JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Photodissociation and Photoionization of Oxygen (02) as Inferred fromMeasured Absorption Coefficients

Po LEEDepartment of Physics, University of Southern California, Los Angeles 7, California

(Received November 15, 1954)

Absorption coefficients, k, of 02 between 1320 A and 200 A were measured using a 2-meter grazing in-cidence vacuum spectrograph. Three weak dissociation continua above 1040 A have been investigated. The"first continuum" between 1320 A and 1270 A had a maximum k-value of 13 cm-l at 1295 A; another onebetween 1220 A and 1096 A was found to have a k=0.23 cm-', and the third one between 1096 A and1040 A had a value of about 3 cm-. The absorption near Lyman a and in various other "atmosphericwindows" in adjacent regions is given. Between 1040 A and 740 A strong absorption was observed due todiffuse bands, which were superimposed over the first ionization continuum with a maximum of 100 cm-'at 920 A. The main ionization continuum had a long wavelength limit at 683 A and a flat maximum of 590cm-' at 510 A. (In addition low absorption coefficients of less than 1 cm-' have been found in N2 between1040 A and 910 A.) As an application of these data, a survey calculation on ionospheric absorption wasmade which showed that the formation of the E layer at 100 km may be due to the absorption of solarradiation by the first ionization continuum of 02. It is also shown that solar radiation between 1220 A and1100 A can reach the height of the D layer by passing through the indicated atmospheric windows.

INTRODUCTION

THE absorption coefficients, k, of 02 for electro-magnetic radiation have been extensively in-

vestigated. In the region of the Schumann-Rungedissociation continuum, U- IF -, it was first meas-ured by Ladenburg and Van Voorhis' and theoreticallycomputed by Stueckelberg.2 Recently, this work wasrepeated by Watanabe et al.1 and by Ditchburn andHeddle.4 Their results showed a maximum absorptioncoefficient of about 490 cm-l at 1450 A. This continuumextended to 1300 A. Below 1300 A the absorptionspectrum was first observed by Price and Collins5 andrecently by Tanaka.6 The absorption of 02 at Lyman ahas been accurately measured by Preston.7 Recently,Ditchburn et al.8 repeated these measurements andobtained the coefficients in the vicinity of Lyman a.Weissler and Lee9 reported k-values in 02 below 1300 A,but their work was designed primarily to measure thestrong ionization continua, and k-values of 20 cm-l orless were not determined accurately. Watanabe et al.1

obtained the absorption contour in 02 between 1050 Aand 1350 A, but in the "poorly resolved region" from1060 A to 1170 A these authors considered their dataas semiquantitative. The present work attempts toestablish the magnitude and range, if possible, ofseveral continua in 02 between 1320 A and 200 A.These measurements together with similar ones in N2above 910 A, are important for the theory of formationof the E and D layers of the ionosphere.

I R. Ladenburg and C. C. Van Voorhis, Phys. Rev. 43, 315(1933).

2 E. C. G. Stueckelberg, Phys. Rev. 42, 518 (1932).Watanabe, Inn, and Zelikoff, J. Chem. Phys. 21, 1026 (1953).

'R. W. Ditchburn and D. W. 0. Heddle, Proc. Roy. Soc.(London) A220, 61 (1953).

5 W. C. Price and G. Collins, Phys. Rev. 43, 887 (1935).6 Y. Tanaka, J. Chem. Phys. 20, 1728 (1952).7 W. M. Preston, Phys. Rev. 57, 887 (1940).8 Ditchburn, Bradley, Cannon, and Munday (private com-

munication, 1954) (to be published).I G. L. Weissler and P. Lee, J. Opt. Soc. Am. 42, 200 (1952).

