JOURNAL OF THE OPTICAL SOCIETY OF AMERICAV

Thorium Lamps and Interferometrically Measured Thorium WavelengthsFRAxNCISCO P. J. VALERO*

Departamtentto de Fisica, Universidad Nacional de La Plata, La Plata, A rgentina, and Consejo Nacionalde Investigaciones Cientificas y Tdcanicas, Buenios Aires, A rgenttina

(Received 14 July 1967)

Wavelengths emitted from an electrodeless thorium iodide lamp are compared with those emitted bywater-cooled and liquid-nitrogen-cooled hollow-cathode lamps. Three hundred forty six thorium lines havebeen interferometrically measured, using a hollow-cathode lamp as a source, in the range from 4574 to6676 A. The accuracy of the measurements has been tested using the combination principle, and was foundto be one part in 6 to 9.5X 107.

INI)Ex HEADINGS: Thorium; Interferometry; Wavelengths; Source.

IT is well known that the thorium spectrum consistsof very many lines free from hyperfine structure and

having small Doppler widths.In 1955 Meggers suggested replacement of the stand-

ard iron arc by a thorium halide "electrodeless" lamp.Secondary standards, superior in sharpness and dis-tribution, could be obtained, as proved by Meggersand Stanley.' These investigators were the first to mea-sure interferometrically the lines emitted by a thoriumhalide lamp. These lines were measured relative to wave-lengths emitted by a Meggers 198Hg lamp. More re-cently, Littlefield and Wood,2 Davison, Giacchetti, andStanley, 3 Giacchetti el al.,4 and Meggers and Stanley5

reported additional measurements of thorium wave-lengths. Littlefield and Wood used a hollow-cathodelight source and a reflection-echelon interferometer. Theothers used electrodeless thorium iodide and Fabry-Perot interferometers. All of these measurements havefound favor among spectroscopists and astronomers,particularly those who specialize in the study of complexspectra.

The work described in this paper was undertaken toprovide a comparative study of the different lightsources to be used for standard purposes, and to provideindependent measurements of thorium wavelengths.As is known, there are many serious discrepanciesbetween values reported by two or more observers; itwas also the aim of this work to seek the source of thelarger differences. The combination principle is used totest the accuracy of wavelength measurements, as sug-gested by Meggers and Stanley.

LIGHT SOURCES

Two different hollow-cathode light sources were de-veloped for the present experiment. One of them was a

* Mailing address: Departamento de Fisica, Universidad Nacio-nal de La Plata, calle 115 y 49 CC 67, La Plata, Argentina.

' W. F. Meggers and R. W. Stanley, J. Res. Natl. Bur. Std.(U.S.) 61, 95 (1958).

2 T. A. Littlefield and A. Wood, J. Opt. Soc. Am. 55, 1509(1965).

3 A. Davison, A. Giacchetti, and R. W. Stanley, J. Opt. Soc. Am.52, 447 (1962).

4 A. Giacchetti, M. Gallardo, M. Garavaglia, Z. Gonzalez,F. P. J. Valero, and E. Zackowicz, J. Opt. Soc. Am. 54,957 (1964).

5 W. F. Meggers and R. W. Stanley. J. Res. Natl. Bur. Std.(U. S.) 69A, 109 (1965).

water-cooled Schiller-type tube and the other a liquid-nitrogen-cooled hollow cathode. The water-cooled Schil-ler-type tube is shown in Fig. 1. The cathode is made ofaluminum with a hollowed-out cylinder 22-mm longand 8 mm in diameter. The inside wall of the cathodeis completely covered with thorium foil 0.3-mm thick.The Pyrex tube is vacuum sealed to the aluminumcathode with wax; the quartz window is also sealedto the Pyrex tube with wax. The aluminum portion ofthe tube is introduced into a Pyrex cylinder and gluedto it (as shown in the figure) with epoxy cement. ThePyrex-glass cylinder mounted externally to the cathodehas a water inlet (A) and outlet (B). In this way, thelamp can be cooled with running water to a temperatureof 60 to 100 C. The life of the tube depends basically onthe quality of the metals used; it should last indefi-nitely if high-purity metals are used. If high-puritymetals are not used, as in our case, the tube should beevacuated and new carrier gas loaded after about 30 huse. This is a simple operation and the high-qualitystopcock and the ball joint are provided to facilitatethe reloading operation. Argon gas was used as acarrier at a pressure of 1 to 2 torr. The spectrum emittedby this lamp is very intense and the lines radiated aresuperior in sharpness to those from an electrodeless dis-charge, even when the light source is run at currentsas high as 200 mA.

