JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Absorption Coefficients of Oxygen in the Vacuum UltravioletG. L. WEISSLER* AND PO LEE

Physics Department, University of Southern California, Los Angeles, California(Received October 1, 1951)

Absorption coefficients of oxygen were measured between 300A and 1300A using a grazing incidencevacuum spectrograph together with a line emission light source. The coefficients near 1300A were less thank= 10 cm-'. The bands in the region between 1300A and 1000A showed strong absorption of the same orderof magnitude as in the Worley bands of nitrogen. No strong continuous absorption was found in the regionbetween 1300A and 740A. The continuous absorption of oxygen below 740A was interpreted as composed ofthe ionization continua of several Rydberg series converging to the excited states of the molecular ion ofoxygen. The continuum had a broad maximum with k-values of about 700 cm-' between 400A and 600Aand, in contrast to nitrogen, still showed a large absorption of k=530 cm-' at 303A. An estimate of theintegrated absorption coefficient was made and the correspondingf-value was found to be 6, in agreementwith the predictions from classical dispersion theory.

INTRODUCTION

ABSORPTION coefficients in 02 had been meas-ured by Ladenburg and Van Voorhis' between

1750A and 1300A using fluorite optics. They havefound that this region, the Schumann dissociation con-tinuum, absorbed strongly and showed a maximumk-value of 490 cm-l at about 1450A. Another region ofstrong absorption2 has been investigated qualitativelybelow 1100A by means of a vacuum spectrograph andwas found to show another maximum at about 500Awith the coefficients in this region "of the same orderof magnitude as in the Schumann region."2 The ab-sorption of 02 in the extreme ultraviolet was linked byLadenburg with its dispersion as measured interfero-metrically between 5900A and 1920A.3 The index ofrefraction has been represented by a three termequation containing three characteristic wavelengths,XA= 1898.6A, 2 = 1467.9A, 3=544.36A, and threecorresponding oscillator strengths, fl=4.04X 10-,

f2=0.202, and f3=5.93. Classical theory establishesthe relation that the integrated absorption coefficientin the vicinity of the characteristic wavelength Xi isproportional to its oscillator strength fi. The predic-tions of absorption regions from dispersion theoryagreed very well with the actual measurements' for X2.

The X3- and f3-values will be referred to later after thepresent data have been given.

Schneider4 has made measurements on the absorp-tion coefficients of air for about 350 lines between 380Aand 1600A, and K. C. Clark' has obtained the coeffi-cients for both N2 and 02 separately for about 25emission lines between 855.6A and 1025.7A and for thehelium lines at 584.3A and 537.OA. Price and Collins6

*The help of the ONR is gratefully acknowledged.'R. Ladenburg and C. C. Van Voorhis, Phys. Rev. 43, 315

(1933).2 Ladenburg, Van Voorhis, and Boyce, Phys. Rev. 40, 1018(A)

(1932).3 R. Ladenburg and G. Wolfsohn, Z. Physik 79, 42 (1932).4 E. G. Schneider, J. Opt. Soc. Am. 30, 128 (1940).5 K. C. Clark, Phys. Rev. 73, 1250(A) (1948). Also by personal

correspondence.6 W. C. Price and G. Collins, Phys. Rev. 48, 714 (1935).

have found and classified a large number of oxygenabsorption bands between 750A and 1300A. Their re-sults together with qualitative observations of absorp-tion continua have been of importance concerning theinterpretation of the data presented here.

These measurements have been based on Beer's law

kToLI=Ioe-k=I0e poT p=IOe-aP, (1)

where x is -the thickness of the absorbing gas layer atN.T.P., p is the pressure of the absorbing gas in the

k (cm')

1000

600

200

000

II 1200 1100 1000 _ 0

A (A)

FIG. 1(A). Absorption of 02. 1300A to 900A.

|k (cm-1000

600

1000 900 800 700 600

A (A)

FIG. 1(B). Absorption of 02. 1000A to 600A.

k (cm-')

1000

. . . i . l.....