APPARATUS AND PROCEDURE

Most of the details on the apparatus and experi-mental procedures have been described previously.9 10

The only modifications made were in the type of lightsource used together with a differential pumpingchamber as shown in Fig. 1. This arrangement facili-tated particularly the measurements of low absorptioncoefficients in the long wavelength region above 900 A.The source consisted of a water-cooled r-shaped dis-charge tube in which a quartz capillary tube, g, 2 mmin diameter and 3 cm in length, was inserted to guidethe soft spark discharge through pure hydrogen at aconstant pressure of 700 microns. The ac sparkbreakdown potential was constant at 600 volts asindicated by the steady trace on an oscilloscope, Oc,during the entire period of measurement. This sourceproduced the many-lined spectrum of hydrogen withconsiderable intensity. Between the light source andthe spectrograph tank a vacuum chamber, V, wasplaced for the purpose of differential pumping. Withthis arrangement one may maintain a pressure of 100

Oc(IT

FIG. 1. Grazing-incidence vacuum spectrograph and lightsource. A, C: gas inlet; Sp: tank of spectrograph; V: differentialpumping chamber; s: slit; c, g: capillary tubes; C,: 0.001 fcondenser; R,: 5000 ohm resistance; R2 : 10 meg ohm resistance;T: transformer; Os: oscilloscope; B, E, D: to pumping systems.

"P . Lee and G. L. Weissler, J. Opt. Soc. Am. 42, 80 (1952);43, 512 (1953).

703

VOLUME 45, NUMBER 9 SEPTEMBER, 1955

PO LEE

k(fr

1000.

500

K

a

5001

Cb 0~ _ 1260 _- i~6 O H660

FIG. 2. Absorption of 02 between 1300 A to 1100 A. The rectanglesindicate regions in which k <2 cm'1.

mm Hg in the tank, which is necessary to measure lowk-values, without change in pressure and therefore inlight intensity in the source chamber. The hydrogenmany-lined spectrum was well resolved and its lineswere closely and evenly spaced. The impurities in thetank oxygen used were less than 0.3% with nitrogen asthe main constituent. Oxygen was introduced into thespectrographs after passing it over a CO2 cold trap anda CaCl2 drying tube. A small dish containing P205-powder was placed in the spectrograph to absorb re-sidual water vapor. For each plate four identicalexposures were taken with four different pressures ofthe absorbing gas. Results showed that the absorptionalways obeyed Beer's law both for high k-values of theorder of 1000 cm 1 as well as for low ones of 0.23 cm-l.In other words, the observed coefficients were foundto be pressure independent within the range from afew microns to 100 mm Hg.

While all individual absorption coefficients wereevaluated from the changes in photographic platedensities of source lines from the hydrogen discharge,the over-all aspects of absorption were supplementedby using also the molecular helium continuum as asource.

Left dinate Riph ordinle0(A)

FIG. 3. Absorption of 02 between 1100 A to 900 A. The semi-circles indicate locations of bands; : data by Wainfan, Walker,and Weissler (see reference 14); X: k-values; 0: low k-values.k-values in the left half of the figure are referred to the left ordi-nate and those on the right to the right ordinate.

no5a aT sUoa6f,0aS= h W0o0Iajm

jL 0XwJ12 I I

I IIII II

[A.

5900 850 00 - 750 700A(A)

FIG. 4. Absorption of 02 between 900 A to 700 A. The rectanglesindicate the positions of bands; A: data by Wainfan, Walker, andWeissler (see reference 14); X: k-values; 0: low k-values; thearrows: the ionization limits; and short lines: Rydberg seriesprogressions.

The absorption contours of the various continuawhich will be discussed later were all found by themethod of connecting together with a smooth line thelowest absorption coefficients found in a specific wave-length region.

EXPERIMENTAL RESULTS AND DISCUSSION

Absorption coefficients of 02 for thousands of sourcelines from 1320 A to 200 A have been investigated,some as large as 1600 cm-l and others less than 0.3cm-l. In certain spectral regions a rapid change in kof several hundred cm-l within a fraction of an ang-strom unit was observed, indicative of resonance bands.On the other hand, regions of nearly constant absorp-tion over a considerable width were also found.

General Aspects of 0 2-Absorption

In order to obtain an over-all impression of k-valuesversus wavelength, those greater than 20 cm-l arepresented in Figs. 2, 3, 4, and 5. Figure 2 shows reso-nance absorption bands which may be recognized bylarge and rapidly varying coefficients. They have beenidentified by Price and Collins.5 Tanaka6 reported thatthe longest band was a close doublet at 1244.3 A and1243.5 A. The values obtained for source lines at

FIG. 5. The main ionization continuum between 683 A to 200 A.A: data by Wainfan, Walker, and Weissler (see reference 14);X: k-values; 0: low k-values; arrow: the fourth ionization limit.