The liquid-nitrogen-cooled thorium hollow cathodeused in the present investigation is of the same designdescribed by Reader and Davis6 with the cathodemachined in aluminum and the internal wall andbottom completely covered with thorium foil. Argon

t~a= 9srm 85m . tQuartz Windo

AFIG. 1. Hollow-cathode lamp.

J. Reader and S. P. Davis, J. Opt. Soc. Am. 53, 431 (1963).

484

VOLUMLE 58, NUMBER 4 APRIL 1908

INTERFEROMETRIC THORIUM WAVELENGTHS

(a) (b)

FIG. 2. Comparison of line widths as emitted by (a) hollow-cathode and (b) electrodeless lamp. Spacer thickness 20 mm.

gas was also used as carrier in this tube at a pressure of1.5 torr. The lines emitted by this source are drasticallysharper than those emitted by a thorium iodide elec-trodeless lamp even when the hollow cathode is run atrelatively high currents (200 mA) and the electrodelessat the minimum power compatible with the emissionof the thorium spectrum. The comparison is shown inFig. 2. Another important advantage of the hollow-cathode lamp is that all of the parameters can beexactly defined without uncertainties of any kind. Thisis particularly important for a standard light source.All of the advantages of the hollow-cathode lamp be-come apparent when the interferometrically measuredlines are tested for accuracy using the combinationprinciple, and compared to the accuracy attainablewhen an electrodeless lamp is used.

Davison, Giacchetti, and Stanley3 discuss the diffi-culties of making accurate measurements on unresolvedblends of lines that appear in a spectrum with as manylines as thorium. These blends are particularly trouble-some because it is necessary to use a very wide slit inthe spectrograph to obtain fringes wide enough foraccurate measurements. Most of the blends that appearunresolved when an electrodeless lamp is used are wellresolved when a thorium hollow cathode is employed.An example is shown in Fig. 3. This fact makes itpossible to increase the number of interferometricallymeasurable lines and reduces in most cases completelythe influence of neighboring lines.

In the report by Giacchetti et al.4 more than 60% ofthe measured lines had to be discarded because con-cordant results could not be obtained from the threedifferent spacers used in the interferometer. This isvery probably due to the many unresolved blendsincluded in the measurements. In the present work,practically all of the measured lines could be includedin the final results.

From the comparison of the lines radiated by a water-cooled thorium hollow cathode against the linesemitted by a liquid-nitrogen-cooled tube it is apparentthat the lines have about the same sharpness in thevisible, and are somewhat sharper for the liquid-nitrogen-cooled source in the ultraviolet.

TABLE I. Variation of the fractional part of the order of inter-ference at the center of ring diameters from some "98Hg lines.

Eta- Wavelength Aion Wvlntmm 5792.2682 5771.1984 5462.2707 3126.5761 2968.1499 2894.4465

20 0.002 0.001 0.002 0.006 0.004 0.00140 0.008 0.001 0.001 0.004 0.00650 0.002 0.004 0.006 0.007 0.006

MD)

FIG. 3. Comparison of blending in lines emitted (a) hollow-cathode and (b) electrodeless lamps. Spacer thickness 20 mm.

In the present experiment, the thorium spectrumwas excited with a powerpack supplying 150 mA at300 V. The actual currents and voltages used for photo-graphing varied from 80 to 120 mA and from 145 to175 V.

