MARCH, 1940

An Estimate of the Absorption of Air in the Extreme Ultraviolet

EDWIN G. SCHNEIDER*Research Laboratories of Physics, Harvard University, Cambridge, Massachusetts

(Received December 11, 1939)

Quantitative measurements on the absorption coefficient of air have been made for aboutthree hundred and fifty points between 380 and 1600A. Because of difficulties in maintaininga constant intensity with extreme ultraviolet light sources, it was necessary to use a many-linespark source rather than a continuous spectrum, with the result that much of the fine structureof the absorption bands has been lost. Photographic measurements with oiled plates showstrong absorption between 1600A and 1300A, a quite transparent region between 1300A and1000A, and a very highly absorbing group of bands at the shorter wave-lengths. Although thesebands below OOOA are poorly resolved, a wealth of fine structure is evident.

ALTHOUGH it has been known for manyyears that air is opaque to extreme ultra-

violet radiation,' quantitative values for theabsorption coefficient have been obtained foronly a few wave-lengths. 2 The data presentedin this paper give a picture of the gross featuresof the absorption structure in the region between1600 and 380A. Since the line source used forthe measurements was a rather poor substitutefor a continuous spectrum, the results giverelatively little information concerning the de-tails of the fine structure.

The measurements in the region above 1300Awere made during the course of another investi-gation4 by introducing air into an absorption cellwith lithium fluoride windows.

For the region below 1260A a one-meter,15,000 line to the inch glass grating was used atnormal incidence. The light source was a "hotspark" between a silver solder and a tungstencarbide electrode. This source was chosen be-cause its wealth of lines approached a continuousspectrum and its intensity could be held reason-ably constant from one exposure to the next.A constant-intensity continuous spectrum, if ithad been available, would have been much moredesirable for these measurements. The excitationfor the spark was furnished by a 25,000-volttransformer. Alternating current from the trans-

* Now at Stevens Institute of Technology, Hoboken,New Jersey.

I T. Lyman, Spectroscopy of the Extreme Ultraviolet(Longmans, Green and Company, 1928).

2 R. H. Messner, Zeits. f. Physik 85, 727 (1933).R. Ladenburg and C. C. Van Voorhis, Phys. Rev. 43,

315 (1933).4 E. G. Schneider, J. Chem. Phys. 5, 106 (1937).

former charged a condenser of proper value togive electrical resonance in the spark circuit.After the spark had been run long enough tooutgas the electrodes, the intensity remainedconstant to about 20 percent provided theseparation of the electrodes was maintained atthe point where the spark would barely runcontinuously or even occasionally fail to jump.Because of the erosive action of the spark, itwas necessary to grind the tips of the electrodesafter about an hour of use in order to keep thegap in line with the slit.

Since the absorbing column of gas was ob-tained by introducing air at pressures between0.1 mm and 0.005 mm into the body of thespectrograph, it was necessary to use a narrowslit and a fast pump to keep the spark chamberwell evacuated. Because a slight change in thepressure in the spark chamber was inevitableunder these conditions, the effect of air on thespark was measured by introducing air into thechamber while keeping the body of the spectro-graph well evacuated. No measurable change inintensity was observed as long as the pressure inthe neighborhood of the electrodes was below0.001 mm. The pressure in the spark chamberwas always well below this value during theexposures used for measurement.

The air used for absorption measurements waspassed through a potassium hydroxide solutionto remove carbon dioxide, and was dried byprolonged exposure to phosphorous pentoxide ina storage bottle. The interior of the spectro-graph was kept dry with activated alumina and adry-ice acetone trap. During an absorption

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J. -. S. A. VOLUME 30

ABSORPTION OF AIR IN EXTREME ULTRAVIOLET

exposure the spectrograph was isolated from thepump except for a small flow of air throughthe slit. The volume of the spectrograph wassufficient to keep the pressure constant withinfive percent without supplying gas to compensatefor that lost through the slit. Since the uncer-tainty in pressure was small compared with thevariation of intensity of the light source, theaverage pressure was used in computing the ab-sorption coefficient.

