MARCH, 1942

The Fluorescence of Phosphors in the Rare Gases

GORTON R. FONDA AND HANS HUTHSTEINERResearch Laboratory, General Electric Company, Schenectady, New York

(Received November 26, 1941)

The spectral distribution for the excitation of phosphors is so erratic that it seems at presentimpossible to predict the location of strong excitation bands. Out of many phosphors examinedin rare gas discharges, none was found to exceed in brightness the calcium tungstate and zincsilicate already in commercial use. The light output of the latter, however, was considerablyimproved by the elimination of the free silica usually present, which evidently has an appreci-able absorption to radiation in the Schumann region. On the reasonable assumption that theresonance radiation is the source of excitation, the efficiency of fluorescence emitted by the zincorthosilicate phosphor in the neon discharge denotes a quantum conversion near unity, just asin the low pressure mercury discharge.

HE present fluorescent lamps take ad-T vantage of the high efficiency of excitationcharacterizing certain phosphors when subjectedto the resonance radiation of a low pressure mer-cury discharge.' These phosphors include silicatesand tungstates. Certain other phosphors, wellknown for their brightness, such as the sulfides,cannot be used in this lamp because their effi-ciency of excitation, though high in other parts ofthe spectrum, is low under the 2536A radiation ofmercury.

Other combinations of a phosphor and a gase-ous discharge are presumably possible. The pri-mary essential is that the radiation of the dis-charge shall be strong in a spectral region capableof exciting the phosphor efficiently. The mostavailable gaseous discharges, in addition to thosein mercury vapor, are those in the rare gases.Their radiation in the ultraviolet, however, ismost prominent in the resonance lines which lieat the very short wave-lengths characterizing theSchumann region. If radiation of such short wave-length is to act as the exciting source of fluores-cence, the energy yield of luminescence emittedby the phosphor will be much more limited thanunder excitation by the 2537A line of mercurybecause of the much more unfavorable quantumratio, i.e., the ratio of the integrated energy perquantum of fluorescence radiation to the energyper quantum of the exciting line. Despite thishandicap, however, Jenkins found it possible toobtain new and useful color effects or an increasein lamp efficiency or both by coating neon or

I G. E. Inman, Trans. I. E. S. 34, 65 (1939).

helium discharge lamps internally with zincsilicate or calcium tungstate phosphors. 2

Jenkins and Bowtell have already presentedthe results of their investigations for the case ofzinc orthosilicate in neon discharge tubes.3 Theiraim was to determine the values of currentdensity, gas pressure and tube diameter thatwould represent the best operating conditions forobtaining optimum luminous output, efficiency,life, and color.

It was the aim of the present work to investi-gate the possibility of further gains by studyingthe luminosity of other phosphors as well as ofzinc silicate and calcium tungstate under excita-tion by discharges in the other rare gases as wellas in neon and by attempting an evaluation of thequantum yield.

The tests were made by coating the phosphoron a glass disk 1.5 cm in diameter which was thenmounted in a 2.6-cm diameter tube with a hotcathode. Gas at various pressures was admittedto the tube on the pump and a direct currentdischarge was operated in it. The intensities ofthe luminescence emitted by the various coateddisks present in the tube were measured byfocusing each in turn upon a photo-voltaic cellwith a filter to correct its sensitivity to that of theeye. The luminescence as measured included notonly (a) the fluorescence of the phosphor but also(b) the luminescence of the gaseous discharge asreflected from the surface of the phosphor. The

2 H. G. Jenkins British Patent 457,486 (1935); A. Rut-tenauer, Zeits. f. tech. Physik 17, 384 (1936); J. T.Randall, Roy. Soc. Arts 85, 354 (1937).

3 H. G. Jenkins and J. N. Bowtell, Trans. Faraday Soc.35, 154 (1939).

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FLUORESCENCE OF PHOSPHORS IN RARE GASES

magnitude of (b) was determined directly bymeasuring the brightness of a disk coated withaluminum oxide as a non-fluorescent material.As the disks were all of uniform size, the value of(b) subtracted from the total brightness emittedby a phosphor yielded the value of (a), thefluorescence alone. The absorption of fluorescenceby the luminescence of the gaseous discharge wasapparently negligible.

