Neodymium two-step optogalvanic spectroscopy in a hollow cathode lamp

Spectrochimica Acta Part B 66 (2011) 748–753

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Research note

Neodymium two-step optogalvanic spectroscopy in a hollow cathode lamp Alessandro R. Victor a, Marcelo G. Destro b,⁎, Maria Esther Sbampato b, Jose W. Neri b, Carlos A.B. Silveira b, Antonio Carlos Oliveira b, Nicolau A.S. Rodrigues b a b

Technological Institute of Aeronautics, ITA 12228-900, Sao Jose dos Campos, SP, Brazil Institute for Advanced Studies, IEAv 12228-970, Sao Jose dos Campos, SP, Brazil

a r t i c l e

i n f o

Article history: Received 12 April 2011 Accepted 25 September 2011 Available online 1 October 2011 Keywords: Optogalvanic spectroscopy Laser multistep excitation Rare earth Neodymium

a b s t r a c t This paper presents the results of Doppler-limited optogalvanic spectroscopy in commercial neodymium hollow cathode lamp, ranging from 580 to 600 nm. Using the laser multistep excitation technique, five transitions for the first step excitation from the neodymium ground state and seven transitions related to the second step of photoionization scheme were observed. Within these results, for the first time, a new line, 596.645 nm, was observed which could be attributed to a possible two-step transition to neodymium energy level from the 16 979.352 cm− 1 to 33 735.4 cm − 1. The simulated (synthetic) spectra of a mixture of neon (Ne I) and neodymium (Nd I) in this region are compared with experiments in order to facilitate data analysis. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The interest in selective photoionization of rare earth isotopes has increased since the past decade due to their many industrial, medical [1], and national security applications together with their use in new energy technologies [2]. The Institute for Advanced Studies [IEAv] has been studying the enhancement in the performance of electrooptic materials, changing their compositions, which implies the use of natural rare earth elements, to a particular grade isotope. The IEAv has also special interest in neodymium, Nd, once it can be used in powerful permanent magnet, in laser medium and in nuclear auxiliary battery for satellites [3–5]. Laser isotope separation of Nd requires knowing its energy levels and absorption frequencies. The Nd atom has an ionization potential of 5.525 eV [6] suitable for a three-step photoionization by laser radiation tuned in visible spectrum. Although there is information on Nd available in the literature [7–9], many details are lacking. Then, to understand and find the best way for photoionizing using a particular laser system it is necessary to carry through many experimental researches. A number of spectroscopic results have been obtained in the IEAv labs through optogalvanic spectroscopy [OGS] by laser multistep spectroscopy technique. The resonant absorption of radiation by atoms or molecules present in a self-sustained discharge changes its electrical properties. Such change is observed as an increase or a decrease in the discharge conductivity and is known as optogalvanic

⁎ Corresponding author. E-mail addresses: [email protected] (A.R. Victor), [email protected] (M.G. Destro). 0584-8547/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2011.09.005

effect [10]. This effect has shown to be a powerful and inexpensive technique for investigation of atomic and molecular species. It is particularly useful in the spectroscopy of refractory elements because associating an efficient vapor source of those elements from the cathode by sputtering with an intrinsically more sensitive technique to obtain atomic or molecular spectra than absorption spectroscopy. The last records a small signal variation superimposed on a large background signal while for the optogalvanic spectroscopy there is no background signal at all [10]. The drawback is that its spectra are more complex than absorption spectroscopy due to mixing of discharge buffer gas and cathode vapor elements spectra. A complete description of the basic principles of Atomic Vapor Laser Isotope Separation (AVLIS) performed at IEAv is described in the literature [11–15]. The present work focuses the Nd photoionization scheme, using the Doppler-limited optogalvanic spectroscopy by laser multistep excitation technique [16] [LME-OGS]. The experiments were carried through with a commercial hollow cathode lamp with direct current stabilized power supply, which is a simple and reliable tool to provide metallic vapor for spectroscopic purposes, in order to identify the first two sequential absorption frequencies, from the Nd ground state in the spectral range between 580 and 600 nm. From the NIST database [7] the possible Nd transitions were calculated and simulated aiming at comparing the synthetic and the experimental spectra obtained, in order to facilitate the investigation and data analysis. Using a dye laser pumped by a Nd:YAG, five transitions from the ground state were observed through single-step OGS, as well as another seven transitions related to the second photon absorption, using a dye laser pumped by a copper vapor laser. As to experimental results, eleven levels coincident with existing data and a new line was observed, which was attributed to the transition from the 16 979.352 cm− 1 to 33 735.4 cm− 1 Nd energy levels.