Cramer Contrast photographic plates sensi-tized with a 25 percent solution of Cenco pumpoil, 11021, in petroleum ether were used torecord the spectrum. Because of the wide varia-tion in the transmission of the air, it was notconvenient to match the density of each lineobtained through the absorbing column with thatobtained through high vacuum. On the otherhand, it was possible to obtain many lines onboth the absorption and comparison exposurewith densities on the straight portion of thedensity-log exposure curve. Adjustment of thegas pressure and the time of the exposure per-mitted the placement of any desired spectrumregion on the linear density portion of the curve.A density-log exposure curve for each plate wasplotted from calibration exposures and was usedfor comparing the intensities of the absorptionand comparison spectra on that plate. By holdingthe time constant for all exposures on the platethe curve could be reduced to a density-logintensity curve, the logarithm of the time beinga common additive constant to all points on theabscissa, by a suitable shift of the origin. Thelogarithm of the intensity corresponding to agiven density was read directly from the lattercurve.

The calibration curve was obtained by ex-posing the plate in air to fluorescent light froma thin film of the oil excited by a quartz mercuryarc. The intensity of the fluorescent light wasvaried by changing the distance of the platefrom the oil film. The inverse-square law wasused to compute the intensity.

Samples of the oil were irradiated with singlemercury lines isolated with a quartz spectro-graph and the resulting fluorescent spectrum wasphotographed. It was found that all wave-lengths shorter than the fluorescent band (3500-4200A) gave the same fluorescence while those

within the lower edge of the band gave a longerwave-length emission. Light of wave-lengthgreater than about 4000A produced no meas-urable fluorescence. Although the shortest wave-length used in these tests was 2537A, it wasassumed that the extreme ultraviolet excited thesame fluorescence. The general radiation from ahydrogen discharge tube excited the same radia-tion from an oil film on a fluorite window as didthe complete mercury arc spectrum so thisassumption is probably valid in the region aboveabout 1250A.5 Thin films of the oil when excitedby the general radiation from the arc did notexhibit the long wave-length emission probablybecause of the transparency of the oil to lightwithin the fluorescent band. The assumption wasmade that these tests made on oil samples inair would apply to oil films in vacuum.

The oil film used for the calibration exposureswas formed by pressing a drop between a quartzplate and the totally reflecting face of a right-angle quartz prism. The exciting radiation wasremoved by total reflection while the fluorescentlight readily escaped. Although no light ofwave-length longer than the desired fluorescentband could be observed, a weak solution ofiodine in carbon tetrachloride was inserted in acell between the oil and the plate to removeresidual long wave radiation.

Within the spectrograph some scattered lightwas evident in the regions where the lines werecompletely removed by the air. Since most ofthis light came from the two surfaces of theglass grating, it was not possible to obtain anabsolutely clear background. A fairly consistentcorrection could be made by subtracting thesmall density due to scattered light from thatof the line, since the scattered light did not varyrapidly along the plate and appeared to belargely visible radiation which would not bechanged by the presence of the absorbingcolumn.

Table I gives the absorption coefficient, k, forair together with the estimated experimentalerror for the measured wave-lengths. k is theabsorption coefficient per centimeter of air whenreduced to 0C and 760 mm of mercury pressure

I G. R. Harrison and P. A. Leighton, J. Opt. Soc. Am.20, 313 (1930).

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130 EDWIN G. SCHNEIDER

and is defined by the equation I = oe-kd where only for air at pressures of a fraction of aIo and I are the incident and transmitted in- millimeter because of pressure broadening of thetensities, respectively, and d is the thickness of bands. An examination of Table I shows thatthe absorbing column when reduced to NTP. in many places wide fluctuations in the absorp-Although the values of k have been reduced to tion occur with a change of wave-length of oneatmospheric pressure, they probably are valid or two angstroms. For this reason no interpola-

TABLE I. Te absorption coefficient per centimeter of air at 0°C and 760 mn pressure. Wave-lengths in angstroms.

WAVE- ABSORPTION WAVE- ABSORPTIONLENGTH COEFFICIENT LENGTH COEFFICIENT

I _ _

382.5384394.5420425426.5428.5431435.5437.5438.5440443445446.5449450451.5452.5453454.5458459.5462.5463.5464.5469472.5474476.5479484487491.5493.5495497498.5499.5502504.5506511.5516519520.5523.5524.5526527.5530532534534.5538.5540.5543544

280 ±50290 a50290 ±50170±50170i50180 ±50140 450340 ±50260 ±+50260 ±50360 ±50260 ±50300 ±50400 ±503204±50330 ±-50300 ±50250±+50370 ±50400 ±50390 ±50270 ±50420 ±503 10 ±50320 ±50330 ±50250 ±50280i50230 ±50220 ±50210 ±50330 ±50270 ±50430 ±50240±50240 ±t50390 ±-50350 ±50400 ±50250 ±450420 ±+50440 ±50370 ±50260 ±50370 ±t503 10 ±50310±50350 ±450310a±503504±50290 ±-50390 ±t50370 ±t50340 ±t50470 ±450330 ±t50350 -503204±50

