Journal of the OPTICAL SOCIETY of AMERICA VOLUME 40, NUMBER 6 Interference Measurements in the Spectra of Neon and Natural Mercury KEiVIN BURNS,* KENNETH B. ADAMS,** AND JEAN LONGWELL* Allegheny Observatory, Pittsburgh, Pennsylvania (Received March 3, 1950) Accepted neon standards cover the region 6929-5852A. We have observed the first spectrum of neon throughout the region 8919-3126A in order to assist in establishing secondary standards in that part of this range where none exist. The first spectrum of natural mercury was observed from 7020 to 2200A in order to discover in what part of this range standards of suitable intensity will be available when lines emitted by Hg' 98 have been measured; and to find out which lines (if any) of the natural element may serve as standards when mercury is accidently or unavoidably present in a low pressure source. It is shown that some lines of natural mercury are suitable for secondary standards, and nearly all may be useful when an accuracy of no more than a part per million is required. THE desirability of having secondary standards of T nearly the same wave-length as that of the unknown line has been stressed by all who have engaged in precision spectroscopy. The rare gases afford convenient sources of suitable lines throughout the re- gion 8900-3400A but to date no wave-lengths of these lines have been designated as standards in the regions 8900-7300A and 4200-3300A. Meggers,' Humphreys, 2 and these two in collaboration, have made observations of the highest precision in these regions of the rare gas spectra. In order to be accepted as a standard a line should be observed in three laboratories. The Allegheny Observatory and the Westinghouse Research Labora- tory present the following table of neon lines which were observed to assist in establishing standards of wave-length shorter and longer than any now accepted in the spectra of the rare gases. The region 8900-5100A was covered by the Porter spectrograph 3 which has glass lenses of 520-cm focal length. The dispersive agent was a 10-cm glass prism. * Allegheny Observatory ** Westinghouse Research Laboratories, East Pittsburgh, Pennsylvania. 1 W. F. Meggers and C. J. Humphreys, Bur. Stand. J. Research 13, 293 (1934), RP 710. 2 C. J. Humphreys, J. Research Nat. Bur. Stand. 20, 24 (1938), RP 1061. ' K. Burns and W. F. Meggers, Pub. Allegheny Obs. 6, 105 (1925). Burns, Meggers, and Kiess, Bur. Stand. J. Research 1, 297 (1928). Two 4X5-in. plates were used in the 10-in. plate holder, 8900-7300A being photographed by the IN emulsion, and the red-green region registered on home-dyed plates or on Eastman's "F" emulsion. The National Bureau of Standards kindly furnished the glass neon tubes of the Nutting type which were used as sources. The gas pressure was 6 mm or less; the current strength was 20 ma furnished by a 2000-v transformer. Inter- ferometer-plate separations of 6.5, 8, 12, 15, 20, and 40 mm were used, the 40 mm carrying over half the weight. Three pairs of plates were used; glass with rather dense aluminum, fused quartz with lighter aluminum, and crystal quartz with dense silver. With a large quartz spectrograph (Hilger El) the region 7000-3120A was observed in one adjustment of *the instrument. The sources were a quartz Nutting-type tube filled with neon to a pressure of 6 mm, and a similar tube to which a trace of mercury was added intentionally. The latter tube emitted the neon spec- trum with about half the brightness of the former and the mercury spectrum in such strength that 5460A of Hg about equaled 5881A of neon in intensity. The relative strength of the two spectra when Ne and Hg are in the same tube depends, of course, on the pressure of neon and on the temperature of the gas. We estimate the quantity of mercury to be in excess of 1 mg/5 cc. By checking against the cadmium standard, both tubes gave the same wave-lengths of the neon lines. In 339 JUNE, 1950
Transcript
Journal of the
OPTICAL SOCIETYof AMERICA
VOLUME 40, NUMBER 6
Interference Measurements in the Spectra of Neon and Natural MercuryKEiVIN BURNS,* KENNETH B. ADAMS,** AND JEAN LONGWELL*
Allegheny Observatory, Pittsburgh, Pennsylvania(Received March 3, 1950)
Accepted neon standards cover the region 6929-5852A. We have observed the first spectrum of neonthroughout the region 8919-3126A in order to assist in establishing secondary standards in that part ofthis range where none exist.
