L-shell absorption spectrum of neutral aluminum
5
Vol. 72, No. 6/June 1982/J. Opt. Soc. Am. 729 L-shell absorption spectrum of neutral aluminum Alberto M. Cantti and Gian Paolo Tozzi* OsservatorioAstrofisico di Arcetri, Unitd Gruppo Nazionale di Astronomia de] Consiglio Nazionale delle Ricerche, 5 Largo E. Fermi, 50125, Firenze, Italy Nissan Spector Soreq Nuclear Research Center, Yavne, 70600 Israel Received November 14,1981 The Al I absorption spectrum obtained with a flash-pyrolysis technique combined with BRV source was observed in the grazing-incidence region between 140 and 650 A. Thirty-three new lines in the range 140-160 A were record- ed. A theoretical interpretation identifies 25% of the observed lines, accounting for half the observed intensity as 2p 6 3s 2 3p-2p 5 3s23p4s transitions. 1. INTRODUCTION Aluminum has a cosmic abundance of 6.39, as contrasted with 12 for hydrogen; it is thus placed among the 12 most abundant elements. Hence its astrophysical importance. Transitions that are due to excitation of the outer 3p electrons have been extensively studied. 1 - 6 Photoionization cross-section and electron transitions in- volving the 3s 2 closed shell have been observed and studied by some authors. Khol and Parkinson, 7 by using an aerody- namic shock tube, measured the photoionization cross section from threshold to 1730 A and Shore-Fano parameters of au- toionized transitions 3S 2 3p 2 P 1 / 2 , 3 / 2 -3s3p 2 2 P1/ 2 , finding the cross section at the first limit much higher than any previous experimental measurement or theoretical prediction. Esteva et al. 8 used two vacuum sparks, one providing the background continuum and the other generating the ab- sorption medium. They observed 10 absorption lines that were due to transitions of the kind 3s 2 3p 2 P 0 -3s3pnp 2 D with 4 < n $ 11 and 2 p with 4 < n < 6. Roig 9 reported absorption spectra in the 1180-1430-A wavelength region obtained by flash pyrolysis to produce the absorption medium. He measured wavelengths of 70 lines that were due to 3s 2 3p 2 P 0 -3s3pnp 2S, 2 p, 2 D, 4S, and 4P transitions, as well as relative photoionization cross section in the 1200-2000-A region and Shore-Fano parameters for the most intense and broad lines. Recently Lombardi et al. 1 0 measured the oscillator strengths and autoionization widths of the neutral aluminum multiplet 3S 2 3p 2 p 0 -3s3p 2 2 p. At wavelengths lower than 925 A, which is the ionization limit 3s3p 'P 0 of aluminum, one-electron transition features are due to transition involving the L shell of the kind 2p 6 3s 2 3p 2 P0_2p 5 3s 2 3pns, nd converging to the 2p 5 3s 2 3p limit at about 143 A. Here an enhancement to 6 Mb for the value of the photoionization cross section is predicted. 11 In this paper we report the work on absorption spectra that are due to such L transitions for which preliminary results have been recently reported. 12 2. EXPERIMENT AND DATA Spectra have been taken at the Osservatorio di Arcetri with the grazing-incidence equipment already used to obtain lithium spectra.1 3 The system consists of a 2-m Hilger and Watts grazing-incidence spectrograph' 4 equipped with a Bausch & Lomb 600-line/mm platinum-coated grating blazed at 3° 31' and used at an angle of incidence of 860. A contin- uum source optically coupled with a toroidal mirror' 5 produces a quasi-stigmatic and high-density background. The mirror, with radii of curvature of 416 and 77 mm in the plane of inci- dence and in the plane orthogonal to it, respectively,was used at an angle of incidence of 800, which represents a good com- promise between surface reflectivity and optical aberrations. With this arrangement the aperture of the spectrograph was filled, and the spectra are nearly stigmatic at about 100 A. Thus well-developed absorption spectra were obtained with only one shot of the source, on Kodak 101-05 photographic plates. The continuum source used is a BRV type' 6 powered by a fast-discharge capacitor of 2.5-yF capacity, charged up to 25 kV. A polyethylene capillary with a hole of 0.7-mm diameter was fitted over the end of the uranium anode. The capillary has two effects: It enhances the emission by pinching the plasma mechanically and fixes its location exactly in space, thus throwing its image, formed by the toroidal mirror, onto the 10-,4m entrance slit. The absorbing medium is realized through the flash py- rolysis technique: a 25-cm-long quartz tube placed between the continuum source and the toroidal mirror is surrounded by a helical quartz flash lamp. The element to be analyzed is placed inside the tube and, because of the strong luminous flux emitted by the flash, the element is heated and vaporized in periods of the order of few microseconds. The maximum energy density emitted by the flash is about 50 J/cm 2 , working with a voltage up to 10 kV supplied by two 60-,4F capaci- tors. Two spark gaps are in series with both the BRV source and the flash to ensure a reproducible start that is independent 0030-3941/82/060729-05$01.00 ©C 1982 Optical Society of America CantU et al.