It was established in the experiments ofRfittenauer2 and by Jenkins and Bowtell3 thatexcitation of phosphors in the rare gas dischargesis in fact to be ascribed to their resonance radia-tion, ranging from 584A in case of helium to1469A in case of xenon. Out of the many com-pounds that we have examined, it developed thatthe extent of their excitation in the various raregas discharges as well as by prominent lines inthe mercury discharge, varied widely from onephosphor to another. Calcium oxide phosphors,for instance, showed in some cases a faintfluorescence under 2536A, a stronger one under3650A, and a low fluorescence in neon. Magne-sium silicate activated with manganese wasunresponsive under 2536A or 3650A but in therare gases developed fluorescence which wasgreatest in the heavy gases, argon, krypton, andxenon. Magnesium borate with manganese wasexcited only in argon and krypton. Fluorescentcalcite was excited under 2536A and 3650A andin helium and xenon, but not in neon or argon.Zinc sulfide activated with copper, which isexcited very strongly under 3650A and onlyweakly under 2536A, showed a very low fluores-cence in neon, krypton, and xenon, but astronger one in helium and argon.

Such observations made it apparent that theexcitation of phosphors does not follow someobvious rule but varies in an erratic manner. Itis at present impossible to predict in whatspectral region a phosphor will respond to exci-tation. The presence or absence of an excitationband at some particular range of wave-length hastherefore no present significance in denoting theextent or location of an excitation band inanother region of the spectrum.

So far as excitation in the rare gases is con-cerned, it was found that zinc silicate andcalcium tungstate, out of many compounds ex-amined, did in fact develop the highest bright-

ness and that this brightness was greater in neonand in helium than in the other rare gases, just asclaimed by Jenkins. 2 The fluorescence fromcadmium silicate and borate, activated withmanganese, and from magnesium tungstate waslikewise high, in accord with recent observationsby Suga and Kamiyama.4

The fluorescence of all varied similarly withchanges in gas pressure and current and developedpeaks or stable values whose location and magni-

LIJ

20

0 20 40 60 0 10

0 20 40 60 80 10

FIG. 1. The relation between the fluorescence of zincsilicate phosphors and gas pressure in rare gas dischargesat an arc current of 200 mA. A-neon; B-helium; C-argon; D-krypton.

tude showed characteristic differences dependingupon whether the gas involved was in the lighteror heavier group. The fluorescence increasedcontinuously with pressure in neon and helium,for instance, as brought out by the curves ofFig. 1 for the typical case of zinc silicate, andapproached a uniform value when the pressurereached the range of 500-1000 microns. Itattained a maximum at a somewhat higher pres-sure and thereafter decreased. This was inmarked contrast to the fluorescence observed inthe heavier gases, argon, krypton, and xenon, theresults for the last being very close to thoseshown in the curves for krypton. In these heaviergases there was a pronounced peak in intensity at

I Taro Suga and Masahide Kamiyama, J. Opt. Soc. Am.31, 592 (1941).

157

G . R. FONDA AND H. HUTHSTEINER

50 100 15CCURRENT MILLIAMPS

FIG. 2. The fluorescence of zinc silicate phosphors in thelow pressure mercury vapor discharge. a-Zn 2SiO,.Mn;b-Zn2 SiO 4 -SiO 2 -Mn.

low pressures, 50-200 microns, where clean-upand sputtering became serious under continuedoperation.

Such an occurrence of a maximum value for theexcitation of phosphors on increase in gas pres-sure is related to similar changes in the intensityof resonance radiation due to collision of thesecond kind which become more prevalent as thepressure rises. This in fact is one of the mainreasons for assuming that it is the resonanceradiation which excites their fluorescence. Theoccurrence of maximum fluorescence, however, atsuch a low pressure as 50 microns in argon,krypton, and xenon, as compared with the highoptimum pressure observed in neon and helium,needs further explanation. One has been sug-gested by C. G. Found of this laboratory on thebasis of the relationship between the efficiency ofexcitation of a gas and the magnitude of theelectron temperature. Excitation begins at acritical electron voltage, rises abruptly to a peakat a somewhat higher voltage-and then tails offgradually under the action of electron velocitieshaving still higher voltage. The distribution ofelectron velocities, however, is displaced to lower