600 500 400 300

A (A)

FIG. 1(C). Absorption of 02. 600A to 300A.

200

VOLUME 42, NUMBER 3 MARCH, 1952

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March1952 ABSORPTION OF OXYGEN IN VACUUM ULTRAVIOLET

TABLE I. Absorption coefficients of oxygen.

Wave number Source linein cm - used k (cm-')

0.07X 1020.24X 1020.24X 1029.09X 1020.54X 1028.72 X 1020.83X 1020.66X 1024.1 X102

1.2 X1020.82X 1020.59X 1020.59X 1027.7 X102

1.0 X10 2

5.4 X10 2

5.0 X102

0.43X 1021.1 X102

1.8 X1021.7 X1021.5 X102

1.7 X102

1.7 X1021.1 X102

4.7 X10 2

2.4 X102

2.4 X102

2.8 X102

4.2 X102

5.9 X1025.4 X102

4.4 X1024.9 X1023.0 X102

3.6 X 1023.4 X102

3.8 X102

Location in theknown band

B.M.

B.M.

B.M.

B.M.

B.M.B.M.

6

6

6

6

66

B.M. (975.324A) 6B.M. (916.338A) 6B.M. (916.338A) 6B.M. (916.338A) 6B.M. 6B.M. 6B.M. 6

B.M. (877.5A) 8B.T. (836.610A) 6B.T. (836.610A) 6B.M. (833.5A) 8B.M. (832.328A) 6

Wave number Source linein cm-, used

120058125524128873129469129550133873134078139182142076142284142382145812145876148600148945151450154999155081155244162061162261166504166778171135172177172294174019185236185341185499185785185931188900188841190188235236232749329187

011

NIIINIIINIIINIINIINilOIlI

NIIINIII

NIII2XII2NII2XNII2XNII2XNII2XOII2XOII2XO1II

2XOII

2XI2XOII2XOII2XNII-2 XOII2XOII2XNII2XOII2XOIII

2XOII

2XOII

3X11eII

k (cm-')

832.93796.661775.957772.385771.901746.976745.836718.483703.850702.899702.332685.816685.513672.348671.391660.380645.167644.825644.148617.051616.291600.585599.598584.331580.967580.400574.650539.853539.543539.186538.256537.830529.8529.547525.795430.041429.647303.779

6.7 X102

6.8 X102

4.9 X102

6.4 X102

5.1 X102

7.6 X102

7.1 X1026.0 X1028.0 X1028.2 X102

7.1 X1027.2 X102

7.2 X102

6.7 X102

7.2 X102

7.8 X102

7.2 X102

7.5 X1026.9 X1028.1 X102

7.4 X1027.9 X102

7.8 X102

5.5 X102

7.2 X102

7.5 X102

8.4 X1027.4 X102

7.6 X102

7.7 X102

7.0 X102

8.6 X102

8.3 X102

8.42X 1028.0 X102

9.3 X1028.9 X102

5.3 X102

Location in theknown band

B.M. (832.328A) 6B.M. (793.903A) 6B.T. 6B.M. 6B.M. 6B.H. (746.7A) 8

B.M. 6B.H. (702.6A) 8B.M. 6B.M. (684.743A) 6B.M. (684.743A) 6

B.M. (671.90A) 8

B.H. (617.4A) 8

spectrograph tank, po is atmospheric pressure, T is theabsolute temperature of the gas, To=0 0C, and L is thegeometrical light path. Equation (1) can be written as

ln1°-lnI= ap, (la)

which enables one to test the validity of Beer's lawby plotting lnI versus p. If a straight line resulted thenits slope would yield the absorption coefficient k. Alldata given here were obtained in this manner using atleast three and often five different pressures. The experi-mental procedure used was the photographic determina-tion of the relative light intensities I° and I with agrazing incidence vacuum spectrograph which was filledwith oxygen at pressures ranging from 0.5 micron to100 microns. The length of the light path L was from79.7 cm at 550A to 103.3 cm at 1300A. A more detaileddescription of the apparatus and the methods ofmeasurements has been presented elsewhere.7