The first spectrum of natural mercury was observed from 7020 to 2200A in order to discover in whatpart of this range standards of suitable intensity will be available when lines emitted by Hg'9 8 have beenmeasured; and to find out which lines (if any) of the natural element may serve as standards when mercuryis accidently or unavoidably present in a low pressure source. It is shown that some lines of natural mercuryare suitable for secondary standards, and nearly all may be useful when an accuracy of no more than a partper million is required.
THE desirability of having secondary standards ofT nearly the same wave-length as that of theunknown line has been stressed by all who haveengaged in precision spectroscopy. The rare gases affordconvenient sources of suitable lines throughout the re-gion 8900-3400A but to date no wave-lengths of theselines have been designated as standards in the regions8900-7300A and 4200-3300A. Meggers,' Humphreys,2
and these two in collaboration, have made observationsof the highest precision in these regions of the rare gasspectra. In order to be accepted as a standard a lineshould be observed in three laboratories. The AlleghenyObservatory and the Westinghouse Research Labora-tory present the following table of neon lines whichwere observed to assist in establishing standards ofwave-length shorter and longer than any now acceptedin the spectra of the rare gases.
The region 8900-5100A was covered by the Porterspectrograph 3 which has glass lenses of 520-cm focallength. The dispersive agent was a 10-cm glass prism.
* Allegheny Observatory** Westinghouse Research Laboratories, East Pittsburgh,
Pennsylvania.1 W. F. Meggers and C. J. Humphreys, Bur. Stand. J. Research
13, 293 (1934), RP 710.2 C. J. Humphreys, J. Research Nat. Bur. Stand. 20, 24 (1938),
RP 1061.' K. Burns and W. F. Meggers, Pub. Allegheny Obs. 6, 105
(1925). Burns, Meggers, and Kiess, Bur. Stand. J. Research 1,297 (1928).
Two 4X5-in. plates were used in the 10-in. plate holder,8900-7300A being photographed by the IN emulsion,and the red-green region registered on home-dyedplates or on Eastman's "F" emulsion. The NationalBureau of Standards kindly furnished the glass neontubes of the Nutting type which were used as sources.The gas pressure was 6 mm or less; the current strengthwas 20 ma furnished by a 2000-v transformer. Inter-ferometer-plate separations of 6.5, 8, 12, 15, 20, and40 mm were used, the 40 mm carrying over half theweight. Three pairs of plates were used; glass withrather dense aluminum, fused quartz with lighteraluminum, and crystal quartz with dense silver.
With a large quartz spectrograph (Hilger El) theregion 7000-3120A was observed in one adjustment of*the instrument. The sources were a quartz Nutting-typetube filled with neon to a pressure of 6 mm, and asimilar tube to which a trace of mercury was addedintentionally. The latter tube emitted the neon spec-trum with about half the brightness of the former andthe mercury spectrum in such strength that 5460A ofHg about equaled 5881A of neon in intensity. Therelative strength of the two spectra when Ne and Hgare in the same tube depends, of course, on the pressureof neon and on the temperature of the gas. We estimatethe quantity of mercury to be in excess of 1 mg/5 cc.By checking against the cadmium standard, both tubesgave the same wave-lengths of the neon lines. In
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JUNE, 1950
BURNS, ADAMS, AND LONGWELL
TABLE I. Wave-lengths in the spectrum of neon.
1 2 3 4 5 6 7Vacuum wave No. Level No.
X A in air Observed Comp. X MH. H v M combinations obs.
TABLE I (continued).
1 2 3 4 5 6 7Vacuum wave No. Level No.
X A in air Observed Comp. X MH, H v M combinations obs.
addition to the interferometer plates used for the longwaves, a pair of crystal-quartz plates coated lightlywith aluminum was employed, and separations of 2,3.75, 5, and 25 mm in addition to those aforementioned.