----r JS .SA__

704 Vol. 45

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= I

L

o.a. I*YL fv4

September1955 PHOTODISSOCIATION AND PHOTOIONIZATION OF OXYGEN 705

A F G

FIG. 6. "First con-tinuum" and "longestband." A: "First con-tinuum ;" F: "longestband;" G: "secondband."

100 1250 t 2LX AI)

HLya

1244.9 A and 1243.8 A were 1200 cm'1 and 540 cm',respectively. In the region between 1100 A and 900 A(Fig. 3) a weak absorption continuum made its appear-ance at a wavelength corresponding to the first ioniza-tion limit, 1040 A, and reached a maximum of about100 cm'l at 920 A. The absorption in bands between1100 A and 1000 A was comparatively weak, but thosebelow 1000 A had k-values in general greater than 600cm'l. A large number of strong and diffuse bands be-tween 840 A and 740 A made it impossible to identifya continuum in this region. Lowest k-values there areconnected by a dashed line in Fig. 4. A strong continu-ous absorption was again observed below 740 A.

From a discontinuity in the contour line it was in-ferred that the main absorption continuum had a longwavelength limit at 683 A. A smooth curve could bedrawn between 683 A and 200 A (Fig. 5).

Details on Absorption Continua

(A) The "First Continuum" at 1295 A

When k-values of less than 20 cm'l were plottedversus wavelength in the region above 1200 A, thefollowing aspects of absorption presented themselvesand are shown in Fig. 6. Source lines in the regions Fand G between dotted lines had all k-values greaterthan 20 cm7-. However, between 1275 A and 1310 Aa continuous absorption region became apparent whichhas been discussed by Tanaka.6 He suggested that the"first continuum" at 1290 A leads to the dissociationproducts 3P+1S. Watanabe et al.3 located a maximumof about 15 cm'- at 1293 A. In this work (Fig. 6) themaximum of this continuum was found to be at 1295 Awith a value of 13 cm'l. (The "longest band" (region F)and the "second band" (region G) were reported byPrice and Collins5 as "somewhat predissociated.") Thestructure of these bands could not be resolved even ina 3-meter grazing incidence spectrograph.6 It may beseen from the data in Fig. 6 and Fig. 2 that the k-values in both bands showed remarkably strong fluctua-tions from 1200 cm'l and 650 cml, respectively downto about 1 cm'l.

TABLE I. Continuous absorption between 1220 A and 1100 A.

Wavelength k (XI0-20 Wavelength k (X0-20(A) (cm') cm2) (A) (cm-') cm2)

H2 1217.2 0.55 2.0 H2 1166.3 0.55 2.0H2 1216.2 0.43 1.6 H2 1157.8 0.54 2.0HI 1215.7 0.23 0.86 H2 1157.5 0.50 1.8H2 1215.0 0.64 2.5 H2 1145.5 0.56 2.0H2 1214.6 0.40 1.5 H2 1143.8 0.30 1.1H2 1188.8 0.48 1.8 H2 1143.0 0.43 1.6H2 1188.6 0.38 1.4 H2 1110.2 0.31 1.1H2 1187.5 0.43 1.6 H2 1110.0 0.43 1.6H2 1187.0 0.35 1.3 H2 1109.2 0.32 1.2H2 1186.6 0.38 1.4 H2 1108.5 0.36 1.3H2 1167.3 0.30 1.1 H2 1096.0 0.44 1.6H2 1167.0 0.28 1.0

(B) Continuous Absorption between 1220 A and1100 A and the Absorption near Lyman a

Watanabe et al.3 reported that a group of low k-valueswas found in the region between 1050 A and 1300 A,e.g., 0.5 cm- at 1166 A, 0.4 cm'l at 1189.5 A, and0.3 cm-' at 1215.6 A. They also pointed out that theirresults might be considered semiquantitative becausetheir H2 -spectrum was "poorly resolved." The lowk-values obtained from the present investigation areshown in Table I and Fig. 7 with a probable error ofabout 0.03 cm7-. Among thousands of source lines inthis region, no one had a k-value less than 0.23 cm-l.Furthermore, absorption work has been done on H2,N2, and argon using identical procedures with the sameinstrument. These gases showed no measurable ab-sorption in this region. Hence the low absorption by02 was not due to scattering or impurities in the instru-ment, but rather to a weak dissociation continuum of02. Since there were numerous strong and weak bandsin this region, this continuum became evident only inseveral narrow sections where bands were weak orabsent. Both its long and its short wavelength limitswere obscured by bands. The possible dissociationproducts were 0('D)+O('D) or 0( 2P)+O('S).

