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Experimental investigations of the Zeeman effect of new fine structure levels of Lanthanum and Praseodymium
Spectrochimica Acta Part B 116 (2016) 16–20
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Experimental investigations of the Zeeman effect of new fine structure levels of Lanthanum and Praseodymium S. Werbowy a,c,⁎, C. Güney b,c, L. Windholz c,⁎⁎ a b c
Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57 PL-80-952 Gdansk, Poland Physics Department, Faculty of Science, Istanbul University, Vezneciler, Tr-34134 Istanbul, Turkey Institut für Experimentalphysik, Technische Universität Graz, Petersgasse 16, 8010 Graz, Austria
a r t i c l e
i n f o
Article history: Received 17 August 2015 Accepted 16 November 2015 Available online 2 December 2015 Keywords: Zeeman effect Lande factors Laser-induced fluorescence
a b s t r a c t Landé-g J data are given for several lines of La I and Pr I in the wavelength range 641.993 nm to 685.364 nm. Spectra were recorded in the presence of magnetic fields of about 800 G for two polarizations of the exciting laser light, perpendicular and parallel with respect to the direction of the magnetic field. We have used a hollow cathode sputtering atom source and laser induced fluorescence and/or optogalvanic detection. In this way we have determined for the first time the Landé-g J factors for some new levels of neutral La and Pr atoms. Additionally, the Landé-g J factors for other known levels of La I and Pr I were reinvestigated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lanthanum (Z = 57) and Praseodymium (Z = 59) are rare earth metals, belonging to the lanthanide group of elements in the periodic table. Because of their electronic properties, both elements have very rich and complex spectra. Lanthanum and Praseodymium in the natural abundance have each only one stable isotope, 139La with nuclear spin quantum number I = 7/2 and 141Pr with I = 5/2, which together with magnetic dipole and small electric quadrupole moments of the respective nucleus causes the hyperfine splitting (hf) of the levels. One of the first studies of the Lanthanum spectrum was performed by King and Carter [1] in the 279.2–830.5 nm range and extended later by Meggers [2]. More than 1500 lines in the wavelength range between 214.281 and 1095.46 nm in the arc and spark spectra were reported, from which about 700 lines were assigned to the neutral La atom. Due to the large density of the lines, a Zeeman effect analysis was made only for the 476 strongest and well resolved lines (279.1– 748.3 nm) [2]. The relative positions of the main Zeeman components at a magnetic field of 33,000 G were given. A more extensive Zeeman analysis was performed later and Landé-g J factors for almost 100 La I levels were determined [3]. Extensive studies of Pr I and Pr II emission spectra in the 350– 1200 nm range and of absorption spectra in the 200–870 nm range were performed by Zalubas and Borchardt [4]. They attributed about
⁎ Corresponding authors at: Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, PL-80-952 Gdansk, Poland. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (S. Werbowy), [email protected] (L. Windholz).
http://dx.doi.org/10.1016/j.sab.2015.11.003 0584-8547/© 2015 Elsevier B.V. All rights reserved.
25,000 lines to neutral atoms of praseodymium. From this observations, 62 high even levels and the 3 lowest levels of the ground configuration were identified. Also the Zeeman effect for some prominent lines of Pr I was reported and Landé-g J factors for 30 of these levels were given. At that time, practically no knowledge on the hyperfine structure of the lines was available, and hyperfine structure splitting was neglected in the analysis of the Zeeman patterns. Since that time, the numbers of known energy levels of La I has reached about 800 of even and 300 of odd parity. For Pr I atoms there are now around 1200 even and 1700 odd levels known. For all these levels the hyperfine constants are also known. The lanthanides with their open nf shells are very difficult elements from the theoretical point of view. Because of the increasing interest of using the lanthanides to a variety of applications, they have been subject of many theoretical studies, i.e. [5,6]. There are several groups that are extensively investigating the electronic structure as well as hyperfine interactions of La I and Pr I spectra.1 In the course of their studies, many new transitions and energy levels were discovered, spectral lines were classified and spectroscopic parameters like energy of the involved levels, their total angular momenta J, their parity and their hyperfine constants A and B were determined. However, despite the ongoing studies of the hyperfine structure of La I and Pr I, we found a big gap concerning the Zeeman effect analysis. The magnetic field has implications on the variety of applications, i.e. by combination of the magnetic field and atomic absorption 1 The group at the Graz University of Technology, Austria; the group of J. Dembczyński at the Poznan University of Technology, Poland; the group of G. Basar at Istanbul University, Turkey; the group of S. Kröger, Hochschule für Technik und Wirtschaft Berlin, Germany.
