RUDOLF PENNDORF Vol. 46

X, y, Y, Ad, and p are, for practical purposes, identical.But, quite the contrary is true for coniferous forests.The luminance factor Y is reduced by 3 to 4 if seendirectly from above into a forest, instead as seen hori-zontally from the ground. This effect is mostly causedby the large shadow areas. Thus, coniferous forests maybe treated as black targets for many practical problems.Furthermore, the dominant wavelength is shifted a bittowards shorter wavelengths, i.e., from the greenishyellow to the yellow green, and a very striking reduc-

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

tion of the purity occurs; it decreases from 36% to18-22%, i.e., by a factor of about 2.

Thus, we can conclude that Krinov's measurementsfrom the ground can be faithfully used to give colors asseen from an airplane as long as horizontal naturalobjects are concerned, namely bare areas, soils, mead-ows, roads, lakes, and rivers; but his values cannot beused for vertical natural objects like forests or buildings.In such cases individual measurements have to becarried out from low-flying airplanes.

VOLUME 46, NUMBER 3 MARCH, 1956

Absorption Bands of Rubidium due to the Presence of Foreign Gases*SHANG-YI CH'EN, ROBERT B. BENNETT, AND OLEG JEFMNKO

Physics Department, University of Oregon, Eugene, Oregon(Received September 19, 1955)

In the presence of heavy foreign gases, such as Kr and Xe, diffused absorption bands were observed atboth the short wavelength and the long wavelength side of the first four absorption doublets of the Rbprincipal series. The red bands were observable when the Rb vapor pressure in the absorption tube wasrelatively low (10-3 to 10- mm Hg). The violet bands were not intense enough until the Rb vapor pressurewas increased to 0.1-1 mm Hg. With other lighter gases (H2, He, Ne, A, N) only violet bands were observed.The positions and the absorption contours of the bands in relation to the member of the absorption seriesand to the nature of the foreign gases are discussed.

THE absorption band appearing on the shortT wavelength side of the second member of the

principal series of rubidium in the presence of H2 , He,N2 , and Ne has been observed.' Due to the growinginterest and experience after the corresponding ob-servations with other alkalis,2 especially with lithium3

and sodium,4 and due to the difficulties of under-standing the existing regularities in the phenomenon,this problem was taken up again, in the present work.The bands appearing in the neighborhood of the reso-nance lines and several higher members of rubidiumprincipal series were observed with A, Kr, and Xe inaddition to the above mentioned gases leading to moreunderstandable data than before.

EXPERIMENT

During the first part of the observation the experi-mental arrangement was essentially the same as before.'

* The research is supported by the National Science Foundation.'T. Z. Ny and S. Y. Ch'en, Nature 138, 1055 (1936); J. phys.

radium 9, 169 (1938).2 S. Y. Ch'en, Phys. Rev. 65, 338 (1944).3 S. Y. Ch'en and C. S. Chang, Phys. Rev. 75, 82 (1949).4 The readings of the positions of the band appeared at the

short wavelength side of the third member of sodium principalseries in the presence of H and N as quoted in reference 2 weretaken from T. S. Ke's M. S. thesis in 1940, and later werefound to be in error [Chinese J. Phys. 7, 421, (1950)]. The positionsof the band in the presence of H, He, N, and Ne were measuredagain by Ch'en as 2848.5, 2848.9, 2849.7, and 2851.4 A respec-tively, giving, consequently, a separation between the band andthe short wavelength component of the third doublet 4.3, 4.1, 4.0,and 1.4 A for the respective Na-foreign gas bands.

When krypton and xenon were used, an absorption tubewas made of Vycor glass about 4 inch in diameter withquartz windows fused at both ends. The whole tube(about 6 inches long) was placed inside a furnace core1 foot long wound with chromel-A pyrometric ribbon.The winding was separated into three sections so thatthe ends of the tube were heated to the same tempera-ture as the center of the tube as measured by threealumel-chromel thermocouples. The convection of airwithin the furnace core was reduced by placing aquartz window at each end inside the heating core.The pressure of foreign gases in the absorption tubewas about one atmosphere.