Lj~l I tI

I I

.5 A

-k(cm

20

10

,9 0

F OS JJD1400 1305 1200 1100 I00 OX(A)

FIG. 7. Continuous absorption of 02 between 1300 A to 1000 A.E: Schumann-Runge dissociation continuum; A: "first con-tinuum;" F: "longest band;" B: weak continuous absorption;C: continuum near 1100 A; D: first ionization continuum; a, b, c:positions of three minor maxima found by Tanaka (see reference6); long arrows: dissociation and ionization limits. The curve inthe middle is referred to the left ordinate and the one on the rightto the right one.

10

5

-. . . . . ..

IiI �I

I

I

a bI

P0 L Jo 45

3cm)

FIG. 8. Absorptionof 02 in the vicinity

l | of Lyman a. The2 For r rectangle indicates

2 the width of atmos-pheric window (k<0.46 cm-'); 0:k-value at Lyman a;X: k-values nearLyman a.

1219 1217 215 123

Particular attention has been paid to the absorptioncoefficient at hydrogen Lyman a. Its k-value as ob-tained by Preston,7 Watanabe et al.,3 and Ditchburnet al.5 was 0.28, 0.30, and 0.226 cm-', respectively. Thebest value in this investigation yielded 0.23 cm-' andwas also pressure independent over a range from 1micron to 23 mm Hg in agreement with Ditchburn,who also computed the intensity of Lyman a in thesolar radiation at high altitudes. As may be seen fromFig. 8 their assumption of a smooth contour of theabsorption coefficients in the vicinity of Lyman a isquite justified. The absorption near its short wave-length wing was stronger than near the long wavelengthwing. Therefore one may expect a shift of the center ofLyman a line at high altitudes as Ditchburn8 pointedout. Obviously, the width of Lyman a encountered inhigh altitude rocket investigations must be limited bythe width of the "atmospheric window," 2 A to 3 A.The significance of this and other atmospheric windowswill be discussed later.

(C) Te Continuous Absorption between1098 A ad 1040 A

Price and Collins' observed a weak continuum start-ing from 1105 A and extending toward shorter wave-lengths. Watanabe et al.3 supported their observationsand estimated the intensities to be about 2 cm-'. Inthe present work hundreds of source lines in this regionwere available and lines with comparable low k-valueswere grouped and are shown in Table II and Fig. 7.

TABLE II. Continuous absorption of 02 between 1098 and 1030 A.

Wavelength k (X10-' Wavelength k (X10-(A) (car 1) cm2) (A) (Cm-') cm2)

H2 1098.8 3.7 1.4 H2 1068.2 4.3 1.6H2 1096.0 2.8 1.0 H2 1067.7 5.8 2.1H2 1092.2 3.5 1.3 H2 1051.2 3.2 1.2H2 1077.4 3.3 1.2 H2 1045.0 5.4 2.0H2 1070.3 3.3 1.2 H2 1044.6 5.6 2.1H2 1070.1 3.5 1.3 H2 1044.0 4.9 1.8H2 1069.7 3.2 1.2 H2 1036.2 10. 3.7H2 1068.4 4.6 1.7 H2 1030.0 13.5 5.0

TABLE III. Lowest absorption cross sections of 02 in theregion of its first ionization continuum.

Wavelength (A) k (cmr-) u (1018 cm2)

H2 1036.2 10 i 2 0.37H2 1030.0 13.54 2 0.50H2 1024.6 17.5± 2 0.65H 2 1016.7 19.5± 2 0.72H 2 970.3 53 i 5 2.0H2 953.0 78 - 8 2.9H 2 945.3 78 ±10 2.9H2 920.0 106 15 4.0.AI 919.8 83 ±20 3.20III 898.7 86 ±15 3.2AIIT 878.7 93 ±20 3.4ATV 850.6 81 ± 10 3.0

The probable error in these measurements was about0.5 cm-'.