values by an increase in pressure. It happens inthe case of xenon, krypton, and argon, all threecharacterized by low excitation potentials, thatthe reduction in electron temperature with pres-sure is so marked that there is a serious decreasein the number of electrons of velocity greaterthan the excitation potential, a decrease which infact predominates over the increase in number ofgas atoms that accompanies the increase in pres-sure. The net result, therefore, is a reduction inthe number of excited atoms formed and conse-quently in the intensity of the resonance radia-tion emitted in these gases. In the case of neonand helium, gases having higher excitationpotentials, this situation would not arise until ahigher pressure were reached. Actually the ob-served peak of fluorescence brightness in theselatter gases occurred somewhat above 1-mmpressure. In neon the brightness at 2 mm was 50percent of its value at 1 mm. In helium thecorresponding decrease was only to 80 percent.

Similar differences between the lighter andheavier gases occurred for variations in current.The fluorescence of zinc silicate can again betaken as typical. It is of especial interest becausethe tests on it brought out also an effect ofcomposition characteristic of the silicates anddenoting a relation between loss of resonanceradiation by absorption and the content of freesilica in the phosphor. Although zinc silicateoccurs only in one form, that of the orthosilicate,Zn2SiO4,5 yet it is generally prepared with anexcess of silica for use in the low pressure mercuryvapor discharge type of fluorescent lamp. Theexcess silica facilitates completion of the reactionduring preparation by assuring consumption ofall of the zinc oxide present. The preparation of asatisfactory form of the orthosilicate alone ismore involved. Fusion of the ingredients does, itis true, secure a ready reaction but the productobtained is low in fluorescence because of thenecessity of grinding. When a mixture of theoxides is fired at a temperature below fusion, thereaction can be brought to completion only bygrinding and refiring and the resulting product,although brighter than that obtained by fusion, isstill not as bright as the normal phosphor. Thecompletion of the reaction in one step is essential

I G. R. Fonda, J. Phys. Chem. 43, 561 (1939).

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FLUORESCENCE OF PHOSPHORS IN RARE GASES

to the attainment of full, normal brightness. Itcan be secured by starting with a colloidal solu-tion of zinc and manganese nitrates in silica,thereby taking advantage of a characteristicallymore rapid rate of reaction. 5

Such a phosphor was prepared and com-parative measurements were made in the variousgases on its fluorescence and that of the ortho-silicate prepared similarly but with one mole ofexcess silica, Zn2SiO4 -SiO2 . Both were activatedwith 0.5 percent manganese. As can be observedfrom Fig. 2, the fluorescence of both phosphorswas essentially the same when tested in the lowpressure mercury discharge. The two phosphorshad likewise the same relative brightness whentested in the open air under a low pressure quartzmercury lamp. When tested in gaseous discharges,however, the phosphor with one mole of excesssilica was invariably much less bright than theorthosilicate phosphor, as is brought out inFig. 3.

It is to be inferred, therefore, that free silicahas an appreciable absorption to radiation in theSchumann region as contrasted with a negligibleabsorption at 2536A.

A similar characteristic difference between thelighter and heavier gases is demonstrated in thecurves of Fig. 3 by the occurrence of a muchearlier saturation effect observable in argon andkrypton with increase in current. The same istrue of the results in xenon, which followed thesame course as the curves shown for krypton. Itoccurs at a much lower current than was observedin neon and helium. It is due presumably to anearlier suppression of the resonance radiation.These curves bring out again the relatively strongorder of fluorescence obtainable in neon andhelium as contrasted with a lower order offluorescence in argon, krypton, and xenon, de-noting a peak in excitation in the range of580-750A.

From two considerations it is probable that theefficiency of excitation of zinc orthosilicate by theresonance radiation in a neon discharge is close tothe maximum that would be possible under aquantum conversion of unity. Under the mostfavorable conditions of current density and gaspressure, the intensity of fluorescence in neon wasfound to be 35 percent of that in low pressuremercury vapor at the same current. If fluores-

cence results in both cases from excitation byresonance radiation and if the same number ofexcited atoms are formed per electron in neon asin mercury, then one quantum of neon resonanceradiation would produce 740/2536 or 29 percentof the fluorescence excited by one quantum ofmercury resonance radiation, close to the 35percent actually observed. The conclusion thenwould be that the quantum efficiency in neon isnear unity, just as it is known to be under the2536A line of the mercury discharge.