Spectra were taken with Bausch and Lomb LargeLittrow spectrograph' in both glass and quartz opticsand with the light from an incandescent tungstenribbon lamp (with ultraviolet transmitting bulb) asbackground. Kodak spectroscopic plates, type I-Nand IV-N, were used for the resonance lines, and type103-0 and III-0 were used for the high members.

RESULTS

The results of observations may be presented in twosections: (a) the bands on the long wavelength side ofeach component of the absorption doublets, hereafter

' The Rb-Kr bands were observed by means of a 35-foot gratingspectrograph in Wadsworth mounting in search for fine structure.The result was negative.

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183ABSORPTION BANDS OF RUBIDIUM

abbreviated as "red bands",6 and (b) the bands onthe short wavelength side of the doublets, abbreviatedas "violet bands".

(a) The Red Bands

A band on the long wavelength side of each com-ponent of the absorption doublet as shown in Fig. 1was perceptible only when heavy foreign gases such asKr and Xe were used. The separation between theband and the associated line decreased with the decreasein the atomic mass of the gas. The corresponding redbands for Rb/A were so close to the atomic lines thatonly a red asymmetry could be seen. For other stilllighter gases (Ne, He, and H2) no such red asymmetrywas observed. The data are given in Table I. Thefigure in parenthesis after "Rb" indicates the memberof the principal series. The second column gives theposition of the peak of the absorption band, LPmstands for the separation between the peak and theatomic line, and Av. gives the separation between theatomic line and the farther edge of the band. Thevalues for the relative width of the bands are onlyrough estimations. A negative sign in front of thefigure for Avm indicates that the band concerned islocated at the lower frequency side (red side) of theline. The accuracy of measurements depended chieflyon how distinctly the peaks or edges of the bandsappeared on the spectrograms. The bands whose peakscould not be definitely resolved are indicated with anasterisk.

The red bands were perceptible when the temperatureof the absorption tube was as low as 120C (Rb vaporpressure 1 mm), and became very conspicuouswhen the temperature was around 140C (Rb vaporpressure 2X10-3 mm Hg) for Rb(1)/Xe. When thetemperature was still higher, the band and the absorp-tion line lapped appreciably, and the band was nolonger resolved from the broadened absorption line.The temperatures given in the last column of Table Iwere optimum values for the present observations.

It is interesting to note how AVm varies with themembers of the principal series. For both krypton andxenon the maximum values of Avm are associated withthe second member of the principal series. The valueof AVm associated with the shorter wavelength (2 P@)component of the resonance lines is considerablygreater than that associated with the longer wavelength(2 P.) component for both gases.

It is quite apparent from Table I that for any givendoublet Avm increases in the order A, Kr, Xe which isalso the order of increasing polarizability and massof the gases. There are not enough points to make anaccurate decision of their relationship.

No apparent change in the position of the maximaof the bands was observed with variation of pressure

6 Such bands for Rb were first observed by Ch'en and Jefimenkowhen saturated hydrocarbons were used as foreign gases. Amanuscript is in preparation.

(a) (c)

(b) (d)

FIG. 1. The diffuse bands appeared on the long wavelength sideof each of the doublet components of the principal series ofrubidium in the presence of xenon. (a) For the resonance lines.(b) For the second doublet. (c) For the third doublet. (d) For thefourth doublet. The corresponding picture for the fifth doubletshowed only some red asymmetry.

of the foreign gas from to 2 atmospheres and withvariation of temperature from 120C to 1900 C.

The bands were much better resolved for highermembers than for the resonance lines. The ones as-sociated with the longer wavelength component of thedoublet had the most pronounced maxima.

(b) The Violet Bands

The bands on the short wavelength side of theabsorption doublet did not appear until the temperatureof the absorption tube was appreciably higher than thetemperature at which the red bands were first per-ceptible. The results of observation are summarizedin Table II where the meaning of the symbols is similarto that in Table I. The results reported previouslyl-4belonged to this class of violet bands.