S. Werbowy et al. / Spectrochimica Acta Part B 116 (2016) 16–20
spectrometry (AAS). The Zeeman technique can be used to achieve accurate background correction of atomic absorbance signals improving the accuracy of the analysis of the elements. Since the group in Graz has large experience related to the spectra of La and Pr, we made an attempt to carry on investigations of the Zeeman effect of lines of these elements using proved successful hollow-cathode sources and laser excitation. We hope that by this we will encourage the scientific community to return to Zeeman effect investigations. The purpose of this study is to investigate the Zeeman effect of some energy levels recently discovered by analysis of the spectra of neutral lanthanum [7] and praseodymium [8]. We use Doppler-limited laser spectroscopy to record the hyperfine patterns of the excited lines under influence of a relatively weak magnetic field. For the analysis of the hyperfine-Zeeman patterns we use most accurate hyperfine structure constants for the investigated levels in order to obtain accurate Landé-g J factors. 2. Experiment For studies of neutral atoms in the laboratory, different experimental techniques were in use, e.g. classical emission spectroscopy of electric arcs between electrodes of graphite, silver, or copper, with a small portion of praseodymium or lanthanum salt being placed on the lower electrode before ignition of the arc [2,3]. A great improvement compared to this method was achieved, when a praseodymium or lanthanum plasma is generated in a hollow cathode lamp cooled with liquid nitrogen (in order to reduce Doppler line broadening). Such lamp can be used as light emission source, and the spectrum is recorded by high-resolution Fourier transform spectroscopy [9,10]. The atoms in the hollow cathode source are produced by a sputtering process from the cathode surface made from the investigated element. The plasma ignition takes place under the presence of argon (ca. 0.5 mbar) but after some minutes the discharge is carried mainly by metal atoms. A detailed description of the used method was given recently in [11]. With this method, the Doppler widths of the recorded hf patterns can be reduced some hundreds of MHz. The line width of a single hf component is caused mainly by the Doppler broadening. The plasma in the hollow cathode has a certain temperature, which is caused by heating of the discharge gas (including the sputtered La or Pr atoms, which carry most of the discharge) due to the electric current, and by cooling of the plasma by the cathode wall. The holder of the cathode is cooled by liquid nitrogen. This cooling of the cathode lowers the observed plasma temperature, but is also essential for a good sputtering efficiency and for operating the discharge with as less current noise as possible.
Fig. 1. Schematic drawing of the hollow cathode lamp. Left: side view, right: front view. K—cathode, made from copper, with an inner layer of La or Pr; A—anode in ceramic holder (holder not shown); H—tube holding cathode and anodes; C—container for liquid nitrogen; M—neodymium magnet.
17
Fig. 2. Magnetic field distribution measured along the magnet bar at distance 2 cm from the magnet.
Such lamp can be used also to perform laser spectroscopic experiments. A laser light beam is passing through the hollow cathode and—if the light frequency is in resonance with an atomic transition—excites atoms from the lower state of the transition to the upper one. The interaction can be detected either by laser-induced fluorescence (LIF) or a socalled optogalvanic (OG) signal (the plasma changes its resistance due to the change of the detailed equilibrium of the plasma). The laser light frequency is scanned over the investigated transition, and a well resolved spectrum of the investigated transition is recorded. If LIF- or OG-detection is better depends on many factors. For Pr, usually LIF is used, while for a special group of La levels OG detection is more sensitive. The disadvantage of the OG method is that it shows the sum of all interactions of the laser light with the plasma. Thus, if the lines are separated less than their total hyperfine splitting, socalled blend situations are observed and the recorded signal shows overlapping patterns of different transitions. In LIF, selecting for detection only one fluorescence channel, also completely overlapping transitions can be separated. To investigate the Zeeman effect we made a slight modification of the experimental apparatus described in [11] by introducing a uniform magnetic field to the laser excitation region, Fig. 1. This was accomplished by placing a strong bar shaped permanent neodymium magnet, placed 2 cm above the cathode. Since the hollow cathode lamp is placed inside a container with liquid nitrogen for cooling reasons, the magnet was immersed directly in the cooling liquid. The very low temperature of the magnet did not have any effect on the properties of the magnetic field. In this way we have generated magnetic fields of about 800 Gauss in the central region of the cathode. Fig. 2 shows magnetic field distribution measured along the magnet at distance 2 cm from the magnet
Fig. 3. (Color online) Recorded Zeeman pattern of the 669.887 nm Argon line used for the magnetic field measurement. Spectral line width is 1600 MHz FWHM.