The dissociation limits O('D)+O('S), 0( 3 P)+O(S),and O('D)+O('D) have been computed from theenergy of the metastable oxygen atom and the dis-sociation energy of 02 into OOP)+O( 3 P). They areindicated in Fig. 8, where one can also see a sharp riseof k-values near the O('D)+O('S) limit. This seems toshow the onset of a dissociation continuum in whichthe reaction O2+hv ->O('D)+O('S) took place. An-other sharp rise of k-value occurred near 1042 A at thefirst ionization limit of 02 which was computed by thecyclic method" to be at 1040 A. While the D+S-dissociation continuum may extend to shorter wave-lengths, the stronger ionization continuum overlappedit at 1040 A and obscured its short wavelength limit.

(D) Te First Ionization Continuum andPreionization Bands

The first ionization limit of 02 was first found fromthe electron impact experiments. 2 Below this limitappeared many diffuse bands (Fig. 3) and thereforethis weak continuum was wholly or partially obscuredin many investigations. By selecting, as before, allsource lines of lowest k-values as shown in Table IIIand connecting them as in Fig. 3 and Fig. 9 a smooth

FIG. 9. The firstionization contin-uum. The arrowmarked by 02+ in-dicates the wave-length correspondingto the first ioniza-tion limit of 02;those marked by 0+and N+ indicate thefirst ionization po-tential of atomicoxygen and nitrogen,respectively (dis-cussed in paragraphon Applications).

rk10150 .

50

. * . . , . l IX(A)1000 900 800

11 R. S. Mullikan and D. C. Stevens, Phys. Rev. 44, 720 (1933).1

2 J. T. Tate and P. T. Smith, Phys. Rev. 39, 27(1932).

706 Vol. 45

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September1955 PHOTODISSOCIATION AND

contour was obtained in the region from 1040 A to900 A with a long wavelength limit at 1042 A, in goodagreement with the computed value. Its absorptionrose to a maximum of 110 cm-l at 920 A and seemed todecrease gradually at 850 A. The position of thismaximum has been estimated to be at about 920 A byapplying the Frank-Condon principle to the potentialenergy curves" of the 02 3Y,- and 02+ 2II states.Since the hydrogen many-lined spectrum was weakbelow 900 A, the measurements were carried out with alight source previously described.9 The f-value of thiscontinuum was estimated to be 0.06.

The photoionization cross sections given in Table IIIwere considerably lower than those obtained by Wainfanet al.14 through direct measurements of photoionizationefficiencies in a normal incidence vacuum monochroma-tor. Their resolving power was comparatively poor(5 A or 10 A band width at exit slit), and preionizedbands might have contributed additional photo-ionization to their results. In Fig. 3, the black trianglesindicate their data on total photoionization cross sec-tions due to all possible processes. It should be noticedthat their values are higher when found in the vicinityof strong bands, in agreement with this explanation.

(E) Absorption of 02 between 835 A and 683 A

No k-value of less than 400 cm-l has been found below827 A. The absorption spectrum of 02, photographedwith the helium molecular continuum as a background,showed strong continuous absorption in this region,superimposed by many diffuse bands. This apparentcontinuum might not be real and rather be due to thesuperposition of these bands The dotted line in Fig. 4represents the upper limit of the apparent continuousabsorption estimated from microphotometer traces ofthe helium continuum. Apparently there was a maxi-mum around 800 A and a flat portion between 780 Aand 740 A. Since source lines were not numerous enoughin this region, the possibility of finding low k-values onthe outside or in the tail of bands was not eliminated.Moreover, in using the helium continuum low k-valuesin very narrow spectral regions might be missed owingto blending.