Transcript of L-shell absorption spectrum of neutral aluminum
Vol. 72, No. 6/June 1982/J. Opt. Soc. Am. 729
L-shell absorption spectrum of neutral aluminum
Alberto M. Cantti and Gian Paolo Tozzi*
Osservatorio Astrofisico di Arcetri, Unitd Gruppo Nazionale di Astronomia de] Consiglio Nazionale delleRicerche, 5 Largo E. Fermi, 50125, Firenze, Italy
Nissan Spector
Soreq Nuclear Research Center, Yavne, 70600 Israel
Received November 14,1981
The Al I absorption spectrum obtained with a flash-pyrolysis technique combined with BRV source was observedin the grazing-incidence region between 140 and 650 A. Thirty-three new lines in the range 140-160 A were record-ed. A theoretical interpretation identifies 25% of the observed lines, accounting for half the observed intensity as2p63s2 3p-2p53s23p4s transitions.
1. INTRODUCTION
Aluminum has a cosmic abundance of 6.39, as contrasted with12 for hydrogen; it is thus placed among the 12 most abundantelements. Hence its astrophysical importance. Transitionsthat are due to excitation of the outer 3p electrons have beenextensively studied. 1-6
Photoionization cross-section and electron transitions in-volving the 3s 2 closed shell have been observed and studiedby some authors. Khol and Parkinson, 7 by using an aerody-namic shock tube, measured the photoionization cross sectionfrom threshold to 1730 A and Shore-Fano parameters of au-toionized transitions 3S23p 2P1/2,3/ 2 -3s3p2 2P1/2, finding thecross section at the first limit much higher than any previousexperimental measurement or theoretical prediction.
Esteva et al. 8 used two vacuum sparks, one providing thebackground continuum and the other generating the ab-sorption medium. They observed 10 absorption lines thatwere due to transitions of the kind 3s23p 2 P0 -3s3pnp 2 D with4 < n $ 11 and 2
p with 4 < n < 6.Roig9 reported absorption spectra in the 1180-1430-A
wavelength region obtained by flash pyrolysis to produce theabsorption medium. He measured wavelengths of 70 linesthat were due to 3s23p 2 P0 -3s3pnp 2S, 2 p, 2D, 4S, and 4P
transitions, as well as relative photoionization cross sectionin the 1200-2000-A region and Shore-Fano parameters for themost intense and broad lines.
Recently Lombardi et al. 10 measured the oscillatorstrengths and autoionization widths of the neutral aluminummultiplet 3S23p 2p0 -3s3p2 2 p.
At wavelengths lower than 925 A, which is the ionizationlimit 3s3p 'P0 of aluminum, one-electron transition featuresare due to transition involving the L shell of the kind 2p 63s 23p2P0_2p5 3s23pns, nd converging to the 2p53s23p limit at about143 A. Here an enhancement to 6 Mb for the value of thephotoionization cross section is predicted. 1 1
In this paper we report the work on absorption spectra thatare due to such L transitions for which preliminary resultshave been recently reported.12
2. EXPERIMENT AND DATA
Spectra have been taken at the Osservatorio di Arcetri withthe grazing-incidence equipment already used to obtainlithium spectra.13 The system consists of a 2-m Hilger andWatts grazing-incidence spectrograph'4 equipped with aBausch & Lomb 600-line/mm platinum-coated grating blazedat 3° 31' and used at an angle of incidence of 860. A contin-uum source optically coupled with a toroidal mirror' 5 producesa quasi-stigmatic and high-density background. The mirror,with radii of curvature of 416 and 77 mm in the plane of inci-dence and in the plane orthogonal to it, respectively, was usedat an angle of incidence of 800, which represents a good com-promise between surface reflectivity and optical aberrations.With this arrangement the aperture of the spectrograph wasfilled, and the spectra are nearly stigmatic at about 100 A.