OPTICAL ARRANGEMENT

The optical arrangement used for the present experi-ment is shown in Fig. 4. A Schimadzu-Ebert-mountingspectrograph of 3.4-m focal length was used. Horizontaldispersion was produced by a 600-grooves-per-mm planegrating blazed at 5000 A. To photograph the range from4500 to 6700 A, the first order of the grating was em-ployed with a reciprocal dispersion of about 5 A/mm.The evacuated and thermostatically controlled inter-ferometer was mounted externally in a parallel beamof light. The interferometer consisted of aluminizedquartz plates separated in turn by three 6talons 20, 40,and 50 mm in length. The quartz-fluorite achromaticprojection lens had a focal length of 50 cm. The inter-ference pattern was focused on the spectrograph slit,which was 20-mm long and 0.3-mm wide. In this way,it was possible to produce six or more interference ringson the photographic plate. The thorium light sourcewas situated on the optical axis and the 198Hg lamp atright angles to it. An aluminum plane mirror was usedto direct the light from the mercury lamp towards theinterferometer and the spectrograph slit. Thus lightfrom each source followed the same optical path throughthe interferometer. The mirror could be removed forphotographing the thorium source. The interferometerwas housed inside a double-wall cylinder with quartzwindows. Thermostatic control was obtained with circu-lating water whose temperature was maintained con-stant to better than 4-0.010C.

EXPERIMENTAL METHOD

Overlapping orders of the "'HHg spectrum were photo-graphed on the same photographic plate with thethorium spectrum; they were filtered to allow only thepassage of wavelengths longer than 4000 A. To excitethe mercury spectrum, an oscillator with a frequency

Th Light l LSoreF .30 e .F =.0 c m .Sperodgruph Shil

II 198

l4Ol Sourr m

FIG. 4. Optical arrangement.

485April 1968

ka)

8 FRANCISCO P. J. VALERO

TABLE II. Interferometrically measured thorium wavelengths. The intensities in Column 3 are taken from Ref. 9.