Below 740 A, the bands were not so crowded as in theabove region and the spectrum showed an obviouscontinuous absorption starting at 740 A, confirmingthe observations of Price and Collins.5 In Fig. 4, k-values are plotted for the source lines OII 718 andAIII 691 which were found to be on the outside ofbands and probably represent the intensity of con-tinuous absorption near the ionization limit 02+ AHUwith k-values of 503 cm'l and 450 cml, respectively.Wainfan et al.'4 reported five values in this region whichlie reasonably close to the contour proposed here.

"3H. S. W. Massey and E. H. S. Burhop, Electronic and IonicImpact Phenomena (Clarendon Press, Oxford, 1952), p. 257.

14 Wainfan, Walker, and Weissler, J. Appl. Phys. 24, 1318(1953). Further data to be published in Phys. Rev.

TABLE IV. Lowest absorption cross sections of 0 inits main ionization continuum.

Wavelength(A)

OII 675.2XOII 644.

OIV 608.NII 582.OIII 580.NII 547.OII 539.OIl 537.OII 518.OII 515.

814148.15,.2.6

k(cm')

490538550580585550590570560590

(X10-'7

cm2)

1.82.02.02.12.12.02.22.12.12.2

Wavelength(A)

OII 458.42XNII 428.2

2XOII 345.32XOII1 328.72XOIII 328.42XOIII 320.92XOIV 306.8

0111 295.72XNIV 247.22XOIV 207.2

k(cm-,)

570540450440415420440420350330

(X10-7cm2)

2.12.01.71.61.51.61.61.61.31.2

(F) The Main Ionization Continuum and Conclusion

In Fig. 5 the low k-values below the fourth ionizationlimit, 02+b 4 _-, formed a smooth curve from 683 Ato 200 A. Their values are given in Table IV with aprobable error of about 40 cm-'. A flat maximum of590 cml was found at 510 A. The f-value of this con-tinuum was measured to be 5.7 and may be comparedto the value calculated by Ladenburg and Wolfsohn,'5

from dispersion data, namely f3= 5.93 at X3= 544 Awhere the subscript 3 indicates the third and dominantterm in their dispersion equation. The data of Wainfanet al.'4 (shown as black triangles in Fig. 5) were inagreement with those presented here within the errorlimits.

In conclusion the information on the absorptioncontinua of molecular oxygen in the ultraviolet regionof its spectrum is summarized in Table V.

ADDENDUM

Absorption Coefficients of N2 in the WavelengthRegion above 910 A

The absorption coefficients of N2 between 1050 Aand 910 A have been remeasured in the same manneras 02 and are presented in Fig. 10. The tank nitrogen

FIG. 10. Absorption of N2 between 1050 A and 900 A. Theshaded regions represent bands, the space between them areN 2-windows. The arrow marked by O+ indicates the ionizationlimit of atomic oxygen, important in atmospheric absorption(see Applications).

15 R. Ladenburg and G. Wolfsohn, Z. Physik 79, 42 (1932).

PHOTOIONIZATION OF OXYGEN 707

TABLE V. The photodissociation and ionization continua in the ultraviolet.

Maximum Maximum crossTransition of k-value section

Spectral region molecular states Reaction (cm-') (cm,) Fig.

1750 to 1350 A 0232 - - 02 32- 02+117 O(ID)+0(3P) 490 1.8 X10-17 reference 11320 to 1270 A ? 02+ - 0(3P)+O('S) 13 4.8 X10-'9 61220 to 1098 A ? 0 2 +ky -1 O(QD)+O('D) 0.23 0.86X10-2° 7

or O0(3P)+0(IS) 3.2 1.2 X 10-19 71096 to 1040 A ? 02+zy O (ID)+0((S)1040 to 850 A 02 32g 02+X2 O2+1 02+X 2Ill 0 e7 100 3.8 X10-18 9740 to 683 A 02 3yg -0 2 +A

211 02+117Y 0 2+A 2 ±e 510 1.9 X10-17 4683 to 200 A 02 1 > 02+b 2 e 2+- 02+b 41 +e 590' 2.1 X 10-17 5

used contained 0.3% impurities of oxygen and raregases. No measurable absorption was observed above1040 A even when the highest pressure of N2 in thespectrograph was 73 mm. This gave its k-values anupper limit of 0.05 cm-l. Below 1040 A there existedstrong bands analyzed by Worley,'6 but source lines onthe outside of these bands or in their tails showed lowabsorption coefficients of less than 1 cm-l. Theseresults might include absorption by impurities. Sincethe absorption of N2 is so low, the presence of 02 insmall amounts, say 1 part in 103, might possibly pro-duce results which are 50% higher than for pure N2.However, the present data at least indicate an upperlimit, and one must conclude that no absorption con-tinuum of N2 greater than 1 cm-' exists above 910 A.