Thus well-developed absorption spectra were obtained withonly one shot of the source, on Kodak 101-05 photographicplates.
The continuum source used is a BRV type' 6 powered by afast-discharge capacitor of 2.5-yF capacity, charged up to 25kV. A polyethylene capillary with a hole of 0.7-mm diameterwas fitted over the end of the uranium anode. The capillaryhas two effects: It enhances the emission by pinching theplasma mechanically and fixes its location exactly in space,thus throwing its image, formed by the toroidal mirror, ontothe 10-,4m entrance slit.
The absorbing medium is realized through the flash py-rolysis technique: a 25-cm-long quartz tube placed betweenthe continuum source and the toroidal mirror is surroundedby a helical quartz flash lamp. The element to be analyzedis placed inside the tube and, because of the strong luminousflux emitted by the flash, the element is heated and vaporizedin periods of the order of few microseconds. The maximumenergy density emitted by the flash is about 50 J/cm 2 , workingwith a voltage up to 10 kV supplied by two 60-,4F capaci-tors.
Two spark gaps are in series with both the BRV source andthe flash to ensure a reproducible start that is independent
0030-3941/82/060729-05$01.00 ©C 1982 Optical Society of America
CantU et al.
730 J. Opt. Soc. Am./Vol. 72, No. 6/June 1982
Table 1. Newly Observed Al I Absorption Lines
I X (A) a (cm-')
123456789
10111213141516171819202122
33
24252627282930313233
62221293122279
102022511422236353233
159.25158.08157.98157.86157.64157.44157.27157.13157.03156.87156.72156.65156.25156.05155.83155.45155.10154.99154.84154.45154.26153.88153.62153.25153.06152.85152.72152.32151.84150.94150.40146.26145.88
627943632591632992633473634357635163635849636416636821637471638081638366640000640820641725643294644745645203645828647459648256649857650957652529653339654236654793656513658588662515664894683714685495
There was no indication of series members converging tothe excited state 2p 5 3s 2 3p of Al II or of any measurable vari-ation of the photoionization cross section in proximity to thislimit around 143 A.
3. THEORETICAL INTERPRETATION
A. Configurations InvolvedThe most likely candidates for the configurations responsiblefor the observed transitions are 2p 5 3s 2 3p4s and 2p 5 3s 23p3d.This can be seen by extrapolating the 2p 6 -2p 5 n1 excitationenergy on the basis of the known Ne I and Mg III values,134000 and 450000 cm-', respectively. This extrapolationputs the corresponding energy in Al I at around 600000 cm-'.If one adds the extra 3p-4s energy interval, which is around30000 cm- 1 , one gets, indeed, transition energies corre-sponding to the 158-A region.
Since the 2p 53s
2 3p4s configuration involves a small numberof outstanding 2p-4s transitions as opposed to a larger butweaker 2p-3d array connected with 2p53s23p3d, our calcu-lations concentrated mainly on the former configurations.
B. Line PositionsIn order to be able to predict the transition array we first hadto calculate the energy positions of the upper configura-tions.
The 2p 53s23p4s configuration has 18 levels that are dividedinto J's as follows:
p5 psJ = Y/2 11/2 21/2 31/2
2SP(2) 4S 4P 4D2p (2) 2p (2) 2D (2)4p 4p 4D4D 2D(2)
4Dof the pressure inside the cell and source; the BRV is delayedcontinuously from 50 to 500 ,usec from the fired flash to fit thebest conditions of absorption.
In the case of aluminum, the sample to be vaporized wassupplied by an Al foil that was 99.99% pure and 0.75 gmthick.
Glass tubes with low transmission cutoff at 3000 A were alsoused to ensure dealing with Al I without any Al HI populationsince the photoionization threshold of Al I is at 2071 A and theflash and cell transmit down to about 1600 A.