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S. Werbowy et al. / Spectrochimica Acta Part B 116 (2016) 16–20
Table 1 La I and Pr I lines investigated by laser spectroscopy. P: Parity (e even, o odd); OG: optogalvanic spectroscopy; LIF: laser-induced fluorescence spectroscopy. Upper level Energy, J, P (cm −1)
Lower level Energy, J, P (cm −1)
Method (Fluorescence wavelength (nm))
La I 641.993 662.261 669.923 669.985 670.316 671.798 674.119 681.853 681.912 685.364
39,276.99, 5/2, e 36,543.48, 5/2, e 39,764.36, 13/2, e 24,841.41, 11/2, o 36,298.20, 9/2, e 36,265.30, 9/2, e 39,079.07, 11/2, e 36,109.7, 5/2, e 23,704.816, 3/2, o 36,034.61, 9/2, e
23,704.816, 3/2, o 21,447.854, 7/2, o 24,841.41, 11/2, o 9,919.826, 9/2, e 21,383.994, 9/2, o 21,383.994, 9/2, o 24,248.994, 9/2, o 21,447.854, 7/2, o 9,044.212, 1/2, e 21,447.854, 7/2, o
OG, LIF (468.6) OG OG, LIF (719.6) OG OG, LIF (713.4) OG, LIF (499.3) OG, LIF (595.3) OG OG, LIF(616.5) OG
Pr I 649.5728 654.5409 654.5421 655.289 655.4134 669.351
18,273.22, 11/2, e 25,191.854, 5/2, o 19,654.709, 13/2, e 18,102.9, 11/2, e 30,600.763, 13/2, e 30,283.139, 15/2, e
2,846.741, 13/2, o 9,918.19, 7/2, e 4,381.072, 15/2, o 2,846.741, 13/2, o 15,347.431, 13/2, o 15,347.431, 13/2, o
OG, LIF (548.2) LIF (533.4) OG LIF (597.4) OG, LIF (563.9) OG, LIF (539.8)
Wavelength (nm)
surface. The magnetic field is calibrated to 0.9% accuracy from the Zeeman effect of the Ar I line at 669.887 nm. A σ(ΔM = ± 1) Zeeman spectrum of the Ar I line is presented in Fig. 3. Spectroscopic data for this transition are [12]: 3s 23p 5( 2P 1/2)6s (E = 121,161.31 cm −1, J = 1, g J = 1.271)→ 3s 23p 5( 2P 3/2)4p (106,237.55 cm − 1, J = 2, g J = 1.305). During typical work with the hollow cathode lamp, we did not observe any special influence of the magnetic field on the parameters of the discharge. The influence of the magnetic field was noticeable
only after dismounting the lamp as an inhomogeneous erosion of the surfaces of the cathode, what is a consequence of deflection of free electrons and ions in a certain direction caused by the magnetic field. In order to analyze the experimental data we have used a software, developed in our group, which was used extensively with success in the analysis of the Zeeman-hyperfine structure of other elements, i.e. obtained in the classical electrodeless discharge [13–17] or in the Doppler free laser spectroscopy of fast ion beams [18,19]. The leastsquare-fitting procedure varies the fitted parameters: the Landé factors,
Fig. 4. (Color online) Recorded Zeeman-hf structure patterns of the La I line 670.316 nm at 835 G for π and σ polarizations. The thin line represents the experimental result and the thick line the computer best fit. The blue dashed line in the lower trace presents the field-free hf pattern of this line. Spectral line width is 600 MHz FWHM.
Fig. 5. (Color online) Recorded Zeeman-hf structure patterns of the La I line 654.5409 nm at 800 G for π and σ polarizations. The thin line represents the experimental result and the thick line the computer best fit. The blue dashed line in the lower trace presents the fieldfree hf pattern of this line. Spectral line width is 800 MHz FWHM.
S. Werbowy et al. / Spectrochimica Acta Part B 116 (2016) 16–20
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Table 2 Landé-g J factors of investigated fine structure levels of La I. Level (cm −1)
Wavelengths of investigated transitions (nm)
Present data
NIST (21)
39,764.36 39,276.99 39,079.07 36,543.48 36,298.20 36,265.30 36,109.70 36,034.61 24,841.41 24,248.99 23,704.82 21,447.85 21,383.99 9,919.826 9,044.212
669.924 641.993 674.119 662.260 670.316 671.798 681.853 685.364 669.985 674.119 641.993, 681.912 662.260, 681.853, 685.364 670.316, 671.798 669.985 681.912
1.060 (3) 1.067 (15) 1.015 (8) 1.415 (50) 1.059 (14) 1.261 (6) 1.307 (20) 1.106 (20) 1.101 (6) 0.948 (14) 1.112 (5) 1.102 (3) 1.239 (11) 1.092 (7) 0.690 (10)
n n n n n 1.22 1.28 1.10 1.15 0.96 1.133 1.103 1.278 1.107 0.69
n—value for a new level.