APPLICATIONS*

The F-Layers and the N2 -Window

If the composition and distribution of particle densityin the upper atmosphere were known, one could calcu-

.300

5

-

.E

*s 100

H contLinum

N.W. l02l

N2 CM6A)

.0 0 08MAc Nit 02D (O910A)

ittlO404)

1600 1200 800Wavelength (A)

400

FIG. 11. Heights of maximum absorption of solar radiation bymajor constituents in the ionosphere. Above 120 km: majorconstituents N2 and 0; constant temperature gradient 7.4 5K/km;inverse square gravity. Below 120 km; composition same, as atsea level except dissociation of 02, corrected by values afterMitra (see reference 21); density distribution calculated fromdata in rocket experiments. F2 represents the absorption by thecombined N2 and 0; F, by 0; E by 02, and D.O. dissociation of02 in Schumann Runge continuum. N.W.Eo inroo: N2 -window;A.W.00EIDEDI: atmospheric windows; and D: height of D-layer.

16R. E. Worley, Phys. Rev. 64, 207 (1943).*The following calculations are merely meant to illustrate

some of the uses to which the absorption coefficients presentedhere may be put. However, since the conclusions are based on aspecific model of the atmosphere, they may not be generally valid.

late the layer height of the maximum rate of absorptionfrom the absorption coefficients. If furthermore therecombination of ions in the ionosphere is a simpletwo-body process, it can be shown that the height ofmaximum ion density is identical with the height of themaximum rate of absorption.'7 The boundary of dis-sociation of 02 occurs approximately at an altitude of120 km; above this height the 02 is completely dissoci-ated into atomic oxygen. Clark gave computed curvesrelating absorption coefficients to layer heights ofmaximum absorption above 120 km. He assumed thatN2 and 0 are the sole components above 120 km andshowed that the combined absorption by N2 and 0 ofthe principal line HeI 584 would have a maximum at293 km, the approximate height of the F2 layer. Usinghis computed curve (for a= 7.360 K/km in his Fig. 2)and k-values of N2 by Weissler et al.18 and of 0 byBates and Seaton," the heights due to the combinedabsorption by N2 in its ionization continuum and 0below 796 A were found to be in a range between 310km and 285 km. This is comparable with the averageobserved height of the F2 layer, 320 km (Fig. 11).Similarly, solar radiations between 910 A and 796 Ashow a maximum absorption in atomic oxygen at layerheights between 215 km and 220 km in agreement withthe observed height of the F, layer, 220 km. Here 910 Aand 796 A are first ionization limits of 0 and N2 ,respectively. Since the absorption coefficients of bothN2 and 0 in their ionization continuum fall off in aslow manner toward shorter wavelengths, probably noradiation below 910 A can reach the level of 120 km.For radiation above 910 A, N2 alone would be theabsorber and therefore its transparency between1040 A (0 2+X 2 lle) and 910 A (+) would be importantbecause solar radiations in this region are capable toionize 02 existing below 120 km. It can be shown fromClark's curve that radiations having k <1.6 cm-' inN2 can penetrate through the upper atmosphere downto a level of 120 km. In Fig. 10 the shaded rectanglesindicate such a "N2-windows" where the absorptionis less than 1.6 cm-', and Table VI list their widthsmore accurately.

17 K. C. Clark, Phys. Rev. 87, 271 (1952).18 Weissler, Lee, and Mohr, J. Opt. Soc. Am. 42, 84 (1952).'9 D. R. Bates and M. J. Seaton, Monthly Notices Roy. Astron.

Soc. 109, 6 (1949).

708 PO LEE Vol. 45

September1955 PHOTODISSOCIATION AND PHOTOIONIZATION OF OXYGEN 709

TABLE VI. "N2-windows." (Regions where k <1.6 cm-'.)