Since the efficiency of the system attains its maximum inthe 130-250-A spectral region, it was possible to record spectrasimultaneously in the first up to the fourth diffraction orders.This feature was employed to improve wavelength measure-ments. The wavelengths were measured by using knownBRV emission lines of 0 v, 0 VI, C III, C IV,17 and uranium' 8
as reference lines. As was the case in lithium, here also theDoppler shift that is due to the motion of the BRV plasma isestimated to be lower than our wavelength precision, 0.01A.
In the 145-160-A spectral region at first order we observed33 new lines of Al I and none that should be assigned to Al11.
Table 1 is a list of wavelengths of the observed Al I linesalong with their relative intensities, which were derived fromvisual estimates at the first order.
6 7 4 1
Since the ground term of Al I, 2 p, involves the levels withJ = 1/2, 11/2 already equally populated at room temperature,only the J = 3'/2 level of p 5ps could not combine with them.Thus the total number of theoretically possible transitions is30. In the first step to calculate these transitions we diago-nalized the energy matrices of 2 p53p4s with the radial pa-rameters given in Table 2.
These parameters were estimated from observed energylevels in Al I and neighboring spectra.
It should be remarked that the very small value-75cm'1-of the spin-orbit parameter tp is derived directly fromthe ground-state interval of 112 cm-. Owing to this small
Table 2. Radial Parameters for 2p53p4 s of Al I
Name Value (cm-')
A 640000F 2 500co 3000G2 100
G2,s 500G3ps 500
t2p 1300t3p 75
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Table 3. Eigenvalues and Eigenvectors for Intermediate (N) and 2p-4s (F) Coupling and Relative Intensities for2p53s 2 3p4s of Neutral Aluminum (Al I)
J = 1/2Eigenvalue (N) (cm-')Eigenvector (N)
Eigenvector (F)
(1lp)
Intensity
J = 1/5Eigenvalue (N) (cm-')Eigenvector (N)
Eigenvector (F)(1P)
('P)
Intensity
J = 21/2Eigenvalue (N) (cm-')Eigenvector
(N)
Eigenvector (F)('P)
Intensity
splitting, lines connecting the two 2P1 /2,3/2 levels to the sameupper level are not resolvable under the present experimentalconditions. Thus most of the lines connecting the groundconfiguration to the 2p5 3s2 3p4s levels (except 4) are double.This fact should be considered in the calculation of intensi-ties.
C. IntensitiesWe notice that in the transitions involved, namely,2p 63s 2 3p-2p 5 3s 23p4s, the 3p electron is 6nly a spectatorelectron and that the actual electron jump is 2 p6-2 p5 4s. Inpure LS coupling only one line is allowed: 'S o-'Pl. Theensuing transitions in the real physical scheme are obtainedfrom this one by two unitary transformations involving the
addition of angular momenta of the spectator electron and thechange to intermediate coupling. However, the relative in-tensities in the 2p6 3s23p-2p5 3s2 3p4s array will be directlyrelated to the percentages of the 2p5 4s 1P component in theeigenvectors of the upper levels of the transitions. Thesepercentages can be determined quantitatively by consideringthe eigenvectors of the upper levels in a coupling scheme inwhich the 'P is a good quantum number.
In order to obtain such eigenvectors, we performed a ficti-tious diagonalization in which we gave the electrostatic pa-rameter of the 2p-4s interaction, G1, a very high value of100000 cm-' and the rest of parameters the value zero (exceptfor a small value, 100 cm-', for Up). It can be seen easily thatin this case the eigenvalue corresponding to the lP parent will
(1S)2S
(3S)2S(1p)2p(3P) 2P
(3p)4p(3D)4D
-99960100000
-100100100000
-100100-100200
(3 S)4 S(1p)2p(3p)2p(3P)4 P(1D)2 D(3D)2 D(3D)4 D
-99900-99900100000
-100200-100100
100000-100100
(3p)4p(1D)2D(3D)2D(3D)4D
-99900-99900100000
-100100
6525750.9900
-0.0003-0.0003
0.05540.06740.0002
-0.5990-0.4978-0.3356
0.0472-0.4165-0.3237
25
6338970.99800.05560.03270.08830.00030.00070.0003
0.76610.25670.05650.2626
-0.50170.00070.1310
6416870.8190
-0.4411-0.2435-0.2756
0.4160-0.3360-0.4301-0.7264