the line shape, background correction and scaling parameters to reproduce the calculated structure to match the experimental profile. For details, see ref. [20]. 3. Results and discussion Zeeman structure patterns for 10 lines of La I and 6 lines of Pr I in the region 641.993 nm–685.364 nm were recorded for excitation with σ (ΔM = ± 1) and π (ΔM = 0) polarized light. Table 1 compiles the investigated lines of La I and Pr I, giving the energy of the levels, quantum number J, parity, and detection method used (either OG spectroscopy or LIF, in the latter case we give also the fluorescence wavelength). Examples of such records together with calculated best fits for the La I line at 670.316 nm (36,298.2 cm − 1 → 21,383.99 cm −1 ) are presented in Fig. 4 and for the Pr I line 654.5409 nm (25,191.85 cm −1 → 9918.19 cm − 1) in Fig. 5. Additionally, in lower boxes of the Figs. 4 and 5 we have presented calculated hf (field free) patterns with assumed similar line widths (blue dashed lines). A summary of the obtained Landé-g J factors for the investigated La I and Pr I levels is presented in Tables 2 and 3, respectively. The tables contains the energy of the level, excitation wavelengths involving the given level, and experimental values of g J factors obtained in this work. For comparison, other known values from the literature are given. Comparing our g J factors with literature values, we find that some of the Landé factors determined earlier are different from our recent results. Especially noticeable is the discrepancy for the level 9918.19 cm −1 of Pr I. This level has very low energy, and the value g J = 0.62 [21] was probably determined from several transitions which
might have been wrongly classified. The present data were obtained with help of a fitting procedure in which both, upper and lower g J factors, were varied. To verify if the discrepancy is somehow related with the fitting procedure, we have assumed a fixed value of 0.62 for the 9918.19 cm − 1 level, and fitted only the g J value for the upper unknown level. The results of the best fits are presented in Fig. 6 as the blue dashed lines. One can see that the calculated patterns differ not only by the relative intensities of the Zeeman components but also by most of their positions. This shows clearly that the earlier value for the 9918.19 cm −1 level of Pr I is incorrect. We have no information on which transitions the value of 0.62 for the 9918.19 cm −1 level was determined in the past. Most of the transitions in which this level is involved are weak or in their neighborhood are strong other lines, so the Zeeman components may have overlapped. In the present investigation, we could isolate the components of only one excited line by LIF detection allowing an accurate Zeeman analysis of this level. We have demonstrated the determination of g J factors applying Doppler-limited laser spectroscopy at moderate magnetic field strengths. If the hyperfine constants A and B of the involved levels are known, our programs allow a reliable determination of g J. The determination of Lande-factors for other levels of lanthanide atoms can be easily performed using our method. We hope that this work will encourage other researchers to collect more of these valuable atomic data. Acknowledgments The present work was supported by Wissenschaftlich-Technisches Abkommen Österreich-Polen, projects no. 21/2012 and 12/2014.
Table 3 Landé-g J factors of investigated fine structure levels of Pr I. Level (cm −1)
Transitions (nm)
Present data
NIST (21)
30,600.76 30,283.14 25,191.85 19,654.71 18,273.22 18,102.90 15,347.43 9,918.190 4,381.072 2,846.741
655.4134 669.351 654.5409 654.5421 649.5728 655.289 655.4134, 669.351 654.5409 654.5421, 681.912 649.5728, 655.289
1.147 (8) 1.171 (21) 0.954 (12) 1.166 (4) 1.000 (18) 1.095 (5) 1.130 (5) 0.964 (3) f f
n n n n n n n 0.62 1.19799 1.10632
n—value for a new level, f—the Landé factor was fixed to the NIST value for transitions in which this level is involved.
20
S. Werbowy et al. / Spectrochimica Acta Part B 116 (2016) 16–20
Fig. 6. (Color online) Recorded Zeeman-hf structure patterns of the Pr I line 654.5409 nm at 800 G for π and σ polarizations. The thick blue dashed line is the computer best fit with fixed g J-Landé factor value 0.62 [21] for the lower 9918.19 cm −1 level. One can immediately see that with g J = 0.62 the experimental pattern can not be reproduced.
References [1] A. King, E. Carter, The electric-furnace spectra of yttrium, zirconium, and lanthanum, Astrophys. J. 65 (1927) 86–107. [2] W.F. Meggers, Wave lengths and zeeman effects in lanthanum spectra, Bureau of Standards Journal of Research, Research Paper 468 (RP468) 9 (1932) 239–268.
[3] H.N. Russell, W.F. Meggers, An analysis of Lanthanum spectra (LaI, La II, La III), Bureau of Standards Journal of Research, Research Paper 497 (RP497) 9 (1932) 625–668. [4] R. Zalubas, B.R. Borchardt, Energy levels of neutral praseodymium (Pr I), J. Opt. Soc. Am. 63 (1973) 102–103. [5] M.S. Safronova, U.I. Safronova, C.W. Clark, Correlation effects in La, Ce, and lanthanide ions, Phys. Rev. A: At. Mol. Opt. Phys. 91 (2015) 022504. [6] B. Karaçoban, L. Özdemir, Electric dipole transitions for La I (Z = 57), J. Quant. Spectrosc. Radiat. Transf. 