EXPERIMENTAL RESULTS

The absorption coefficients k in 02 were obtained for76 wavelengths between 300A and 1300A and are sum-marized in Table I and Fig. 1. Many of the source lines

I Weissler, Lee, and Mohr, J. Opt. Soc. Am. (to be published).

from which the coefficients were calculated lie withinmolecular absorption bands. Such bands, if identifiedand measured, have been listed in the last column ofTable I. The notations B.H. indicate location of thesource line near the band head where band absorptionis usually strongest, B.M. near the middle of the band,and B.T. near the tail of the band. The wavelength ofthe band head is given in parenthesis followed by theliterature reference.6' 8 Even though all absorptioncoefficients reported here were found from plots oflnI versus p it is felt that the accuracy of k-values whichfall within molecular absorption bands is less than thosewithin a continuum. Furthermore, their physical sig-nificance is not clear and the data within the bands arepresented without interpretation. Elsasser9 and Niel-sen'" have studied the problem of band absorptiontheoretically, but unfortunately their refinements couldnot be applied here because they assume a detailedknowledge of the line structure within a band. If severalof the measured coefficients blended with a molecular

8 Y. Tanaka and T. Takamine, Sci. Pap. Inst. Phys. Chem.Res., Tokyo 39, 437 (1942).

9 W. M. Elsasser, Phys. Rev. 54, 126 (1938).10 Nielsen, Thornton, and Brock Dale, Revs. Modern Phys.

16, 307 (1944).

76567.476636.476793.680431.182125.782891.283284.383364.885054.586795.188107.388150.188170.190090.092201.796432.096497.197942.8100050100849100929100988101030100929101135102352109086109170109175110561110602110667113815113924119718119746119940120060

OI01OINII0ISuIiiNINICIIIOINININISiIIINIICIICIIHI0INIII0I01NIII0I0ICIIINIINIINIICICICI0I010III

OIIi0111,

1306.0381304.8641302.1921243.1701217.6451206.41200.7061199.51175.7161152.1391134.9801134.4191134.1711110.01083.9911037.0201036.3321025.717999.494991.579990.797990.213989.8990.797988.775977.020916.708916.01915.96904.482904.144903.952886.618877.778835.293835.10833.749833.332

201

G. L. WEISSLER AND PO LEE

absorption band a rapid change in absorption occurred(see Table I).

The following general absorption trend was observed.At 1300A the k-value was found to be less than 10 cm-lin support of the well-known "air window." This wasalso the value of the coefficients in the region between1300A and 1000A if outside of molecular bands. Theabsorption in the bands was strongest for those atlonger wavelengths, probably due to predissociation,6

and decreased gradually for successive bands towardshorter wavelengths in agreement with Price andCollins. Between 1000A and 740A the coefficientsfluctuated rapidly as was the case in nitrogen.7 Thelowest k-value in this region (k= 110 cm-') which wasfound not to fall within a band indicated that continu-ous absorption was of this magnitude or smaller, whereasthe coefficients within the bands of this region wereabout 600 cm-' or greater. Below 740A the absorptiontook on values between k=500 cm-l and k= 1000 cm'lpointing to a strong continuous absorption region witha broad maximum of k= 700 cm-l between 400A and600A and still being strong at 303A, namely k=550cml, in contrast to N2 at this wavelength.' The dottedline in Fig. 1 has been drawn such as to connectroughly the lowest measured k-values between 1000Aand 300A. Below 740A this line is probably a reasonableapproximation to the continuous absorption of 02 asjudged from qualitative absorption spectrograms takenwith a wavelength continuous Lyman source. However,above 740A this line may not be representative becauseof various diffuse, unclassified bands.