A.R. Victor et al. / Spectrochimica Acta Part B 66 (2011) 748–753

2. Setups Fig. 1 shows the experimental setup used to investigate Nd absorption frequencies, using LME-OGS. A commercial hollow cathode lamp (HCL) with neon (Ne) as buffer gas and Nd as cathode was used. A 10 mA/200 V electrical discharge was supplied by a stabilized voltage source through a 1.8 kΩ ballast resistor. The optogalvanic signal was coupled to a Tektronix Oscilloscope model R7603 (OSC) and to a lock-in Stanford Research Systems model SR530 (LOC) through a 1.2 nF capacitor. The optogalvanic signal was recorded by an acquisition system developed at IEAv using a 10 bit analogical–digital (A/D) converter connected to a computer (REC). In the single-step OGS, the laser used was a Radiant Dye Laser model NarrowScan DR (DL) pumped by an Edgewave model IS8II-E pulsed laser (PL). This laser system delivers 9 W average power at 5 kHz repetition rate with 7 ns pulsewidth and 2.4 GHz (~0.003 nm @590 nm). This laser has an internal device consisting of an HCL of neon, a stabilized voltage source to supply the HCL and a boxcar averager allows the wavelength calibration by means of optogalvanic spectroscopy yielding ±0.002 nm precision in wavelength measurements. Furthermore, this laser tunes its wavelengths using a step motor controlled by a computer that yield both wavenumber and vacuum wavelength within the precision of the calibration realized. For two-step OGS, two Molectron Corporation model DL II Series dye lasers were pumped by two synchronized copper vapor lasers. This laser system delivers 35 mW average power at 5 kHz repetition rate with 35 ns pulsewidth and 700 MHz linewidth (~0.001 nm @595 nm). The wavelength scan can be continuously tuned by varying the SF6 pressure in the scanning chamber from 0 to 760 Torr yielding a 0.462 nm spectral range at 590 nm. As the dye laser has a manual gas control, it means that a variation up to ±5 Torr at read display introduce up to ±0.003 nm in wavelength determination. This laser had your wavelength calibrated using a Jobin Yvon model Triax 550 monochromator, 2400 l/mm grating and coupled to an

LSB

HCL

BS

L

M

DL2

PL

DL1

PL

LOC C

R PS

TR OSC

REC

Fig. 1. LME-OGS setup. BS: beam splitter; C: capacitor; DL (1; 2): dye laser (1; 2); HCL: neodymium hollow cathode lamp Ne buffered; L: lens; LOC: lock-in amplifier; LSB: laser safety barrier; M: mirror; PL: pumping laser; PS: power supply; OSC: oscilloscope; R: resistor; REC: recorder; TR: trigger.

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ICCD Dicam Pro with 0.025 nm resolution at 585 nm, was used for a rough wavelength calibration and for a thin adjustment Ne spectral lines were used as a reference line to calibrate the spectra. Different kinds of laser systems were used because they must be triggered into a delay time synchronization to observe the two-step optogalvanic signal. In both stages a sulforhodamine B dye was used, which allows covering the 580–600 nm wavelength range. Before carrying through the experiments, synthetic spectra were simulated by a program developed in Matchcad 2001i Professional, using only Nd wavelengths and relative intensities and Ne neutral atoms (Nd I and Ne I) taken from NIST database [7]. The simulation takes into account the instrumental broadening due to the laser and the HCL temperature used in the experiments. The temperature assumed in HCL was approximately 1000 K. The Voigt profile obtained by convolution of Lorentz and Gauss profiles for lineshape is closer to the experimental spectrum obtained in the lab. 3. Results and discussions The results are presented and discussed in two sections: the first one presents single-step OGS experiments and the second one shows the results of two-step OGS. All of the wavelengths are given in the air and Nd electronic configurations are xenon (Xe) like and were taken from NIST. Once intensities depend on emission source, the spectral line intensities shown are reported using completely arbitrary units, even for synthetic spectra. 3.1. Single-step optogalvanic spectroscopy To identify the transitions corresponding to the first step of the multistep photoionization a scan was made varying the wavelength of dye laser in a region previously set to investigate. The Nd spectra obtained by single-step OGS are shown in Fig. 2, where five transitions from Nd ground state to an excited level were observed. The electronic configuration of Nd ground state is represented by [Xe] 4f 46s 2 5I4 [7]. The optogalvanic spectrum, due to absorption of laser radiation from ground state to the 17 162.930 cm − 1 excited state, with electronic configuration [Xe] 4f 46s6p 3H5° [7], is shown in Fig. 2(a). Note that there are two transitions at 582.019 nm and 582.494 nm; so, checking the synthetic spectra for the same region (Fig. 2(a) non continuous line), the peaks were easily distinguished between Ne and Nd transitions, respectively. Ne transition was used as a reference line to calibrate the spectrum wavelength. Similarly to Fig. 2(a), two optogalvanic spectrum for Nd transitions were observed at 586.967 nm and at 593.493 nm. The former populates the 17 032.146 cm − 1 excited state with electronic configuration [Xe] 4f 3( 4I°)5d 2(3P) ( 6K°)6s 7K4°, and the last populates the 16 844.843 cm − 1, represented by [Xe] 4f 3( 4I°)5d 2(3P) ( 6K°)6s 7K5°. Fig. 2(b) shows the two behaviors of optogalvanic effect, the positive and negative signals, caused by the transition from metastable and non-metastable states, respectively. The mechanisms of these different effects are explained due to the variation of electron-ion pair production and electron and gas temperatures produced by superelastic collisions with excited atoms [10]. Nd transition occurs when laser was tuned in the 588.792 nm wavelength yielding the 0 → 16 979.352 cm − 1 transition ([Xe] 4f 3( 4I°)5d 2(3P)( 6K°)6s 7K3°). Note that the synthetic and experimental spectra are different with respect to Ne transition. The experimental spectrum (continuous line) gives negative signal, while the synthetic spectrum gives positive signal. The program developed to simulate the synthetic spectra uses only the emission wavelengths of elements in HCL and their relative intensities. Any special function to distinguish metastable from non-metastable transitions was not included, so the synthetic spectra do not provide the real behavior of the optogalvanic signal. Fig. 2(c) shows how spectra simulations were quite useful tools for preliminary examination stage and experimental result analysis.