109 (2008) 1968–1985. [7] F. Güzelçimen, I. Siddiqui, G. Başar, S. Krüger, L. Windholz, New energy levels and hyperfine structure measurements of neutral lanthanum by laser-induced fluorescence spectroscopy, J. Phys. B Atomic Mol. Phys. 45 (2012) 135005. [8] Z. Uddin, D. El Bakkali, B. Gamper, S. Khan, I. Siddiqui, G.H. Guthöhrlein, L. Windholz, Laser spectroscopic investigations of praseodymium I transitions: new energy levels, Adv. Opt. Technol. (2012) 639126 v2012. [9] B. Gamper, Z. Uddin, M. Jahangir, O. Allard, H. Knöckel, E. Tiemann, L. Windholz, Investigation of the hyperfine structure of Pr I and Pr II lines based on highly resolved Fourier transform spectra, J. Phys. B Atomic Mol. Phys. 44 (2011) 045003 (with supplementary data). [10] F. Güzelçimen, Gö. Başar, M. Tamanis, A. Kruzins, R. Ferber, L. Windholz, S. Kröger, High-resolution Fourier Transform Spectroscopy of lanthanum in Ar discharge in the near-infrared, Astrophys. J. Suppl. Ser. 208 (2013) 18. [11] K. Shamim, I. Siddiqui, L. Windholz, Experimental investigation of the hyperfine spectra of Pr I – lines: discovery of new fine structure levels with low angular momentum, Eur. Phys. J. D 64 (2011) 209–220. [12] L. Minnhagen, Spectrum and the energy levels of neutral argon, Ar I, J. Opt. Soc. Am. 63 (1973) 1185–1198. [13] S. Werbowy, J. Kwela, The E2 admixtures in mixed multipole lines 459.7 nm and 461.5 nm in the spectrum of Bi I, Eur. Phys. J.-Spec. Top. 144 (2007) 179–184. [14] S. Werbowy, J. Kwela, M1–E2 interference in the Zeeman spectra of BiI, Phys. Rev. A 77 (2008) 023410. [15] Ł.M. Sobolewski, S. Werbowy, J. Kwela, Hyperfine and Zeeman structure of lines of Bi I, J. Opt. Soc. Am. B 31 (2014) 3038–3041. [16] S. Werbowy, J. Kwela, M1–E2 interference in the Zeeman spectra of Pb I and Pb II, J. Phys. B 42 (2009) 065002. [17] S. Werbowy, J. Kwela, Observation of the M1–E2 interference in the Zeeman spectra of isotopes 121Sb and 123Sb, J. Phys. B 43 (2010) 065002. [18] S. Werbowy, J. Kwela, N. Anjum, H. Hühnermann, L. Windholz, Zeeman effect of hyperfine-resolved spectral lines of singly ionized praseodymium using collinear laser–ion-beam spectroscopy, Phys. Rev. A 90 (2014) 032515. [19] S. Werbowy, H. Hühnermann, J. Kwela, L. Windholz, Investigations of the Zeeman effect of some 142Nd ionic levels, using collinear laser ion beam spectroscopy, J. Quant. Spectrosc. Radiat. Transf. 166 (2015) 102–107. [20] S. Werbowy, J. Kwela, R. Drozdowski, J. Heldt, M1–E2 interference in the Zeeman effect of the 461.5 nm line of Bi I, Eur. Phys. J. D 39 (2006) 5–12. [21] W.C. Martin, R. Zalubas, L. Hagan, Atomic Energy Levels—The Rare Earth Elements, Nat. Stand. Ref. Data Ser., NSRDS-NBS, 60, Nat. Bur. Stand, U.S., (1978) (422 pp.).
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Experimental investigations of the Zeeman effect of new fine structure levels of Lanthanum and Praseodymium S. Werbowy a,c,⁎, C. Güney b,c, L. Windholz c,⁎⁎ a b c
Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57 PL-80-952 Gdansk, Poland Physics Department, Faculty of Science, Istanbul University, Vezneciler, Tr-34134 Istanbul, Turkey Institut für Experimentalphysik, Technische Universität Graz, Petersgasse 16, 8010 Graz, Austria
a r t i c l e
i n f o
Article history: Received 17 August 2015 Accepted 16 November 2015 Available online 2 December 2015 Keywords: Zeeman effect Lande factors Laser-induced fluorescence
a b s t r a c t Landé-g J data are given for several lines of La I and Pr I in the wavelength range 641.993 nm to 685.364 nm. Spectra were recorded in the presence of magnetic fields of about 800 G for two polarizations of the exciting laser light, perpendicular and parallel with respect to the direction of the magnetic field. We have used a hollow cathode sputtering atom source and laser induced fluorescence and/or optogalvanic detection. In this way we have determined for the first time the Landé-g J factors for some new levels of neutral La and Pr atoms. Additionally, the Landé-g J factors for other known levels of La I and Pr I were reinvestigated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lanthanum (Z = 57) and Praseodymium (Z = 59) are rare earth metals, belonging to the lanthanide group of elements in the periodic table. Because of their electronic properties, both elements have very rich and complex spectra. Lanthanum and Praseodymium in the natural abundance have each only one stable isotope, 139La with nuclear spin quantum number I = 7/2 and 141Pr with I = 5/2, which together with magnetic dipole and small electric quadrupole moments of the respective nucleus causes the hyperfine splitting (hf) of the levels. One of the first studies of the Lanthanum spectrum was performed by King and Carter [1] in the 279.2–830.5 nm range and extended later by Meggers [2]. More than 1500 lines in the wavelength range between 214.281 and 1095.46 nm in the arc and spark spectra were reported, from which about 700 lines were assigned to the neutral La atom. Due to the large density of the lines, a Zeeman effect analysis was made only for the 476 strongest and well resolved lines (279.1– 748.3 nm) [2]. The relative positions of the main Zeeman components at a magnetic field of 33,000 G were given. A more extensive Zeeman analysis was performed later and Landé-g J factors for almost 100 La I levels were determined [3]. Extensive studies of Pr I and Pr II emission spectra in the 350– 1200 nm range and of absorption spectra in the 200–870 nm range were performed by Zalubas and Borchardt [4]. They attributed about
⁎ Corresponding authors at: Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, PL-80-952 Gdansk, Poland. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (S. Werbowy), [email protected] (L. Windholz).
http://dx.doi.org/10.1016/j.sab.2015.11.003 0584-8547/© 2015 Elsevier B.V. All rights reserved.