546548.5551553.5558560560.5561.5562.5564571.5572.5574.5575.5578581583584585.5589.5595597598.5602.5604.5606609612613.5617623.5624.5627.5636639.5641.5649651.5657665.5668.5670672.5676677.5682687.2688690.5691.5693.5700.5703710714715.5716.5718.5

400 ±50410 ±t50380i50340 ±450330 ±450350--50360 j50370 ±t50330 ±t50330 ±t50270 ±t50280 ±50430 ±50320 ±t50270±=50260 ±t50280=±50320 ±t50290 ±50300 ±=50290 --50320±i50280 ±=50260 ±50250 ±=50420 ±50290=±50220 ±=50220 ±=50230 ±=50230 ±=50160 ±50220=±50180 ±t50440±t50270 ±=50190 ±=50340 ±t50200 ±=50230 ±=50220 ±=50210=±50250 ±50450 ±=50440 ±t50570=±50440 ±=50450±=50340±t50390 ±t50290±t50410 ±t50430 ±50280=±50360=±50350 ±50340 ±450360 ±=50

WAVE- ABSORPTIONLENGTH COEFFICIEN

719.5 400==50724 540±=50726 470±=50727 440 ±=50730.5 4904±50732 420±=50733.5 330 ±=50735.5 380±=50737 4204±50738.5 360±=50741 260±=50745 460 ±=50748.5 230 ±t50753 200±=50755 310±=50757.5 310 ±450761 720±=100762.5 230±=50764 340=±50765.5 470 ±=50768 370±=50772.5 410±t50775.5 370±=50777 260±=50778.5 310±=50780.5 540--50783 250 ±=50786.5 580 ±t50788.5 310 ±=50790 440 ±50791.5 640 ±t50792.5 320±=50797.5 260 ±50799.5 270 ±t50801 570±=50803 220±t50806.8 850--50810.5 280 =50812 270±t50814 490 ±50815 190±t50818 190±t50821 200-50824 100=±40829.5 120±4t40833.5 100±=40835.5 180±t50838.5 150±t50841.5 160±t50842.5 150±t50844 160±t50845.5 110±4t50848 60±t30849 100=±40850 310±t50851.5 150±t50854 320-50856 160±t40

WAVE-LENGTH

.-~~~858.5864866867.5869.5873878880881884.5886887.5889.5892.5894896.5899900901.5902907908909.5911912.5913.5915.5917921923925926927.5929.5933.5935.5939.5940942943944.5946948949950.5953954955.5957960.5962964965966969971972

ABSORPTIONCOEFFICIENT

90±44080±440

130 40220 4±50110±44060 430

120 40110±+402204±5080 40

240 50150 440180 40170 40160 ±-40130 ±40130 ±40130 ±401404±40140 ±40200 ±40120 ±40150 ±40190 ±40100 ±40230 ±40130 ±-40190 ±40120 ±40140 ±40150 ±40110±440140 ±40300 45045 ±2024 ±10

300 ±450140 ±4040±-2080 ±43090 ±3060 ±430

130 ±40100 ±3040 2010 ±10

100 ±30120±440110 ±30170 ±4090 ±3080 ±130

100 4±40230 ±t50110 ±-4022 ± 10

120 ±40

WAVE- ABSORPTION WAVE- ABSORPTIONLENGTH COEFFICIENT LENGTH COEFFICIENT

973974.5981982.5983.5987988990.5991.5993.5995.5

100010021003.51006100910101011101310151019.5102510261031.510411046104810491050.51055.5106110631066.51072.510741077.51078.51079108010831084.5108810931100.51102.51106110811121113.51121.511251130.511331135113611371138

200 ±50160=±4060 ±20

140 ±402 10 ±5050=±2050 ±208±10

80±t30150=±4014O1014=t1040 -2035 ±2030 ±2080 ±3035 ±20

120 -406±107±10

15 -1040 ±308-10

26-t2028 ±2013 ±1035 1050 ±3012 -105±4106±10

20=±1013 1025 ±2012 ±104±10

10 ±41010t 1011±1012 1014 -101±1O

22 101±410

11-104±10

- 2 1025 ±204-109±109±105±106±10

13 t 1013 ±t1O13 ±1010 ±10

11401141.5114411481150.51152.511541155.511581159.51161.5116411651169.511801183.5118611891192.51196.51197.51200.512031204.512061207.512091210.51214121712191225.512271228.5123412351239.51241.51242.512441245.512501251125212551258.51263.5134314031436145014641496151715771596