x (A) M A) x (A) x (A)Vacuum Air Intensity Spectrum Vacuum Air Intensity Spectrum4574.9852 4573.7034 200 I 4874.2777 4872.9167 300 I4589.7119 4588.4262 400 I 4875.7255 4874.3641 200 I4593.9526 4592.6658 400 I 4879.3714 4878.0090 75 I4596.7079 4595.4203 600 I 4880.0951 4878.7325 200 I4600.9935 4599.7048 15 I 4882.5679 4881.2046 50 I4604.4342 4603.1446 175 T 4896.3216 4894.9546 350 I4614.8968 4613.6044 125 I 4900.6080 4899.2400 200 II4616.3165 4615.0237 125 I 4903.4234 4902.0546 250 I4616.6266 4615.3338 70 I 4906.0611 4904.6914 50 I4620.8269 4619.5330 400 II 4911.5283 4910.1573 150 I4622.4571 4621.1626 200 I 4912.1628 4910.7917 175 I4628.5938 4627.2980 125 I 4912.7502 4911.3790 150 II4629.4975 4628.2011 125 I 4914.0212 4912.6496 300 II4639.9836 4638.6846 100 I 4921.1891 4919.8156 600 II4641.3458 4640.0464 500 II 4922.9873 4921.6133 400 II4648.5523 4647.2510 200 I 4924.3185 4922.9442 150 II4656.5158 4655.2124 150 I 4929.1561 4927.7804 140 I4664.5080 4663.2025 200 I 4935.2291 4933.8519 100 II4669.4784 4668.1716 700 I 4938.1523 4936.7742 300 I4671.2915 4669.9842 400 I 4941.0205 4939.6421 350 I4674.9688 4673.6605 600 I 4944.4433 4943.0637 140 I4681.5476 4680.2376 150 I 4946.8387 4945.4582 140 I4681.9560 4680.6459 250 I 4948.0443 4946.6637 75 II4684.6625 4683.3517 250 I 4967.1171 4965.7314 250 I4687.5060 4686.1944 400 I 4981.5751 4980.1856 600 I4691.9342 4690.6215 120 I 4983.8773 4982.4871 300 I4692.9486 4691.6356 150 I 4986.7631 4985.3722 300 I4695.4048 4694.0912 400 II 4990.7003 4989.3084 200 14696.3514 4695.0375 400 I 4995.1416 4993.7482 50 I4705.3057 4703.9895 500 1 5003.4920 5002.0968 400 I4707.0771 4705.7604 400 II 5004.9938 5003.5980 75 I4709.6115 4708.2942 175 I 5005.5235 5004.1276 200 I4714.1596 4712.8410 150 I 5010.6373 5009.2401 15 I4724.7594 4723.4379 400 II 5017.2881 5015.8891 400 II4730.4508 4729.1279 250 I 5018.6540 5017.2547 500 II4733.4326 4732.1087 15 I 5021.2058 5019.8057 300 I4737.3150 4735.9902 70 I 5023.4057 5022.0050 250 I4741.8554 4740.5294 400 II 5030.0578 5028.6554 400 II4746.6633 4745.3361 100 I 5040.6354 5039.2301 200 14750.5280 4749.1998 200 I 5046.1259 5044.7192 400 I4753.7431 4752.4140 500 II 5048.4508 5047.0435 160 I4767.9333 4766.6005 200 I 5051.2041 5049.7961 400 I4774.5755 4773.2409 100 I 5052.1921 5050.7838 200 I4775.6092 4774.2742 100 II 5053.2968 5051.8882 200 I4776.6486 4775.3134 150 I 5056.7568 5055.3473 400 II4777.1292 4775.7939 150 I 5061.2713 5059.8606 200 I4779.6295 4778.2936 300 I 5066.0136 5064.6017 160 I4784.0983 4782.7611 180 II 5069.3862 5067.9734 900 I4787.8689 4786.5307 150 I 5082.8623 5081.4459 150 I4788.4859 4787.1476 100 I 5086.4093 5084.9920 150 I4790.7255 4789.3866 300 I 5091.9637 5090.5448 50 I4794.5844 4793.2445 100 I 5097.9050 5096.4845 200 I4797.2536 4795.9130 150 I 5099.4641 5098.0431 200 II4801.5141 4800.1724 100 II 5102.0425 5100.6209 200 I4809.4773 4808.1334 350 I 5116.4699 5115.0445 250 I4810.9581 4809.6138 300 I 5127.3785 5125.9501 150 I4824.2025 4822.8548 300 I 5129.9185 5128.4896 125 I4828.0490 4826.7002 300 I 5136.1765 5134.7459 150 I4832.4710 4831.1210 350 I 5138.9043 5137.4729 7 I4832.9478 4831.5977 200 I 5144.7000 5143.2671 150 II4834.1530 4832.8027 200 II 5145.3494 5143.9163 200 I4842.1953 4840.8428 400 I 5153.0468 5151.6117 400 II4846.5160 4845.1623 100 I 5155.6787 5154.2428 400 I4849.7166 4848.3621 250 I 5160.0411 5158.6041 700 I4851.7949 4850.4398 600 II 5162.9772 5161.5394 300 I4854.2240 4852.8683 200 I 5164.8965 5163.4582 300 I4859.6897 4858.3325 300 IT 5178.4029 5176.9610 400 I4862.5745 4861.2165 150 I 5195.9038 5194.4572 250 I4863.0750 4861.7169 50 I 5197.2605 5195.8136 400 I4864.5313 4863.1729 1000 II 5200.2477 5198.8003 400 IT4866.8362 4865.4771 350 I 5200.6115 5199.1638 800 I4870.2412 4868.8813 100 1 5206.6016 5205.1523 200 I

4S6 Vol. 58

April1968 INTERFEROMETRIC THORIUM WAVELENGTHS 487

TABLE II (continued)