25,000 lines to neutral atoms of praseodymium. From this observations, 62 high even levels and the 3 lowest levels of the ground configuration were identified. Also the Zeeman effect for some prominent lines of Pr I was reported and Landé-g J factors for 30 of these levels were given. At that time, practically no knowledge on the hyperfine structure of the lines was available, and hyperfine structure splitting was neglected in the analysis of the Zeeman patterns. Since that time, the numbers of known energy levels of La I has reached about 800 of even and 300 of odd parity. For Pr I atoms there are now around 1200 even and 1700 odd levels known. For all these levels the hyperfine constants are also known. The lanthanides with their open nf shells are very difficult elements from the theoretical point of view. Because of the increasing interest of using the lanthanides to a variety of applications, they have been subject of many theoretical studies, i.e. [5,6]. There are several groups that are extensively investigating the electronic structure as well as hyperfine interactions of La I and Pr I spectra.1 In the course of their studies, many new transitions and energy levels were discovered, spectral lines were classified and spectroscopic parameters like energy of the involved levels, their total angular momenta J, their parity and their hyperfine constants A and B were determined. However, despite the ongoing studies of the hyperfine structure of La I and Pr I, we found a big gap concerning the Zeeman effect analysis. The magnetic field has implications on the variety of applications, i.e. by combination of the magnetic field and atomic absorption 1 The group at the Graz University of Technology, Austria; the group of J. Dembczyński at the Poznan University of Technology, Poland; the group of G. Basar at Istanbul University, Turkey; the group of S. Kröger, Hochschule für Technik und Wirtschaft Berlin, Germany.
S. Werbowy et al. / Spectrochimica Acta Part B 116 (2016) 16–20
spectrometry (AAS). The Zeeman technique can be used to achieve accurate background correction of atomic absorbance signals improving the accuracy of the analysis of the elements. Since the group in Graz has large experience related to the spectra of La and Pr, we made an attempt to carry on investigations of the Zeeman effect of lines of these elements using proved successful hollow-cathode sources and laser excitation. We hope that by this we will encourage the scientific community to return to Zeeman effect investigations. The purpose of this study is to investigate the Zeeman effect of some energy levels recently discovered by analysis of the spectra of neutral lanthanum [7] and praseodymium [8]. We use Doppler-limited laser spectroscopy to record the hyperfine patterns of the excited lines under influence of a relatively weak magnetic field. For the analysis of the hyperfine-Zeeman patterns we use most accurate hyperfine structure constants for the investigated levels in order to obtain accurate Landé-g J factors. 2. Experiment For studies of neutral atoms in the laboratory, different experimental techniques were in use, e.g. classical emission spectroscopy of electric arcs between electrodes of graphite, silver, or copper, with a small portion of praseodymium or lanthanum salt being placed on the lower electrode before ignition of the arc [2,3]. A great improvement compared to this method was achieved, when a praseodymium or lanthanum plasma is generated in a hollow cathode lamp cooled with liquid nitrogen (in order to reduce Doppler line broadening). Such lamp can be used as light emission source, and the spectrum is recorded by high-resolution Fourier transform spectroscopy [9,10]. The atoms in the hollow cathode source are produced by a sputtering process from the cathode surface made from the investigated element. The plasma ignition takes place under the presence of argon (ca. 0.5 mbar) but after some minutes the discharge is carried mainly by metal atoms. A detailed description of the used method was given recently in [11]. With this method, the Doppler widths of the recorded hf patterns can be reduced some hundreds of MHz. The line width of a single hf component is caused mainly by the Doppler broadening. The plasma in the hollow cathode has a certain temperature, which is caused by heating of the discharge gas (including the sputtered La or Pr atoms, which carry most of the discharge) due to the electric current, and by cooling of the plasma by the cathode wall. The holder of the cathode is cooled by liquid nitrogen. This cooling of the cathode lowers the observed plasma temperature, but is also essential for a good sputtering efficiency and for operating the discharge with as less current noise as possible.
Fig. 1. Schematic drawing of the hollow cathode lamp. Left: side view, right: front view. K—cathode, made from copper, with an inner layer of La or Pr; A—anode in ceramic holder (holder not shown); H—tube holding cathode and anodes; C—container for liquid nitrogen; M—neodymium magnet.
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Fig. 2. Magnetic field distribution measured along the magnet bar at distance 2 cm from the magnet.