0±4102=107±107±10

10=t1041=±2028=±109±102±4104±10

31-2013 ±109±107±104±10

17±10- 2 1016=±105±10

21± 1O7±10

11±tO37 ±t1095 ±2080 ±2070 ±2018-t1010=±102-103±105±1O8±102±103±108±105±102±4105±1O

70 ±20130 ±4019-O105-103±105±105±103±1O

14=±1030=±10

100=±20100=±20100 ±41085 1075 ±41070=t1040=±1030 -10

l-- -

ABSORPTION OF AIR IN EXTREME ULTRAVIOLET

P=0.08mm

FIG. 1. Spectra taken through 200 cm of air at indicated pressures. Pressure about 10-5 mm for comparison exposures.

tion between points may be considered valid.Furthermore, since much of the fine structuremust be essentially monochromatic, it is unlikelythat some of the extremely high absorptionpeaks, such as the one at 806.8A, are actually somuch higher than others in the immediateneighborhood. These extremely high narrowpeaks probably indicate a fairly exact coin-cidence between lines in the light source andstrong absorption lines in the air.

Since the values for the absorption coefficientin the region above 1300A are in agreement withthe measurements of Ladenburg and Van Voorhis3

on oxygen, when allowance is made for thepartial pressure of oxygen in air, the bulk ofthe absorption in this region may be attributedto oxygen.

A comparison of the bands in Table I withthe published photographs of Price and Collins6

shows that most of the strong absorption be-tween 825 and 1300A is also due to oxygen.Although much of the broad oxygen structurein the photographs of Price and Collins may beidentified in Table I, much of the fine structurehas been missed.

Below 800A most of the absorption is probablydue to nitrogen, but the only series which hasbeen identified is one reported by Hopfield7

which extends from 723 to 665A and only a fewof the members of this series are sufficientlyresolved by the tungsten carbide line source tomake identification reasonably certain.

Figure 1 shows the general characteristics ofthe absorption of air, the most striking feature

6 W. C. Price and G. Collins, Phys. Rev. 48, 714 (1935).7 J. J. Hopfield, Phys. Rev. 36, 789(A) (1930).

being the relatively transparent region between1250 or 1300A and about 1000A. Lyman8 hasreported experiments in which spectrogramswere obtained through as much as 4.5 cm ofair at atmospheric pressure in the region be-tween 1245 and 1185A. The lower limit of thisrange was set by the window on the spectrographrather than by the air. A crude guess at theabsorption coefficient from Lyman's plates indi-cates a value of 0.5 per centimeter of air atatmospheric pressure or less. The best value ofthe absorption coefficient in this region has beenobtained by Dr. William Preston of HarvardUniversity by using the 1216A atomic line ofhydrogen. By introducing dry, carbon dioxidefree, air at a few centimeters pressure into a cellabout 20 cm long with lithium fluoride windows,an absorption coefficient of 0.063 per centimeterwhen reduced to NTP was obtained. A photo-electric cell with a lithium fluoride window wasused to measure light intensities.

The values for the absorption coefficient in thisregion in Table I are too uncertain to give morethan an indication of the order of magnitude ofthe absorption compared with that in otherspectral regions. The air pressure in the spectro-graph needed to obtain accurate measurements ofabsorption coefficients less than about 25 percentimeter was so great that the pressure in thespark chamber could not be kept low enough toinsure constant intensity of the spark. The figuresfor the small absorption coefficients may beassumed to give, therefore, only an approximateupper limit of the absorption.

Furthermore, because of the extreme com-

8 T. Lyman, Phys. Rev. 48, 149 (1935).

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EDWIN G. SCHNEIDER

plexity of the absorption structure of air andthe low resolving power of the line spectrumused for measurement, the data presented in thispaper are useful only in giving the absorptioncoefficient of air at the measured points and inpresenting a general picture of the amount and

type of absorption to be expected in the differentspectral regions.

In conclusion the author wishes to express hisappreciation to Professor Theodore Lyman forhis aid which made this investigation possibleand for his interest in the progress of the work.

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