In Table I the first and second columns contain,respectively, the measured wave-lengths in standardair and the corresponding wave numbers in vacuum.4
The third column shows the fractional part of thedifferences of the levels of column six. A letter in thiscolumn indicates that this level was found only once.These levels have been adjusted so that agreementbetween observed and computed is best for lines of
4 W. F. Meggers and C. G. Peters, Sci. Papers Bur. Stand. 14,697 (1918), SP 327.
greatest weight. The fourth column contains the frac-tional wave-length as measured by Meggers andHumphreys' (8865-4334A), or by Humphreys2 (3754-3369A). Column five contains the fractional differencesof levels as published by Charlotte Moore,5 "compiledfrom an unpublished manuscript kindly furnished byEdl6n." Column seven shows the number of observa-tions. An "S" in this column indicates that the wave-length is that recommended by The InternationalAstronomical Union6 and the wave number in columntwo corresponds to this value.
In setting up the levels, is 4 was adopted as given byCharlotte Moore.5 The difference s5-s 4 occurs innine well-observed line pairs; the mean of these ninevalues was applied to is4 to obtain s5. Similarly, 1S3
was derived from five pairs and is 2 from fifteen. Insetting up the p levels, it was seen that the level s3was systematically 0.002 too low, the result of includingone discordant observation in comparing this level with1s4. The p levels were derived from combinations withthe s levels. The 2s and all higher odd levels werefound from combinations with the 2p group. Anasterisk indicates that the level was found in only onecombination. The level 2p9 was derived from only onepair-difference, both lines being standards. This levelis checked against other 2p levels but the accuracy ofthe other combinations is not great enough to add muchweight to the single difference here used.
In the region of wave-lengths longer than the stand-ards, 8865-6929A, the mean difference AOW-M is-0.0005A; the median difference is 0.0005A, a part insixteen million. In the region of the standards, 6717-5852A, for lines other than the standards the differenceAOW-MH is -0.0006A. From 5820 to 5100A weused both the glass and the quartz spectrographs;here the well-observed lines show the negligible system-atic difference +0.0004A between results from the twolaboratories. From 5100 to 4300A, the end of the MHtable, AOW-MH= +0.0OiOA. In the region measuredby Humphreys, 3754-3369A, the difference AOW-His -0.0007A; after applying this systematic differencethe largest AOW-H is 0.0005A, the median is 0.0003A.The presence of these discrepancies suggests an exami-nation of the manner in which the correction for phasewas derived. We assume that the wave-length deter-mined from the thickest interferometer is most nearlyfree of error and that the error is inversely proportionalto the order number. On these assumptions the correc-tion applicable to a thickness of 20 mm for the crystalquartz thinly aluminized is -0.0012A from 3750 to3360A; for the fused quartz plates with medium coatof aluminum the correction is zero at the beginning and+0.0002A at the end of this range; for the crystalquartz with heavy silver the correction is zero at 3750A
5 C. Moore, "Atomic energy levels. Vol. I," Circ. Nat. Bur.Stand. 467, 76 (1949).
6 Trans. Int. Astr. Union 5, 86 (1935).
341
BURNS, ADAMS, AND LONGWELL
and +0.0012A at 3360A; the glass plates with heavyaluminum have correction zero throughout the range.
Between 3400 and 2300A where all rare gas spectraare weak the mercury spectrum has several lines strongenough to serve as standards; many of these lines,especially the strongest at 2536A, are complex and notto be recommended when natural mercury is the source.For this region, in fact throughout the whole range5790-2300A, the perfect source of standards has beenintroduced by Meggers 7 whose efforts have resulted inmaking the use of Hg188 practical. It would be difficultto overestimate the benefits which the availability of asingle even-numbered isotope of mercury will bring toprecision spectroscopy. Wherever a line of sufficientstrength is emitted by this source there is no point inlooking for any other standard. Consequently we haveexamined the mercury spectrum from 7000 to 2100Ato locate the lines which are of sufficient strength when
the source is the neon-mercury tube mentioned earlier.By using interferometer films of moderate density underthe observing conditions described above, the exposuretime necessary to register a mercury line is: One secondor less for 4358 and 2536A; five to ten seconds for 5460,4046, 3650, 3341, 3131, and 3125A (and Ne 5881A);one minute for 3021 and 2967A; two minutes for 2893,2803, and 2652A (four minutes for Ne 5400A); tenminutes for 2464, 2446, 2399, and 2378A. These figuresgive only a rough indication of the required exposuretime since mercury lines of sufficient sharpness may beobtained at intensities which vary by a factor of ten asthe temperature of the tube and the current strengthare changed.