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Nd 596.603 nm and Ne 596.621 nm transitions are too close, the last one complicating the observation of Nd optogalvanic signal. This transition appeared convoluted with Ne transition and operating the HCL in nominal current 10 mA it was impossible to identify Nd transition. Even so, according to the synthetic spectra, both lines are detectable considering the laser resolution used in the experiments. Besides, there are some works in the literature [7, 8] relating the use of this line for Nd isotopic separation. Thus, changing to the maximum operating current of used HCL, 15 mA (in order to increase Nd vapor density), allowed the distinction between Nd and Ne signals, which gives the transition 0 → 16 757.037 cm − 1 with the higher energy level represented by [Xe] 4f 3( 4I°)5d6s 2 7H5°. Table 1 summarizes all observed Nd transitions, as well as their assignments taken from NIST [7]. 3.2. Two-step optogalvanic spectroscopy The absorption measurements for two-step OGS were performed by tuning DL1 to an atomic transition from ground state observed and described in the previous section. Then, different kinds of routes, using the transition related to 586.967 nm ( 5I4 → 7K4°, J = 4) and 588.792 nm ( 5I4 → 7K3°, J = 3), populating the energy levels in 17 032.146 cm − 1 and 16 979.352 cm − 1, respectively, were studied. The procedure held to obtain the wavelengths corresponding to the second-step, that can be used to obtain the Nd multistep photoionization, is described as follows: (i) DL1 laser was tuned and locked in the first transition and using DL2 a scan was made varying its wavelength in a spectral region previously set to investigate transitions from an excited state; (ii) in case of any detection of the optogalvanic signal the DL1 was blocked, then DL2 scan was repeated; (iii) both spectra were compared. If the spectra were identical (unless a DC voltage level due to the DL1 signal), it meant that there were no transitions from the first excited level populated by DL1 in the region scanned by DL2. On the other hand, if the spectrum was different from the one obtained by (i), then the disappeared-peaks corresponded to the absorption from the first excited state, yielding the second step excitation. Despite detailed information on Nd atom energy levels presented in NIST for the majority of even ones over 23 000 cm − 1 and odd ones over 17 000 cm − 1 neither the configuration nor the terms are found, only parity and J values are known [17]. In literature is found rare information on energy levels above 30 000 cm − 1, and for the great majority the J values are unknown [15, 18]. Therefore, this section reports only the identification of transitions wavelengths yielding the energy of upper levels populated by DL2 and the possible J values must obey the selection rules (ΔJ = 0, ±1). In that stage of experiments, synthetic spectra were also used to help the investigation of transitions resonant with the excited states cited above, and only transitions from these excited states and not from ground state were taken into account for the Nd spectra simulation. Then, the DL1 was tuned in 588.792 nm resonant with 5I4 → 7K3°, in conformity with Zyuzikov et al. [8], and four transitions were observed. Fig. 3(a) presents two-step optogalvanic spectrum in the range of 593.850 to 594.100 nm, and shows the optogalvanic signal at 594.031 nm, which corresponds to absorption of the second resonant wavelength, i.e., the second step transition 16 979.352 to