Region (A) Width (A) Region (A) Width (A)

913.0 to 915.5 2.5 963.0 to 965.5 2.5917.0 to 919.0 2.0 968.7 to 971.0 2.3932.0 to 934.5 2.5 976.0 to 980.0 4.0943.5 to 945.3 1.8 984.0 to 992.0 8.0950.5 to 955.0 4.5 above 1000 A

The E-Layer and Atmospheric Windows

Assuming the composition of the atmosphere below120 km to be the same as at sea level except for the dis-sociation of 02, one computes the particle density, p,as a function of the altitude, x, from the pressures andtemperatures measured in rocket experiments.20 Fromthis distribution of particle density and its slope,dp/dx, one can compute the height of maximum ab-sorption for a given absorption cross section, a-, by agraphic method using the relation dp/dx= - p2o-. Thisis true only if Beer's law holds. A correction of thedensities for the dissociation of 02 was made by usingvalues given by Mitra.2 ' A survey calculation using theabove procedure and the absorption cross sections of02, showed that the layer heights of maximum absorp-tion in the first ionization continuum of 02 above 910 Alie in a range between 90 km and 102 km. If one takesk=32 cm-l at X=1000 A (Fig. 9), where N2 is verytransparent (Fig. 10), the corresponding height ofmaximum absorption was found to be at 101 km. Thiswas comparable with the observed height of the Elayer, 100 km.

Radiations in the previously mentioned pre-ionizationbands are also capable of ionizing 02 below 120 km.Since their corresponding k-values are high, maximumionization may be expected at about 110 km.

Similar calculations showed that no wavelengthsbelow 1040 A could reach 85 km; however, wavelengthsabove 1090 A with k <0.46 cm'I could penetrate to80 km. N2 is transparent above 1040 A (see Addendum)and 02 has several low k-value regions or "windows"

20 G. P. Kuiper, The Atmospheres of the Earth and Planets(The University of Chicago Press, Chicago, 1952), second edition,p. 140.

21 S. K. Mitra, The Upper Atmosphere (The Royal AsiaticSociety of Bengal, Calcutta, India, 1947), p. 138.

between 1100 A and 1220 A, where all k-values wereequal to or less than 0.46 cm-l (Table VII).

Hopfield2 2 once suggested that "uv spectra of celestialbodies may be observed from high-altitude rocketsthrough atmospheric windows in the region 1000 A-1300 A." (See Table I.) Lyman a falls on such a windowand since it is a strong component of the solar radiation,considerable ionization near 80 km produced by ioniz-ing minor constituents such as NO is to be expected.In addition photodissociation of 02 in the weak con-tinuum between 1220 A and 1100 A also takes place atthis height which would produce metastable atoms inthe O (ID) and/or O (IS) states as well as atoms in the3P ground state.

TABLE VII. Atmospheric windows in 02.

(Regions where k <0.46 cm-1.)

Region (A) Width (A) Region (A) Width (A)

1214.7 to 1217.0 2.3 1142.4 to 1144.9 2.51186.5 to 1188.5 2.0 1108.0 to 1111.2 3.21166.4 to 1167.4 1.0

In summary, the above results strongly support thetheory of formation of the E, F1, and F 2 layers suggestedby Bates and Massey.2 They ascribed the F2 layer toionization of N2 at its first ionization potential (796 A),the F, layer to ionization of 0 at its first ionizationpotential (910 A) and the E layer to ionization of 02

at its first ionization potential (1040 A). Radiationabove 1100 A may penetrate through the atmosphericwindows to form the D layer probably by ionizingNO.2 4 The layer heights of the maximum absorptionof solar radiation by major ionospheric constituentsdiscussed previously are summarized in Fig. 11.

ACKNOWLEDGMENTS

I wish to thank Professor G. L. Weissler for hisfriendship and helpful suggestions and also to acknowl-edge gratefully the aid of the Office of Naval Research.

22 J. J. Hopfield, Astrophys. J. 104, 208 (1946).23 D. R. Bates and H. S. W. Massey, Proc. Roy. Soc. (London)

A187, 261 (1946).24 D. R. Bates and M. J. Seaton, Proc. Phys. Soc. (London)

63, 135 (1950).