X((A) x(.) A()Vacuum Air Intensity Spectrum Vacuum Air Intensity Spectrum

5207.9455 5206.4957 100 II 5588.5775 5587.0263 500 I5211.1746 5209.7240 200 I 5595,1673 5593.6142 100 II5212.6811 5211.2300 400 I 5596.6169 5595.0635 200 I5214.8008 5213.3492 300 I 5603.1584 5601.6032 150 I5220.5629 5219.1098 500 I 5641.3117 5639.7464 250 II5229.6799 5228.2244 200 I 5650.5589 5648.9910 75 II5232.6156 5231.1593 900 I 5659.4955 5657.9253 100 I5234.6823 5233.2254 350 II 5666.1931 5664.6211 50 II5240.2718 5238.8134 300 I 5666.7530 5665.1808 140 I5241.0105 5239.5520 300 I 5668.7006 5667.1279 125 I5241.6555 5240.1968 300 II 5676.5612 5674.9864 120 I5255.1530 5253.6907 10 I 5678.6281 5677.0527 125 I5265.8018 5264.3367 10 I 5686.7696 5685.1921 150 I5297.1328 5295.6593 125 I 5702.0397 5700.4582 200 II5297.7523 5296.2786 200 I 5702.4995 5700.9177 150 II5299.2168 5297.7428 250 I 5708.6868 5707.1035 150 II5299.7561 5298.2819 200 I 5721.2094 5719.6226 200 15301.9974 5300.5223 250 I 5721.7697 5720.1829 400 I5308.9423 5307.4657 300 II 5726.9767 5725.3884 250 I5311.7440 5310.2666 350 II 5734.5651 5732.9748 200 II5313.4795 5312.0017 400 I 5743.4218 5741.8291 50 I5314.0069 5312.5289 300 I 5750.3353 5748.7407 150 I5314.3826 5312.9045 300 I 5750.9831 5749.3884 100 II5318.9735 5317.4942 300 I 5751.3807 5749.7859 75 I5328.4576 5326.9757 400 I 5754.6220 5753.0264 100 I5331.5631 5330.0804 150 I 5762.1482 5760.5503 600 I5345.0674 5343.5811 500 I 5765.1272 5763.5287 80 I5349.4585 5347.9710 100 I 5769.3782 5767.7786 75 I5352.6146 5351.1263 20 I 5769.7811 5768.1814 150 I5361.6403 5360.1496 250 I 5775.5478 5773.9465 150 I5374.1968 5372.7027 200 I 5791.2504 5789.6450 200 I5376.3164 5374.8218 400 I 5794.0365 5792.4303 150 15376.8473 5375.3526 200 II 5797.6752 5796.0682 150 I5380.3312 5378.8355 100 I 5802.4380 5800.8295 175 I5380.6061 5379.1104 200 I 5805.7503 5804.1410 300 I5384.4244 5382.9276 250 II 5814.5842 5812.9725 150 I5388.1084 5386.6106 300 1 5817.0343 5815.4219 175 II5391.9645 5390.4658 500 II 5833.9874 5832.3705 125 I5396.2606 5394.7607 400 1 5840.5689 5838.9502 50 II5400.2032 5398.7022 125 I 5842.2594 5840.6403 75 I5400.6758 5399.1747 200 I 5854.3034 5852.6811 200 I5409.1570 5407.6537 200 I 5855.0973 5853.4747 50 I5412.2729 5410.7687 180 I 5855.7432 5854.1205 100 I5418.9917 5417.4857 200 I 5865.3437 5863.7185 125 I5425.5153 5424.0075 180 I 5887.3328 5885.7016 120 I5427.1863 5425.6782 250 II 5893.0839 5891.4512 70 I5432.6212 5431.1116 300 1 5901.4785 5899.8435 75 I5435.6614 5434.1510 125 I 5907.2072 5905.5705 100 I5437.4035 5435.8926 400 II 5910.5627 5908.9253 125 I5438.8993 5437.3880 200 II 5916.3099 5914.6709 140 I5444.6317 5443.1189 300 II 5920.5848 5918.9446 75 I5448.8160 5447.3020 15 II 5927.0450 5925.4035 50 I5450.9932 5449.4788 150 II 5927.8744 5926.2326 100 I5453.7338 5452.2186 250 I 5946.2944 5944.6474 75 II5465.7235 5464.2050 75 I 5975.3193 5973.6645 250 I5472.2793 5470.7590 100 I 5976.7200 5975.0649 250 I5493.9443 5492.4183 100 I 5990.7041 5989.0452 150 II5497.6637 5496.1367 80 I 5992.6665 5991.0071 120 I5500.7828 5499.2550 250 I 5995.7890 5994.1288 200 I5511.5242 5509.9935 300 I 6002.8654 6001.2033 60 I5516.4048 5514.8728 160 I 6006.8282 6005.1650 50 I5540.8001 5539.2616 400 I 6008.7360 6007.0723 180 I5541.4493 5539.9106 200 II 6011.8251 6010.1606 90 I5544.4295 5542.8900 160 I 6017.0881 6015.4221 75 II5549.7168 5548.1759 300 I 6022.7032 6021.0358 140 I5558.5886 5557.0453 200 I 6032.1150 6030.4450 50 I5559.8858 5558.3422 400 I 6039.3695 6037.6976 140 I5561.4351 5559.8910 60 I 6040.3522 6038.6802 75 I5572.7384 5571.1914 300 I 6046.1067 6044.4330 75 II5574.0122 5572.4648 140 I 6050.7260 6049.0511 100 I5574.9012 5573.3536 350 I 6055.0571 6053.3810 300 I5577.7532 5576.2048 100 I 6074.7852 6073.1038 50 II