The error in the coefficients presented here is esti-mated to be about ten percent for k-values greaterthan 100 cm-' and fifteen percent for lower values.tSimilar conclusions can be drawn from Table II whereabsorption coefficients are presented for wavelengthsin their first and second order. By measuring k for twowidths of the primary spectrograph slit, errors due toslit width have been found to lie within the statedlimits.7

DISCUSSION

It has not been possible to analyze' the far ultra-violet absorption bands in 02 in quite as much detail asin nitrogen" because of more complexity and also be-

TABLE I. Comparison of absorption coefficients in oxygen,first- and second-order lines. Plates LE 25.

PercentWavelenigth First order Second order difference

OII 644.188 640 cm-' 670 cm-' 7.8NIl 644.825 710 cm-' 750 cm-' 5.6NII 645.167 750 cm-' 720 cm' 4.0NII 660.280 740 cm-' 780 cm' 5.3

t The experiment wvas not designed to measure k-values lessthan 20 cm-' accurately.

11 R. E. Worley, Phys. Rev. 64, 207 (1943).

cause of somewhat less experimental accuracy due to agenerally greater diffuseness of the 02 bands. An energylevel diagram of the molecule is shown in Fig. 2, con-structed from the data of Price and Collins6 and Tanakaand Takamine.8 In order to understand the mechanismof the continuous absorption of 02, an attempt is madeto investigate all those transitions which are likely togive rise to either a dissociation or an ionization con-tinuum. To this purpose the electronic configurationsof certain critical levels are needed. Molecular oxygenwith an internuclear distance ro"= 1.2A in the groundstate has the arrangement

kk(o-2S)2(ou2S)2(oa,2p)(ru2p)4(7r,2p)2 , X2-

This changes for the Schumann-Runge bands to

kk(o-,2s)2(o.02s)2(cr2p)2(ru2p)3 (r,2p)3 , Blu-

where an electron goes from the bonding orbital(ru2p) to the antibonding one (r2p), resulting in anincreased r'= 1.6A. Therefore this transition gives riseto the strong Schumann dissociation continuum.'

Those transitions which approach the first ioniza-tion limit X2II are due to the excitation of a 7r,2p-elec-tron giving in the limit

kk(ag2s)2(o.u2s) 2(o2p)2 ( 7r2p)4(7rg2p), 02+X2 I,

and an r'= 1.12A, smaller than ro", due to the loss of anantibonding electron. No dissociation continuum canbe expected from these bands converging to the 2llstate at 1030A. Price and Collins did not observe anyRydberg progressions above 1000A even though thebands in that region "appear to converge to a limit,"6

2II , at 1030A. "The first bands in the region 1250A to1200A are somewhat pre-dissociated, probably due toproximity of the potential curve of the 02 B'2;,- stateor other predicted states"6 and, because of their diffusecharacter, the absorption values of those source lineswhich fall into these bands should not be seriouslyfalsified by blending. "The intensities of these bandsfall off much more rapidly than for a series going to anionization continuum."6 This decrease is also apparentfrom the data in Fig. 1. "No ionization continuum couldbe found for these bands, although a slight weakeningof the background was observed to extend from 1105Atoward shorter wavelengths, probably due to transitionsto repulsion curves."6 NO and CO are examples ofsimilar gases for which no ionization continuum corre-sponding to the first ionization limit has been observed.Although not enough absorption data are availablebetween 1200A and 1000A, there seems to be a weakcontinuous absorption in this region of the order ofk -50 cml to k = 100 cm'l. No strong continuous ab-sorption was observed by Price and Collins above 740A.This seems to be substantiated indirectly by the meas-ured absorption coefficients between 1030A and 740Awhich, though large in magnitude, nearly all lie withinthe bands of this region. If any continuum does exist

Vol. 42202

March1952 ABSORPTION OF OXYGEN IN VACUUM ULTRAVIOLET

here it would be of magnitude k= 100 cm- and probably of the same origin as previously mentioned for theweak continuum above 1030A.