It is interesting to note that in contrast to red bands:(1) violet bands were observed for all gases used (He,Ne, A, Kr, Xe, H2, N2), (2) the separation between thebands and associated doublets increased with thedecrease in the atomic mass of the gas, (3) the violetbands were evidently associated only with the shortwavelength component of the doublet (the values forseparation of the bands given in Table II representmeasurements from short wavelength component of

March 1956

CH'EN, BENNETT, AND JEFIMENKO

TABLE I. The Rb-noble gas bands on the long wavelength side of each of the doublet components of Rb principal series.

Band near longer wavelength component Band near shorter wavelength componentRb/foreign Maximum Arm AP, Rel. width Maximum Avrn AP, Rel. width Temperature

gas cm'1 cm-, cm-' cm-, cm'1 cm-, cm-, cm-, C

(I) For Rb resonance lines (7800.29, 7947.64 A or 12816.5, 12578.9 cm-')Rb (1)/A Red asymmetry Red asymmetryRb(1)/Kr *12571+3 - 8 -15 * *12799.243 -17.3 -25 -15 140Rb(1)/Xe *12561.5+2 -17.4 -25 -15 *12783.243 -33.3 -44 -21

(II) For Rb second absorption doublet (4201.82, 4215.56 A or 23792.5, 23715.0 cm-')Rb(2)/A Red asymmetry Red asymmetryRb (2)/Kr 23694.541.5 -20.5 -25.5 -10 *23765.342 -27.2 -32.5 -12 180Rb (2)/Xe 23669.8+1.5 -45.2 -56.1 -20 *23749.0+2 -34.5 -53.2 -21

(III) For the third doublet (3587.08, 3591.59 A or 27869.9, 27834.9 cm')Rb(3)/A Red asymmetry Red asymmetryRb(3)/Kr 27817.0i1 -17.9 -22 - 8 27854.5+41.5 -15.4 -25 -19 190Rb(3)/Xe 27800.5+1 -34.4 -42 -15 27848.9+1.5 -21.0 -33 -20

(IV) For the 4th doublet (3348.72, 3350.89 A or 29853.6. 29834.3 cm-')Rb(4)/A * ...Rb(4)/Kr Red asymmetry Red asymmetry 190Rb(4)/Xe 29823.642 -10.7 -15 - 8 *29844.1+42 - 9.5 -15 -10

the corresponding doublet), and (4) the violet bands at divided into three groups: (1) the Rb(l)/He bandthe resonance lines exhibited several maxima (the which was very broad, symmetrical, and stretched verymost prominent one is designated in Table II as Band I, far from the resonance lines, (2) Rb (1)/Ne and Rb (1)/Athe fainter one as Band II; the separations are desig- bands which were much closer to the atomic line withnated as Avmi and AVm2 accordingly). not much different Ai, values and very similar

The values of APm for the resonance lines ranged from secondary maxima with almost equal AVm 2, and (3)7 to 11 times greater than the corresponding values for Rb(l)/Kr and Rb(l)/Xe bands with other smallerthe second member of the series. For the third member but quite close values of APm and very alike secondaryonly violet asymmetry was observed. bands with almost equal AVm 2.

Figure 2 gives a direct comparison of the violet bands The bands with greater Avm had greater width butnear the resonance lines of rubidium for various noble lower intensity. A higher temperature was usuallygases.7 It appears that these five spectra could be required to bring a wide band into appreciable intensity.

TABLE II. The Rb-foreign gas bands on the short wavelength side of the various members of Rb principal series.

Band I Band IIRb/foreign Maximum Av,0 , Rel. width Maximum APm2 Temperature

gas cmj cm-' cm-' cm-' cm-' C

() For Rb resonance linesRb(1)/He 13595410 778 -150 290Rb(1)/Ne 1326545 448 75 *13189i5 372 270"Rb(1)/A 13250+5 433 - 65 *13188+5 371 270bRb(1)/Kr 13159+5 342 - 58 *13108+5 291 250Rb(1)/Xe 13151+5 334 - 56 *13107+5 290 250Rb(1)/H2 13738410 921 -225 300Rb(1)/N2 13535410 718 -100 13786+t10 969 300

(II) For the second doubletRb(2)/He 23890+5 98 - 60 240Rb(2)/Ne 23830+5 38 - 34 285Rb(2)/A 23814+5 22 275aRb(2)/H 2 *23925+15 133 - 85 345Rb(2)/N 2 *23904+10 112 - 40 330