18
6353830.00000.9924
-0.06730.0781
-0.0676-0.0002
0.39300.85970.10350.03440.27180.1409
74
642402-0.0710
0.7759-0.2460
0.4888-0.1536
0.26210.0801
0.27910.44480.17470.63700.41580.14990.3040
5
6402660.56300.72910.32050.2214
0.7600-0.0541
0.64210.0806
41
641795-0.0404
0.0172-0.6303-0.1472
0.7200-0.2480
0.5090-0.0054
0.0910-0.4402
0.3713-0.6329
18
643131-0.0620
0.23440.90780.22610.1609
-0.18910.0602
0.1108-0.1992
0.9033-0.0893
0.1453-0.0829-0.3073
82
6393930.0390
-0.42910.90240.0067
-0.4820-0.5686
0.5672-0.3501
32
643567-0.0550-0.0940-0.2500
0.9578-0.0173
0.0500
0.3771-0.1074-0.5105
0.70100.0511
-0.2001
50
6416770.04300.47420.0792
-0.70650.13690.22660.2881
-0.43000.40070.3057
-0.4661-0.0345
26440.5155
16
6382410.1080
-0.2995-0.1541
0.9354
0.1240-0.7489-0.2836
0.5859
8
642296-0.0550
0.07790.69400.19850.67730.0997
0.06220.03840.61930.5193
-0.57300.1148
27
639951-0.0100-0.2403-0.0284
0.26330.63790.55990.3910
0.2533-0.2140-0.1451-0.2661
0.35570.8034
-0.1616
67
6392190.0002
-0.00330.22110.1074
-0.1161-0.9623
-0.2831-0.0018
0.46450.20410.5327
-0.6152
4
6407350.0003
-0.22310.3239
-0.0393-0.6047
0.6863-0.0802
-0.0505-0.5892
0.16920.36450.41960.29180.4777
11
6386390.00020.08640.0482
-0.11230.40040.2446
-0.8715
-0.26120.36950.08500.3103
-0.49490.4125
-0.5258
18
CantU et at
732 J. Opt. Soc. Am./Vol. 72, No. 6/June 1982
be 100000. After performing the real diagonalization, werecoupled both transformation matrices to obtain a new ei-genvector matrix in which the 'P components could be tracedby using the above-mentioned eigenvalue, and their per-centages were obtained directly from this new matrix. In thecalculation of the relative intensities, we took into account thestatistical weight of the two absorbing levels plus the fact thatthe lines connecting them to upper levels of J = 1/2, 11/2 aredouble, whereas the transitions involving the J = 21/2 levelsare single.
In Table 3 we give the results of our diagonalizations for J= 1/2, 11/2, 21/2 of the 2p 5 3s 2 3p4s configuration.
4. DISCUSSION
In Table 4 we give a comparison between the predicted andobserved spectra of Al I in the 153-157-A region. It is seenthat in the wavelength interval involved there is good agree-ment between the observed and calculated spectra both forpositions and for intensities. The theoretical relative inten-sities are normalized to correspond to the observed visualestimates. As can be seen by comparing Tables 1 and 4, al-though the 2p63p-2p 53p4s transition array permits theidentification of about 25% of the observed lines in this region,it accounts for 50% of the observed intensity. Thus theargument proposed in Section 3.A is confirmed.
Table 4. Identified 2p63p-2p 5 3p4s Transitions in Al I
a Calc. (cm-') Rel. Int. X Calc. (A) X Obs. (A) Lobs
635383 8 157.38 157.27 9638241 1 156.68 156.72 2638639 2 156.58 156.65 2639219 0 156.44639393 2 156.40639951 7 156.26 156.25 7
6407356 3 156.071 156.05 96416778 155.84641687 155.84641795 8 155.81 155.83 10642296 155.69642402 155.67
64353671 19 1155.34981 155.45 20
652575 3 153.24 153.25 2
I 77 b
159.2515727
In Fig. 1 we plot the theoretical spectrum-bar positionscorrespond to wavelengths and heights to relative intensi-ties-against a recording of the experimental spectrum.Quite good agreement is obtained at the part of the spectrumcovered by the calculations.
5. CONCLUSIONS
In the present work, the flash pyrolysis technique was suc-cessfully applied to obtain the absorption spectrum of Al i inthe grazing-incidence region. A group of absoption linesbelonging to neutral aluminum was observed in the region145-160 A. It has been shown that the outstanding featuresof this group could be accounted for by transitions from theground configuration to the upper one, 2p53s23p4s, involvingthe inner shell excitation 2p6-2p5 4s. It has also been shownthat the inner shell jump supplies the key to the under-standing of the intensity ratios of the lines by using a processof recoupling to include the spectator-electron quantumnumbers.