FRANCISCO P. J. VALERO

TABLE II (conltinued)

Intensity SpectrumxWk)

Vacuum)A)

Vacuum

6078.78866079.55606080.10396080.90596087.05966088.947 i6089.71616100.77196103.41456104.28416109.22476114.53016123.10206126.17596153.69566155.77166157.28476163.05866171.52976180.14156182.41586184.33286189.83776193.61896199.93786205.20926208.93786221.73246226.2497

AirX (X)Air

6077.10616077.87336078.42116079.22296085.37486087.26246088.03076099.08356101.72526102.59486107.53416112.83806121.40766124.48076151.99306154.06856155.58116161.35366169.82246178.43196180.70556182.62206188.12546191.90566198.22296203.49286207.22046220.01166224.5277

Intensity Spectrum

50757580

10075

12575759050

1257575

125757550

50010075

40016010080

10016075

100

IIIIIIIIIIIII

IIIIIIIIIIIIIIIIIIII

of 250 MHz was used. The Hg-198 lamp was cooledto a temperature of 100C. To avoid confusing over-lapping of thorium and mercury lines, the interfero-grams were photographed in the following sequence:(1) '9 8 Hg; (2) 2 3 2 Th; (3) '9 8 Hg; (4) 2 3 2 Th; (5) 1 9 8 Hg. Thestability of the interferometer was checked for each Thexposure by measuring the order of interference at thecenter of the ring diameters for every measurable '95Hgline before and after each Th interferogram. Only thoseinterferograms were measured where the variation ofthe fractional order of interference was negligible (de-pending on the length of the 6talons employed in eachcase. Representative values are shown in Table I. Eightring diameters were measured for each line, and eachline was measured three times on each plate. The 198Hgwavelengths used are those reported by Kaufman.7

The green 5462.2707 A line was used as the standard.The data-reduction procedure is described in differentpublications.8 3 The thorium wavelengths have beencalculated by measuring six ring diameters. Eachthorium line was photographed at least two times witheach 6talon (20, 40, and 50 mm) in the Fabry-Perotinterferometer, a minimum of six interferograms foreach line. It was not necessary to increase the number ofinterferograms measured, owing to the excellent agree-ment obtained between the different measurements.

To determine the integral order of interference at thecenter of the ring diameters, the wavelength values

1 V. Kaufman, J. Opt. Soc. Am. 52, 866 (1962).8 K. W. Meissner, J. Opt. Soc. Am. 31, 405 (1941).

given by Zalubas9 were used. Owing to some inaccuraciesin these values it was impossible to determine directlythe correct integral order of interference for many lines.This problem has been solved by computing every Thwavelength three times for each measurement: firstusing the integral value obtained directly with the wave-length given by Zalubas, second using the first valueplus one order, and third the first value for the integralorder minus one. By comparing the results obtained forthe three different spacers it was possible to decidewhich was the actual wavelength for most linesmeasured.

The measurements of the mercury and thorium linessuggested that the dispersion of phase change in thealuminum films of the interferometer was very small.No significant differences were found between theresults from the 20, 40, and 50 mm 6talons. In theregion above 5792.2682 A, where there are no mercurylines that could be used to determine whether appreci-able correction was necessary for dispersion of phasechange of the thorium wavelengths, many of thethorium lines were measured with particular care. Forthese lines, eight ring diameters were measured and eachline was measured three times. No appreciable correc-tion for dispersion of phase change resulted from thisadditional set of measurements.