Table 1 Nd transitions from ground state obtained by single-step OGS in the spectral range from 580 to 600 nm. Fig. 2. Nd single-step excitations observed by optogalvanic and synthetic spectra from ground state. (a) Transition 5I4 − 3H5°; 0 → 17 162.930 cm− 1 (582.494 nm); (b) transition 5I4 − 7K3°, 0 → 16 979.352 cm− 1 (588.792 nm); (c) transition 5I4 − 7H5°, 0 → 16 757.037 cm− 1 (596.603 nm).

λair [nm]

Ji–f

Ei–f [cm− 1]

Termsi–f

582.494 586.967 588.792 593.493 596.603

4–5 4–4 4–3 4–5 4–5

0–17 162.930 0–17 032.146 0–16 979.352 0–16 844.843 0–16 757.037

5

I4 − 3H5° I4 − 7K4° 5 I4 − 7K3° 5 I4 − 7K5° 5 I4 − 7H5° 5

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related to the 596.645 nm wavelength is not shown neither in synthetic spectra, obtained from NIST, nor in Ref. [8]. However, the scan was carried out several times and in all of them this transition was observed. Besides, when the DL1 was blocked, this transition disappeared proving to be a second step Nd transition. For the 580–600 nm range, using a tantalum oven at 1373–1573 K, for vaporize Nd atoms, 22 transitions from the same level (E1 = 16 979.352 cm − 1 @588.792 nm) were reported by Zyuzikov et al. [8]. Under our conditions, 4 transitions were observed due to low Nd vapor density produced by HCL used in the experiments. Three levels, at 33 832.6, 33 808.9 and 33 726.7 cm − 1, coincided with their data. To the present work author's acknowledgement and from available information that were consulted, the 33 735.4 cm − 1 energy level was not observed before. Changing the DL1 tuning to the transition 5I4 → 7K4° (J = 3, λ = 586.967 nm), the transitions from the excited level at 17 032.146 cm − 1 were investigated. Similarly to the other trials, the wavelengths related to the possible transitions from this level were calculated and simulated. Fig. 4(a) shows the spectra carried out by DL2 in the presence and absence of DL1 in 593.800 to 594.100 nm spectral range. The optogalvanic signal was detected when the DL2 was tuned in 593.879 nm, giving the transition from 17 032.146 to 33 866.0 cm− 1. By simulation in this figure, it was expected to observe two Nd transitions, the second one occurring with the absorption of 593.992 nm wavelength. The

Fig. 3. Nd two-step excitations observed by optogalvanic and synthetic spectra from 16 979.352 cm− 1 level, for DL1 and DL2 laser on and only laser DL2 on. (a) to 33 808.9 cm− 1 (594.031 nm); (b) to 33 735.4 cm− 1 and to 33 726.7 cm− 1 (596.645 and 596.947 nm, respectively).

33 808.9 cm − 1. According to this figure, in the absence of DL1 the optogalvanic signal corresponding to the absorption of second laser at 594.031 nm wavelength disappears. On the other hand, the optogalvanic signal related to the Ne transition (593.931 nm) was still present. As expected, Fig. 3(a) reproduced the transitions appearing in the same region scanned by DL2 in the presence of DL1, being useful to the line identifications. Scanning another region between 593.200 and 593.600 nm a second step transition was observed, where the 16 979.352 to 33 832.6 cm− 1 transition was similarly obtained in Fig. 3(a). In the absence of DL1, the peak at 593.196 nm disappeared resulting in a second-step transition. Besides the transitions at 593.196 and 594.031 nm, another two transitions that can be seen in Fig. 3 (b), were identified. These ones correspond to the energy levels 33 726.7 and 33 735.4 cm− 1, when the DL2 was tuned in the 596.947 and 596.645 nm wavelengths, respectively. After the detection of the optogalvanic signal at 596.947 nm a peak shifting to 0.038 nm was observed, related to the same transition that occurred at 596.947 nm. This peak is due to laser mode jump, a characteristic of multimode lasers, especially the dye laser used in this work. The shift corresponds exactly to the value of the etalon free spectral range used to decrease the dye laser linewidth. Comparing the synthetic spectra with the experimental spectrum there was no transition close to 596.947 nm. By the way, the transition 16 979.352 →33 735.4 cm− 1

Fig. 4. Nd two-step excitations observed by optogalvanic and synthetic spectra from 17 032.146 cm− 1 level, for DL1 and DL2 laser on and only laser DL2 on. (a) to 33 866.0 cm− 1 (593.879 nm); (b) to 33 721.7 cm− 1 (599.014 nm).