ACKNOWLEDGMENTS
The authors would like to thank Paolo Stefanini for histechnical assistence and Tito Grisendi for helping in the dataprocessing.
This research was supported by Consiglio Nazionale delleRicerche through the Gruppo Nazionale di Astronomiagrant.
* Temporary address, Institut fur Astronomie, ETH Zen-trum Zurich, CH, Switzerland.
REFERENCES
1. K. B. S. Eriksson and H. B. S. Isberg, "The spectrum of atomicaluminum, Al'I," Ark. Fys. 23, 527-542 (1963).
2. K. B. S. Eriksson and H. B. S. Isberg, "Comments on the sharpand diffuse series in the spectrum of atomic aluminum," Ark. Fys.33, 593-595 (1967).
3. K. B. S. Eriksson, "The 2D term of the configuration 3s 3p 2 in AlI," Ark. Fys. 39, 421-424 (1969).
4. A. W. Weiss, "Series perturbations in atomic spectra: superpo-sition-of-configurations calculations on Al I and Al II," Phys. Rev.A. 9, 1524-1536 (1974).
5. C. E. Lin, "Theoretical analysis of the Al I absorption spectrum,"Astrophys. J. 187, 385-387 (1974).
6. P. S. P. Wei, "Emission spectrum of aluminum in a laser-producedplasma," J. Chem. Phys. 64, 1531-1532 (1976).
7. J. L. Kohl and W. H. Parkinson, "Measurement of the neutral-aluminum photoionization cross-section and parameters of the3p 2 P 0
-3s3p2 2
S1"2 autoionization doublet," Ap. J. 184, 641-652
(1973).8. J. M. Esteva, G. Mehlam-Balloffet, and J. Romand, "Spectres
d'absorption dans l'ultraviolet lointain de Be, B, C, N, Mg, Al etSi," J. Quant. Spectrosc. Radiat. Transfer 12, 1291-1303(1972).
9. R. A. Roig, "The photoionization spectrum of neutral aluminum,Al I," J. Phys. B. 8, 2939-2947 (1975).
10. G. G. Lombardi, B. L. Cardon, and R. L. Kurucz, "Measurementof the oscillators strengths autoionization widths of the neutral-aluminum multiplet 3 s23p 2 P'-3s3p 2 2 p," Ap. J. 248,1202-1208(1981).
11. E. J. McGuire, "Photo-ionization cross-sections of the elementshelium to xenon," Phys. Rev. 175, 20-30 (1968).
12. A. M. Cantui, N. Spector, and G. P. Tozzi, "New absorption
ib0 155 o) 160
Fig. 1. Comparison between (a) recorded and (b) theoretical spectra.(Intensities are normalized to the same arbitrary scale.)
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spectrum of aluminum (Al I) in the VUV," presented at Thir-teenth European Group for Atomic Spectroscopy Conference,Heidelberg, Germany, April 6-10,1981.
13. A. M. Canta, W. H. Parkinson, G.Tondello, and G. P. Tozzi,"Observations of Li I and Li II absorption spectra in the grazingincidence region," J. Opt. Soc. Am. 67, 1030-1033 (1977).
14. A. H. Gabriel, J. R. Swain, and W. A. Waller, "A two-metregrazing-incidence spectrometer for use in the range 5-950 A," J.Sci. Instrum. 42, 94-97 (1965).
15. A. M. CantU and G. Tondello, "Continuum source and focusing
technique for the 80-500 A spectral range: improvements," Appl.Opt. 14, 996-998 (1975).
16. G. Balloffet, J. Romand, and B. Vodar, "An emission source ofcontinuous spectrum extending from the visible to the extremeultraviolet," C. R. Acad. Sci. 252, 4139-4151 (1961).
17. R. L. Kelly, "Atomic emission lines below 2000 Angstroms-hydrogen through argon," Naval Research Laboratory Rep. 6648(U.S. Government Printing Office, Washington, D.C., 1968).
18. G. H. Newsom, "Wavelength standards for use with a BRVsource," Appl. Opt. 13, 2712-2715 (1974).
Canta et al.