Such lamp can be used also to perform laser spectroscopic experiments. A laser light beam is passing through the hollow cathode and—if the light frequency is in resonance with an atomic transition—excites atoms from the lower state of the transition to the upper one. The interaction can be detected either by laser-induced fluorescence (LIF) or a socalled optogalvanic (OG) signal (the plasma changes its resistance due to the change of the detailed equilibrium of the plasma). The laser light frequency is scanned over the investigated transition, and a well resolved spectrum of the investigated transition is recorded. If LIF- or OG-detection is better depends on many factors. For Pr, usually LIF is used, while for a special group of La levels OG detection is more sensitive. The disadvantage of the OG method is that it shows the sum of all interactions of the laser light with the plasma. Thus, if the lines are separated less than their total hyperfine splitting, socalled blend situations are observed and the recorded signal shows overlapping patterns of different transitions. In LIF, selecting for detection only one fluorescence channel, also completely overlapping transitions can be separated. To investigate the Zeeman effect we made a slight modification of the experimental apparatus described in [11] by introducing a uniform magnetic field to the laser excitation region, Fig. 1. This was accomplished by placing a strong bar shaped permanent neodymium magnet, placed 2 cm above the cathode. Since the hollow cathode lamp is placed inside a container with liquid nitrogen for cooling reasons, the magnet was immersed directly in the cooling liquid. The very low temperature of the magnet did not have any effect on the properties of the magnetic field. In this way we have generated magnetic fields of about 800 Gauss in the central region of the cathode. Fig. 2 shows magnetic field distribution measured along the magnet at distance 2 cm from the magnet
Fig. 3. (Color online) Recorded Zeeman pattern of the 669.887 nm Argon line used for the magnetic field measurement. Spectral line width is 1600 MHz FWHM.
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Table 1 La I and Pr I lines investigated by laser spectroscopy. P: Parity (e even, o odd); OG: optogalvanic spectroscopy; LIF: laser-induced fluorescence spectroscopy. Upper level Energy, J, P (cm −1)
Lower level Energy, J, P (cm −1)
Method (Fluorescence wavelength (nm))
La I 641.993 662.261 669.923 669.985 670.316 671.798 674.119 681.853 681.912 685.364
39,276.99, 5/2, e 36,543.48, 5/2, e 39,764.36, 13/2, e 24,841.41, 11/2, o 36,298.20, 9/2, e 36,265.30, 9/2, e 39,079.07, 11/2, e 36,109.7, 5/2, e 23,704.816, 3/2, o 36,034.61, 9/2, e
23,704.816, 3/2, o 21,447.854, 7/2, o 24,841.41, 11/2, o 9,919.826, 9/2, e 21,383.994, 9/2, o 21,383.994, 9/2, o 24,248.994, 9/2, o 21,447.854, 7/2, o 9,044.212, 1/2, e 21,447.854, 7/2, o
OG, LIF (468.6) OG OG, LIF (719.6) OG OG, LIF (713.4) OG, LIF (499.3) OG, LIF (595.3) OG OG, LIF(616.5) OG
Pr I 649.5728 654.5409 654.5421 655.289 655.4134 669.351
18,273.22, 11/2, e 25,191.854, 5/2, o 19,654.709, 13/2, e 18,102.9, 11/2, e 30,600.763, 13/2, e 30,283.139, 15/2, e
2,846.741, 13/2, o 9,918.19, 7/2, e 4,381.072, 15/2, o 2,846.741, 13/2, o 15,347.431, 13/2, o 15,347.431, 13/2, o
OG, LIF (548.2) LIF (533.4) OG LIF (597.4) OG, LIF (563.9) OG, LIF (539.8)
Wavelength (nm)
surface. The magnetic field is calibrated to 0.9% accuracy from the Zeeman effect of the Ar I line at 669.887 nm. A σ(ΔM = ± 1) Zeeman spectrum of the Ar I line is presented in Fig. 3. Spectroscopic data for this transition are [12]: 3s 23p 5( 2P 1/2)6s (E = 121,161.31 cm −1, J = 1, g J = 1.271)→ 3s 23p 5( 2P 3/2)4p (106,237.55 cm − 1, J = 2, g J = 1.305). During typical work with the hollow cathode lamp, we did not observe any special influence of the magnetic field on the parameters of the discharge. The influence of the magnetic field was noticeable
only after dismounting the lamp as an inhomogeneous erosion of the surfaces of the cathode, what is a consequence of deflection of free electrons and ions in a certain direction caused by the magnetic field. In order to analyze the experimental data we have used a software, developed in our group, which was used extensively with success in the analysis of the Zeeman-hyperfine structure of other elements, i.e. obtained in the classical electrodeless discharge [13–17] or in the Doppler free laser spectroscopy of fast ion beams [18,19]. The leastsquare-fitting procedure varies the fitted parameters: the Landé factors,
Fig. 4. (Color online) Recorded Zeeman-hf structure patterns of the La I line 670.316 nm at 835 G for π and σ polarizations. The thin line represents the experimental result and the thick line the computer best fit. The blue dashed line in the lower trace presents the field-free hf pattern of this line. Spectral line width is 600 MHz FWHM.
Fig. 5. (Color online) Recorded Zeeman-hf structure patterns of the La I line 654.5409 nm at 800 G for π and σ polarizations. The thin line represents the experimental result and the thick line the computer best fit. The blue dashed line in the lower trace presents the fieldfree hf pattern of this line. Spectral line width is 800 MHz FWHM.