A.R. Victor et al. / Spectrochimica Acta Part B 66 (2011) 748–753

E [cm–1]

Ionization energy

44 562.0

J= ?

33 721.7

J= ?

17 162.930 J= 5

3Ho

5

17 032.146 J= 4

3 o K

16 979.352 J= 3

7Ko

16 844.843 J= 5

7Ko

16 757.037 J= 5

7

596.947

33 726.7

596.645

J= ?

594.031

J= ?

33 735.4

4

3

5

Ho5

596.603

33 792.8

593.196

J= ?

593.493

33 808.9

599.014

J= ?

588.792

33 832.6

596.473

J= ?

586.967

33 866.0

593.879

transitions depend on parameters such as cross section, radiative lifetime, and oscillator strength, and there is no information on these values available in the literature. Thus, the observation of some transitions depends on instrumental apparatus detection sensitivity. In some cases, transitions predicted by synthetic spectra (Fig. 4(b)) also were not observed. The synthetic spectra, presented by this figure, yield three transitions at 598.835, 598.989 and 599.014 nm for Nd, but only one was observed. In this case, the Ne transition was very important to allow the spectrum calibration to distinguish Nd lines absorptions occurring at 598.989 and 599.014 nm. Using the calibrated spectrum, transition 17 032.352 → 33 721.7 cm−1 (599.01 4 nm) was identified. Similarly, the observation of 33 972.8 cm− 1 level by tuning the DL2 in 596.473 nm obtained by excitation of 17 032.146 cm− 1 was also observed. Due to the operating current (~10 mA) limited by the HCL manufacturer, giving a low Nd vapor density, a greater effort was needed to find the best adjustment for the tuning and detection equipment of the optogalvanic signal for each investigated transition. It is noteworthy that all observations and measurements described in this work were obtained despite the lack of information about parameters of Nd transitions, such as cross section, radiative lifetime and oscillator strength. New transitions have arisen from the other three transitions from ground state ( 5I4 → 3H5°, 5I4 → 7K5°, 5I4 → 7H5°) are being studied and they are not described in this paper. Table 2 summarizes all observed Nd transitions, and the transition indicated by an asterisk (*) represents the new energy level only found and identified in this work. Finally, Fig. 5 also summarizes as a diagram all observed transitions, discussed in this work, obtained by single and two-step OGS, using an Nd–Ne HCL for the spectral region between 580 and 600 nm.

582.494

752

4. Conclusions

0 The results of neodymium absorption spectra using a conventional Doppler-limited optogalvanic spectroscopy by laser multistep excitation technique in a commercial neodymium hollow cathode lamp buffered with neon gas were reported. In the 580–600 nm spectral range, five transitions from Nd ground state and seven transitions related to second step for multistep photoionization were observed. For the first time a new line, 596.645 nm, was observed which could be attributed to a possible two-step transition to neodymium energy level from the 16 979.352 cm − 1 to 33 735.4 cm − 1. The spectra simulation allowed us to predict interference between lines of Ne and Nd atoms, considering the resolution of laser used, and to calibrate the experimental spectra. Finally, optogalvanic spectroscopy is a powerful and inexpensive technique, able to reproduce the results obtained with more complex apparatus, also allowing the observation of a new possible neodymium atom energy level.

Table 2 Nd transitions from excited state (17 032.146 cm− 1 and 16 979.352 cm− 1) obtained by two-step OGS in the spectral range from 580 to 600 nm. λair [nm]

Ji–f

Ei–f [cm−1]

593.879 596.473 599.014 593.196 594.031 596.645* 596.947

4–? 4–? 4–? 3–? 3–? 3–? 3–?

17 032.146–33 866.0 17 032.146–33 792.8 17 032.146–33 721.7 16 979.352–33 832.6 16 979.352–33 808.9 16 979.352–33 735.4a 16 979.352–33 726.7

? J values must obey the selection rules (ΔJ = 0, ± 1). a Transition line observed only in the present work.

[nm] Ground State: J= 4

Configuration: 4f 4 6s2 5I4

Fig. 5. Energy level diagram obtained by single and two-steps OGS for neodymium atom.

Acknowledgment The authors gratefully acknowledge the partial financial support given by CNPq — Conselho Nacional de Desenvolvimento Científico e Tecnológico (National Council of Scientific and Technological Development).

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