Mercury shows no strong red lines; here neon andcadmium are already available. Between 3261 and2141A cadmium has some possible standards which arestronger than the mercury lines of this region with the
exception of 2536A. Our measurements of the cadmiumlines will be reported later.
The mercury lines 5790 to 3125A were compared withneon standards, the same tube being the source of bothspectra, assuring simultaneity of exposure and identityof optical path. The strong mercury lines all appear tobe complex; it is therefore of interest to see whataccuracy can be expected in the measurement of theselines when the source is a low pressure tube. Whenweakly exposed these lines are usually single, or onecomponent is notably stronger than the others. Atsome orders of interference the components of a lineare grouped so as to show an unsymmetrical patternwhich may be shaded to either shorter or longer values.2
In such cases an effort was made to set on the centerof gravity of the pattern, the apparent position of whichshifts with the strength of exposure. The exact positionof the apparent center depends not only on the strengthof exposure, but also on the order of interference, onthe reflectivity of the interferometer plates, on thegrain of the photographic plate, and on the adjustmentof the optical system. To evaluate the effect of each ofthese factors would require more observation than iswarranted. The tables as published are the means ofmeasurements of lines on exposures so short that thelines are barely visible, on normal exposures, and onexposures ten times as long as required for best results.All measurements were given equal weight regardlessof strength of exposure, but the results from smallseparations were given half-weight if those from 25and 40 mm were good.
TABLE III. Wave-lengths in the spectrum of mercury (Hg I)relative to neon standards.
The lines 4358, 3650, and 3341A were observed astraces in a neon tube during the first one or two hoursof its use, and in a cadmium lamp. A few results from3.75 or 5-mm separations of plates are compared withthe adopted values in angstroms:Adopted 4358.3277A 3650.1533A 3341.4766ATrace .3271(wt. 2) .1536(wt. 2) .4779(wt. 1).
The agreement of "adopted" and "trace" is withinthe error to be expected of such low weights.
In the region compared directly with neon standardsthere are three well observed pairs of mercury lines
TABLE III-A. Additional lines in the spectrum of Hg I.Various standards.
whose wave numbers differ by the interval 6p 3 P 30-
6p 3P 20; in view of the experience of previous observers
we were surprised and gratified to find the agreementshown here.
X A Wave No.5460 18307.4794358 22938.156Diff. (cm-')
MA36543125
4630.677
Wave No. X A27353.242 334131983.920 2893
4630.678
Wave No.29918.32134548.9974630.676
The close agreement is of course accidental.Some of the short wave neon lines have been observed
by means of nine different separations, 3.75 to 40 mm.The median variation of wave-length with thickness isabout a part in seven million, the extreme range being-a part per two million. There is no systematic variationas great as the accidental error. With the exception ofthose involving an s level the well observed mercurylines are nearly as good in this respect as those of neon.These exceptional (and strongest) lines in the spectrumof natural mercury vary more widely than the otherswith change of interferometer; the 6p 3P2
0, 1, o-7s IS,and 6p 3 Pi 0-7s 'So of natural mercury are not suitablefor standards when an accuracy exceeding a part permillion is required.