The interferograms were measured at the PhysicsDepartment of the University of California, Berkeley,using a rotating-prism photoelectric comparator. The

9 R. Zalubas, Nati. Bur. Std. (U. S.) Monograph 17 (1960).

6228.09256236.58036259.15486262.79616263.15036275.85256328.11646329.02776339.37286344.61406373.70576378.69426408.21736413.67196415.38816417.87536439.54136448.55356459.06766492.53116533.14696579.03126579.47386585.72516590.35986593.30556595.76096664.10906676.5399

6226.37006234.85556257.42406261.06426261.41846274.11726326.36706327.27816337.62046342.86026371.94536376.93126406.44646411.89966413.61536416.10196437.76206446.77186457.28316490.73756531.34276577.21456577.65686583.90646588.54006591.48496593.93956662.26946674.6970

502001004018010060

18040

30030

35075

250200155050

5001204005075

20020010020025030

III

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Vol. 58

INTERFEROMETRIC THORIUM WAVELENGTHS

TABLE III. Examples for lower-energy-levels differences for Th I.

DeviationWavenumber from

Wavelength Wavenumber difference Mean meanA cm-1 cm-1 cm-1 10-3 cm-

4622.4571 21633.5161689.2458 +0.6

4774.5755 20944.27035472.2793 18273.9211

689.2453 +0.15686.7696 17584.67585574.9012 17937.5376

689.2447 689.2452 -0.55797.6752 17248.29295769.7811 17331.6800

689.2447 -0.56008.7360 16642.43536078.7887 16450.6458

689.2457 +0.56344.6140 15761.4001

4687.5060 21333.30602092.4002 -0.4

5197.2605 19240.90584842.1953 20651.7899

2092.4009 +0.35388.1084 18559.38905003.4920 19986.0417

2092.4006 2092.4006 0.05588.5775 17893.64115136.1765 19469.7359

2092.3999 -0.75754.6220 17377.33605465.7235 18295.8395

2092.4013 +0.76171.5297 16203.4382

4946.8387 20214.93042869.2594 -0.1

5765.1272 17345.67105127.3785 19503.1438

2869.2600 2869.2595 +0.56011.8251 16633.88385603.1584 17847.0771

2869.2590 -0.56676.5399 14977.8181

measurements to study the dispersion of phase changewere made with a Carl Zeiss comparator at the Uni-versity of La Plata. The computation work was doneusing an IBM 1620 computer.

DISCUSSION

The combination principle was applied as a rigid testof the accuracy of measured wavelengths, which arepresented in Table II. Two radiations resulting from

transitions from the same high energy level to twodifferent low energy levels will show in their wave-numbers the energy difference of the low lying levels.In Table III examples for lower-energy-levels differ-ences for Th I 10 are presented.

The average deviation of all tested pairs of lines isless than40.0005 cm-'; if we assume that the errors areshared equally by all lines, the individual average erroris about ±40.00025 cm-', that is, about one part in 6 to9.5X 107 for the wavelength range measured. This isless than half the average error obtained by Meggersand Stanley5 and less than one-fourth the average errorobtained by Littlefield and Wood.2

We have compared our wavelengths with those givenby Meggers and Stanley' and the systematic wave-length difference (1/n)2 (NM S-NV) is + 1.4X 10-4 A for173 lines in the range from 4574 to 6676 A. If we con-sider that Meggers and Stanley used the value 5462.2706A for their "'1Hg standard line and that I used thevalue 5462.2707 A as standard, the systematic wave-length difference should be increased by +±X1 4 A,resulting in a systematic difference of +2.4X 104.

This shift is consistent with a higher pressure in theelectrodeless discharge tube used by Meggers andStanley.

The average deviation of our values from those ofMeggers and Stanley is 3.1X 104 A. If the systematicdeviation is taken into account, this agreement is evenbetter and reduces to less than 2.4X 10- A, that is, theaverage deviation from the mean reduces to -4h1.2 X 10-4A. The average deviation of our values from the valuesof Littlefield and Wood (1/n)Z I XLW-NV I is 9.9X 10-4 Awhich is more than four times as great.

ACKNOWLEDGMENTS

This work has been possible thanks to the encourage-ment, advice, and help received from Professor SumnerP. Davis of the University of California, Berkeley. Heprovided the "'1Hg lamp, the thorium metal used forthe construction of the sources, and made availablethe comparator and computer facilities of the PhysicsDepartment. The author also thanks Dr. Mario J.Garavaglia of the University of La Plata and RichardJ. Wolff of the University of California for assistingwith the computer programing.

10 R. Zalubas, J. Res. Natl. Bur. Std. (U. S.) 63A, 275 (1959).

489April 1968