A high value for the quantum efficiency wasalso derived from consideration of the energyefficiency of light formation. The theoreticalvalue for zinc silicate, the peak of whose fluores-cence lies at 5300A, would be 520 LPW. Under

0zwn,

0-JLL

I-zwI-z

50 100 15(CURRENT- MILLIAMPS

'0

FIG. 3. The fluorescence of zinc silicate phosphors in raregas discharges. a-Zn 2 SiO4 -Mn; b-Zn 2SiO4 SiO2 Mn.

I

159

wMr. A. SHURCLIFF

excitation by 740A, this would be reduced to740/5300 X 520 = 72 LPW. The average efficiencyreported by Randall2 for a commercial type ofneon lamp coated with zinc silicate, is 22 LPWas compared with 15 LPW for a neon dischargealone under the same conditions. This gives avalue of 7 LPW for the efficiency of the fluores-cent light alone, 10 percent of the theoretical onthe basis of a quantum conversion of unity. Whenallowance is made for the reduction in resonanceradiation at the relatively high pressure andcurrent used commercially and also for itsincomplete utilization in the necessarily thin film

MARCH, 1942

of phosphor, this value would become about 20percent. The proportion which the energy emittedas resonance radiation bears to the total energyof the neon discharge is not known, but it isreasonable to suppose that it is not more than 30percent. Here again therefore a high value for thequantum yield is apparent.

Both of these evaluations are necessarily ap-proximate. They indicate, however, that theprospect is not favorable for the attainment ofappreciably higher efficiencies by combination ofa fluorescent phosphor with a neon dischargeunless some radically new factor is introduced.

J. 0. S. A. VOLUME 32

Curve Shape Index for Identification by Means of Spectrophotometric Curves

Wzi. A. SHURCLIFFResearch Laboratories of the Calco Chemtical Division of the American Cyanamid Company, Bound Brook, New Jersey

(Received October 25, 1941)

A simplified curve-shape indexing system for spectrophotometric absorption curves isdescribed by means of which the tentative identification of an "unknown" (whose curve isalready at hand) may ordinarily be made in 2 or 3 minutes even in the presence of smallamounts of colored impurities. Quantitative solutions are not required.

INTRODUCTION

W HEN there are spectrophotometric absorp-tion curves on file for 500 to 1000 dyes,

rapid identification of unknowns requires that asimple curve shape index be available. If therewere but a relatively small number of referencecurves, it would, of course, be a simple matterto compare the curve of an unknown with allthe reference curves. However, when the numberof reference curves is large, such a procedurebecomes too laborious.

Various systems of curve shape symbols havebeen proposed. Holmes et al.1 discuss a number ofthese, including one of their own involving an"absorption ratio," the ratio of extinction coeffi-cients at two arbitrary wave-lengths pre-selectedfor a given group of related dyes. However, itis not apparent how such a system could beextended to include all dyes in the same manner.

I W. C. Holmes, J. T. Scanlan and A. R. Peterson, "Thevisual spectrophotometry of dyes," Dept. AgricultureTech. Bull. 310 (June, 1932).

More recently, Godlove2 has proposed a systemof symbols in which, for a given curve, one firstadjusts the data so that the minimum trans-mission value is exactly 10 percent, and thenindicates the approximate transmission at eachof nine, fixed, (more-or-less equally spaced)wave-lengths, starting at 400 millimicrons. How-ever, this type of symbol presupposes eitherspecial experimental conditions for providing 10percent minimum transmission or making graphi-cal transformations to reduce the curves toequivalent form.

Further, a system employing prescribed wave-lengths tends in practice to fail to give adequateemphasis to the most characteristic features ofthe absorption curve, and is, therefore, con-siderably disturbed by the presence of smallamounts of colored impurities. Thus, for ex-ample, the very first number in the symbol fora blue, green, or black would by changed by

2 I. H. Godlove, paper in the Intersociety Color CouncilNews Letter 26 (December, 1939).

160