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Table 2 Landé-g J factors of investigated fine structure levels of La I. Level (cm −1)
Wavelengths of investigated transitions (nm)
Present data
NIST (21)
39,764.36 39,276.99 39,079.07 36,543.48 36,298.20 36,265.30 36,109.70 36,034.61 24,841.41 24,248.99 23,704.82 21,447.85 21,383.99 9,919.826 9,044.212
669.924 641.993 674.119 662.260 670.316 671.798 681.853 685.364 669.985 674.119 641.993, 681.912 662.260, 681.853, 685.364 670.316, 671.798 669.985 681.912
1.060 (3) 1.067 (15) 1.015 (8) 1.415 (50) 1.059 (14) 1.261 (6) 1.307 (20) 1.106 (20) 1.101 (6) 0.948 (14) 1.112 (5) 1.102 (3) 1.239 (11) 1.092 (7) 0.690 (10)
n n n n n 1.22 1.28 1.10 1.15 0.96 1.133 1.103 1.278 1.107 0.69
n—value for a new level.
the line shape, background correction and scaling parameters to reproduce the calculated structure to match the experimental profile. For details, see ref. [20]. 3. Results and discussion Zeeman structure patterns for 10 lines of La I and 6 lines of Pr I in the region 641.993 nm–685.364 nm were recorded for excitation with σ (ΔM = ± 1) and π (ΔM = 0) polarized light. Table 1 compiles the investigated lines of La I and Pr I, giving the energy of the levels, quantum number J, parity, and detection method used (either OG spectroscopy or LIF, in the latter case we give also the fluorescence wavelength). Examples of such records together with calculated best fits for the La I line at 670.316 nm (36,298.2 cm − 1 → 21,383.99 cm −1 ) are presented in Fig. 4 and for the Pr I line 654.5409 nm (25,191.85 cm −1 → 9918.19 cm − 1) in Fig. 5. Additionally, in lower boxes of the Figs. 4 and 5 we have presented calculated hf (field free) patterns with assumed similar line widths (blue dashed lines). A summary of the obtained Landé-g J factors for the investigated La I and Pr I levels is presented in Tables 2 and 3, respectively. The tables contains the energy of the level, excitation wavelengths involving the given level, and experimental values of g J factors obtained in this work. For comparison, other known values from the literature are given. Comparing our g J factors with literature values, we find that some of the Landé factors determined earlier are different from our recent results. Especially noticeable is the discrepancy for the level 9918.19 cm −1 of Pr I. This level has very low energy, and the value g J = 0.62 [21] was probably determined from several transitions which
might have been wrongly classified. The present data were obtained with help of a fitting procedure in which both, upper and lower g J factors, were varied. To verify if the discrepancy is somehow related with the fitting procedure, we have assumed a fixed value of 0.62 for the 9918.19 cm − 1 level, and fitted only the g J value for the upper unknown level. The results of the best fits are presented in Fig. 6 as the blue dashed lines. One can see that the calculated patterns differ not only by the relative intensities of the Zeeman components but also by most of their positions. This shows clearly that the earlier value for the 9918.19 cm −1 level of Pr I is incorrect. We have no information on which transitions the value of 0.62 for the 9918.19 cm −1 level was determined in the past. Most of the transitions in which this level is involved are weak or in their neighborhood are strong other lines, so the Zeeman components may have overlapped. In the present investigation, we could isolate the components of only one excited line by LIF detection allowing an accurate Zeeman analysis of this level. We have demonstrated the determination of g J factors applying Doppler-limited laser spectroscopy at moderate magnetic field strengths. If the hyperfine constants A and B of the involved levels are known, our programs allow a reliable determination of g J. The determination of Lande-factors for other levels of lanthanide atoms can be easily performed using our method. We hope that this work will encourage other researchers to collect more of these valuable atomic data. Acknowledgments The present work was supported by Wissenschaftlich-Technisches Abkommen Österreich-Polen, projects no. 21/2012 and 12/2014.
Table 3 Landé-g J factors of investigated fine structure levels of Pr I. Level (cm −1)
Transitions (nm)
Present data
NIST (21)
30,600.76 30,283.14 25,191.85 19,654.71 18,273.22 18,102.90 15,347.43 9,918.190 4,381.072 2,846.741
655.4134 669.351 654.5409 654.5421 649.5728 655.289 655.4134, 669.351 654.5409 654.5421, 681.912 649.5728, 655.289
1.147 (8) 1.171 (21) 0.954 (12) 1.166 (4) 1.000 (18) 1.095 (5) 1.130 (5) 0.964 (3) f f
n n n n n n n 0.62 1.19799 1.10632
n—value for a new level, f—the Landé factor was fixed to the NIST value for transitions in which this level is involved.
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Fig. 6. (Color online) Recorded Zeeman-hf structure patterns of the Pr I line 654.5409 nm at 800 G for π and σ polarizations. The thick blue dashed line is the computer best fit with fixed g J-Landé factor value 0.62 [21] for the lower 9918.19 cm −1 level. One can immediately see that with g J = 0.62 the experimental pattern can not be reproduced.
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