Kayser's Handbucli8 records the comparisons ofmercury lines with the fundamental standard whichwere made before 1927. More recently, P6rard9 hascompared certain mercury lines with the cadmiumstandard with higher accuracy than was attained earlier.P6rard varied the resolution from moderate to highand considered the published wave-lengths to refer tothe strongest component of each line. We have deter-mined accurate wave-lengths for four of these lines;ours are probably center of gravity values. Perard andTerrien'0 report the preliminary results of the observa-tion of three of these lines with Hg'98 as source. Themeans of the measurements made by Meggers, byBarrell, and by P6rard and Terrien are shown in thefourth column below.
The close agreement of observations made underdifferent conditions of excitation and with differentisotopes as sources suggests that the 6p 'P, 0-6d 'D2and 6p 'Pi-6d 'D2 may be used as standards; thewave-lengths of 6p3P2 , 1-7s.3S1 vary so much withobserving conditions that no precise value can beassigned to them when natural mercury is the source.
The region shorter than 3100A was observed bymeans of one pair of interferometer plates, crystalquartz thinly coated with aluminum. Etalons of length2, 3.75, 5, and 8 mm were used. The thickness was
8 H. Kayser, landbitch der Spectroscopic 7, 669 (1934).9 A. Pbrard, Rev. d'Optique 7, 1 (1928).1 A. P6rard and J. Terrien, Comptes Rendus 228, 964 (1949).
TABLE IV. The lower levels of Hg I.
6s2 1SO7s ISo8s 'So9s 'So
lOs 'So
6d 'Di7d 3Di8d Di9d 3Di
0.00063928.243*74404.59078404.38780365.653*
71336.16477084.63279678.70881071.027
Even
7s aSi 62350.4568s 3S, 73961.2989s 3S, 78216.261
lOs 'Si 80268.05611s 3S1 81416.352*12s 3Si 82124.081*
* No check.Note.-The odd levels and the even levels 7s So, 7s 3SI, 8s So, 6d ID2,
6d 3D2 and 6d 3D3 were evaluated from lines which have been directlycompared with neon twelve or more times. These levels are probablysufficiently accurate to warrant the use of eight places. The relative valuesof some other levels appear to be sufficiently consistent to justify the useof eight places. The level designations follow those of R. F. Bacher andS. Goudsmit. Atomic Energy States (McGraw-Hill Book Company, Inc.,New York, 1932).
determined by use of lines recently measured and twolines given by Meggers and Burns," viz., 3080.827 and2980.622A of cadmium. The new lines are 3261.054,3252.524, and 3133.167 of cadmium; 3125.668 ofmercury; 3148.610, 3153.411, and 3167.575 of neon.The agreement between standards was not entirelysatisfactory and agreement from plate to plate wasalso rather poor. The phase change 3200 to 2300A islarge and poorly determined. But it does not seemwise to expend any more effort on this region of thespectrum of natural mercury since 2536A is probablythe only line that will show as a trace in low pressureobservations of other elements and this line is rathertoo complex for precise measurement with naturalmercury as source. Because of uncertainty in the phasechange, wave-lengths in the region shorter than 2893Awere adjusted to agree with levels derived from longerwave-lengths. In this region our adjusted values areprobably more accurate than our observed.
In Table III the column headed "M.I.T." containswave-lengths as published by Harrison'2 and others in1939. The editors of this table selected the best availablevalues; some were measured at M.I.T. Some of thesewave-lengths apply to the arc in air, some to the arc atreduced pressure, and some to low pressure (vacuum).In general, the M.I.T. table lends support to ourwave-lengths where we have no check. In four or fivecases the reality of our line and the level depending onit may be questioned.
We are indebted to the National Bureau of Standardsfor the loan of interferometer plates, neon tubes, andseparators; and to E. U. Condon, W. F. Meggers, andCharlotte Moore Sitterly for encouragement and assist-ance. We thank R. F. Mehl and the Carnegie Instituteof Technology for the loan of the quartz spectrographwhich was necessary for much of our work.
"W. F. Meggers and K. Burns, Sci. Papers Bur. Stand. 18,185 (1922), SP 441.
12 G. R. Harrison, MIT Wavelengthi Tables (John Wiley andSons, Inc., New York, 1939).
