am. 50,617 (1960).8. G. Herzberg, Infrared and Raman Spectra of Polyatomic

Molecules (Van Nostrand, Princeton, 1945), p. 274.9. D. F. Eggers and B. L. Crawford, J. Chem. Phys. 19, 1554

(1951).10. L. D. Kaplan and D. F. Eggers, J. Chem. Phys. 25, 876

(1956).11. A. M. Thorndike, J. Chem. Phys. 15, 868 (1947).12. D. Weber, J. R. Holm, and S. S. Penner, J. Chem. Phys. 20,

1820 (1952).13. G. Yamamoto and T. Sasamori, Sci. Repts. Thuku Univ.,

Fifth Ser. 10, No 2 (July 1958).

continued from page 766

the longer wavelengths of the visible and infrared. This poses aserious limitation on the field (outdoor) uses of the ultraviolet, atleast in a conventional atmosphere where scattering must beconsidered.

This particular recitation of facts suggests that the ultraviolet

will not experience as much of a resurgence of interest as hasoccurred for the infrared. However, there will surely be someincrease in the utilization of this spectral region, as well as aconcern about protection from ultraviolet rays in space flights.Therefore, the ultraviolet does indeed represent another area of ex-pansion of the already rapidly growing field of applied optics.

November 1962 / Vol. 1, No. 6 / APPLIED OPTICS 765

I I I I I I I '1

CO2

mm Hg -

1500 -760250 -

. . I , I,, , I