Between 1030A and 723A the Rydberg progressionsconverging to the excited II-states of 02+ result from theexcitation of a bonding 7r2p-electron, giving rise to theconfigurations

kk(o7g2S)2 (o2S)2(o,2p) 2(7ru2p) 3 (7r,2 p)2, aII and A211.,

with increased internuclear distances ra'= 1.381A andrA'= 1.409A, respectively. Starting at 740A an ioniza-tion continuum was observed corresponding to thesecond ionization limits 02+a4IIu or the third one02+A211 or both. "However, the possibility that thecontinuum is a dissociation continuum cannot be elimi-dated,"6 particularly in view of the larger internuclearinstances of the 411 and 2JII states. In the data pre-sented here the source lines OII 718.5A (a close doublet)could not be found to fall within any distinguishableband as judged from a reprinted spectrogram of Tanaka.Their k-value of 600 cm-l probably is indicative of themagnitude of this continuum.

Bands approaching the fourth ionization limit 4 9

have been analyzed6' 8 as Rydberg progressions and aredue to the excitation of a o-2p-electron, resulting in theconfiguration

kk(o-,2s)2(ou2S)2(g2p) (7ru2p)4 (7rg2p)2 , 02+b427,

with an increased r'= 1.279A. Tanaka and Takamineobserved 15 members each for the (0, 0), the (1, 0), andthe (2, 0) series. In 02 the (1, 0) bands are even strongerthan in nitrogen where in the Worley-Jenkins Rydbergprogressions to N2+X22;+ the (0, 0) bands were strongerthan the (1, 0) bands. This is explained by the fact thatfor this transition in oxygen the internuclear distancechanges by 0.066A whereas in nitrogen the correspond-ing change is only 0.019A because in N2 the antibonding7rg2p-electrons are missing. Because of these intense andlong progressions in oxygen one should expect a strongionization continuum adjoining the 02+b42;- limit be-low 682A. An additional contribution to the observedcontinuum in this region may be due to a separate ioni-zation continuum starting at 610A. Tanaka8 found asecond group of Rydberg series with a limit at about610A having features similar to the 4z - one.

E ( VOLTS)

I

ONIZATIONLIMITS

FIG. 2. Energy level diagram of the 02 molecule. The full hori-zontal lines give some of the observed electronic states. The dashedhorizontal lines indicate ionization limits. Vertical arrows signifytransitions: (1) first negative bands; (2) second negative bands;(3) Schumann-Runge bands; (4) H-, I-Rydberg series (see refer-ence 6); (5) M-. N-Rydberg series (see reference 6) ; (6) P- Q-Ryd-berg series (see reference 6); (7) Tanaka-Takamine (see reference8) (0, 0)-, (1, 0)-, (2, 0)-Rydberg series; (8) unclassified bands inthe region between 1000A and 700A; (9) unclassified bandsbetween 1050A and 1300A; (10) Tanaka-Takamine (see reference8) Rydberg series converging on 610A,

Finally it should be mentioned that Herzberg' 2

points out that "a second charge-transfer transition in02 is expected to be one in which an electron goes fromthe bonding o2p to the antibonding o-f2p orbital. Thistransition would be at shorter wavelengths than theSchumann-Runge bands, but has not yet been definitelyidentified.' 2 If one considers this in the light of thecharge-transfer spectrum of the Schumann-Runge bandswhich has a strong absorption continuum' with a maxi-mum k-value of 490 cm-l, one would have a sizeablecontributing mechanism to account for the magnitudeof the observed continuum in the extreme ultraviolet.

When integrating graphically the dotted curve inFig. 1 representing continuous absorption, a meanoscillator strength of approximately f= 6 was found.This is in agreement with the value f3= 5.9 predictedfrom dispersion measurements. 3

12 G. Herzberg, Molecular Spectra and Molecular Structure,(D. Van Nostrand Company, Inc., New York, 1950), 2nd Edition,p. 385.

203

..,t

I