(III) For the third doubletRb(3)/He 220Rb(3)/Ne Violet asymmetry 220Rb(3)/H 2 300Rb(3)/N 2 *27888+10 18 20 240

a In addition to the two bands, a third band was observed at *13130±5 cm-' with a AzPm 313 cm-'.b In addition to the two bands, two more bands were observed at 13135±5 cm-, and *13105±5 cm-' with APm 318 and 288 cm-', respectively.' The value in the second column is for the "edge" of the band and that in the third column is for the distance between the edge and the atomic line

7 Spectrograms for the violet band at the second doublet of Rb principal series can be found in reference 1.

184 Vol. 46

ABSORPTION BANDS OF RUBIDIUM

Rb (1)/He

g X X g 0 | g | ~~~~~~~~~~~~~~~~~~~Rb 1/K

Rb (1 )/Xe

1~b K Rb ~ os b~fl~J5Rb 2 bnd0004 ;;0 t/50 0 i00 00 ;0 Rb0 0 Rb ; 0Rb/ ;;; R io bnd;;

FIG. 2. Spectra showing the diffuse bands on the short wavelength side of Rb resonance lines in the presence of raregases. The lines marked K and Cs are the respective potassium resonance lines and the shorter wavelength componentof cesium resonance lines which appeared as impurities. Note also the red band of Cs in the presence of Kr and Xe.

Microphotometer traces of the bands showed anasymmetry towards the atomic line. Asymmetry wasmore conspicuous for the bands with smaller AVmand there was a general absorption in the region betweenthe band and the atomic line.

With Kr and Xe, spectra of the Rb resonance lineswere taken at different temperatures of the absorptiontube. Their microphotometric curves showed clearlythat although the intensity of the band and the atomiclines increased rapidly with the increase of temperature,the violet edge of the band did not appreciably changein position.

Although one would anticipate that the values ofAVm depend on the member of the series, the mass ofthe foreign gas and the polarizability of the gas, a plotof these quantities against AVm did not suggest a simplerelationship.

DISCUSSION OF RESULTS

The study of the positions and the intensities of thebands due to different absorbing and perturbing atomsis leading to important information about van derWaals interactions between the particles. No detailedquantitative analysis of the intensity distribution ofthe bands is as yet available. However, following theideas of previous investigators8 the red bands and theviolet bands can be interpreted qualitatively in termsof potential curves as shown in Fig. 3.

The potential curves in Fig. 3 are drawn for theRb-foreign gas combination for which both the red andthe violet bands were observable. When the curve forthe excited state, Spe, is lowered to coincide with theground-state curve, soo, for large values of r (curve(Pe' in Fig. 3), it is easy to see that a red band is expectedfor transitions at internuclear distances in the neighbor-

s W. M. Preston, Phys. Rev. 51, 298 (1937) and papers cited inhis article.

hood of r, and a violet band for transitions in theneighborhood of r. (For Rb(l)/Xe the red band hada AVm of about -30 cm-l while the violet band a vof 330 cm-l.)

The red bands which were due to transitions occurringat relatively large internuclear distances (rr) shouldbe more intense than the violet ones which were due totransitions at smaller distances () because the distri-bution function falls off rapidly with decreasing r.The present experiment confirmed that both the redand the violet bands appeared with red bands havinggreater intensity.

The fact that the red bands were observed only forheavy gases may be due to their considerable polariza-bility, in consequence of which an appreciable depth ofminimum of the potential curves was caused. For the

FIG. 3. Potential curves for the ground and the excited states ofRb in the presence of a foreign gas.

11 11

185March 1956

CH'EN, BENNETT, AND JEFIMENKO

cases of Rb-foreign gas combinations which gave onlyviolet bands (such as Rb(1)/He or Rb(l)/Ne) thepotential curves for both Spe and o will have practicallyno minimum because the polarizabilities of these gasesare very small, so no red band can be resolved fromthe unperturbed atomic line.

In terms of the above picture the maximum of thered band would occur where the curve soil lies lowerthan oo and has the same slope. The fact that neitherred nor violet bands were observed for members higherthan the fifth suggests that the shape of the potentialcurves for these higher states is probably similar tothat of the ground state, or that the slopes are notsimilar enough to produce a band.

It is to be noted that the red band associated withthe second member (5s-6p) of the series had the

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

largest A, and there was evidently a difference ofAPm for the two associated doublet components. Thefirst point must mean that the deviation of the potentialcurves for the p states from the potential curves of theground state was greatest for the 6p state in theattractive portion of the curve. The second pointindicates a difference in potential curves of the twofine structure p levels. More experimental observationsare needed for the study of these two fine structurepotential curves. The fact that the short wavelengthwing of the violet band fell off quite rapidly in intensityand the fact that the edge of the band did not change inposition for various temperatures of the absorptiontube indicated that there was a maximum in theseparation between the potential curves in the regionof closer collisions.

VOLUME 46, NUMBER 3 MARCH. 1956

Infrared Transmission of Synthetic Atmospheres.* I. InstrumentationJ. N. HOWARD,t D. E. BuRCH, AND DuDLEY WiLLiAms

Department of Physics and Astronomy, The Ohio State University, Columbus, Ohio(Received October 11, 1955)

A technique for investigation of the infrared absorption of water vapor and carbon dioxide under simu-lated atmospheric conditions has been developed. The "total absorption" or area under the curve givingfractional absorption as a function of frequency can be determined for each spectral region in which charac-teristic absorption occurs. The apparatus includes a 22-m multiple-traversal absorption cell which permitscontrolled variation of the following parameters: (1) geometrical path length, (2) pressure of absorbing gas,(3) pressure of the nonabsorbing gases nitrogen and oxygen, and (4) the temperature of the gaseous mixture.A prism spectrometer is used to measure fractional absorption as a function of frequency under variousexperimental conditions. Although the observed shape of a given absorption band depends upon the effectiveslit widths of the spectrometer, the total absorption of a band depends, within wide limits, only on the fore-going listed parameters. For the range of temperatures encountered in the lower atmosphere, the influenceof temperature variation on total absorption is so small that it can be neglected. On the basis of results ob-tained by the techniques described, it is possible to make accurate predictions of absorption of infraredradiation in the earth's atmosphere.

A KNOWLEDGE of the infrared transmission of theearth's atmosphere is of importance to astro-

physics, meteorology, and the aeronautical sciencesand is of interest in its bearing on the basic physicalprocesses involved in the absorption and scattering ofradiation. Infrared radiation of certain wavelengths isabsorbed by the molecules of various atmospheric con-stituents, chiefly water vapor, carbon dioxide, andozone; near infrared radiation also experiences attenua-tion by fog, haze, dust, and smoke. The present studydeals with absorption by carbon dioxide and watervapor and its dependence on path length, concentration,

*The research reported in this document has been madepossible through support and sponsorship extended to The OhioState University Research Foundation by the Geophysics Re-search Directorate of the Air Force Cambridge Research Center.It is published for technical information only and does not repre-sent recommendations or conclusions of the sponsoring agency.

t Present address: Air Force Cambridge Research Center.

and pressure at temperatures normally encountered inthe lower atmosphere.

Following the pioneer work of Fowle,l there have beenmany studies of infrared absorption in the atmosphere.An excellent review of the prewar studies has beenprepared by Gaertner,2 and Elder and Strong3 havegiven an excellent critical review of more recent work.

At the present time work on atmospheric absorptioninvolves two types of approach to the problem:

(1) One approach is to determine in detail themolecular mechanisms causing absorption of radiationand their relations to the exact shapes of spectral linesand to the variation of line strengths and half-widths

I F. E. Fowle, Astrophys. J. 35, 149 (1912); 37, 359 (1913);38, 392 (1913); 40, 435 (1914); 42, 394 (1915). See also Smith-sonian Misc. Coll. 68, No. 8 (1917).

2 H. Gaertner, NAVORD Report 429 (1947). U. S. GovernmentPrinting Office.

3T. Elder and J. Strong, J. Franklin Inst. 255, 189 (1953).

186 Vol. 46