- Email: [email protected]
Three-step resonance ionization of zirconium with Ti:Sapphire lasers
Spectrochimica Acta Part B 158 (2019) 105640
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Three-step resonance ionization of zirconium with Ti:Sapphire lasers☆ a,⁎
a
b
b,c
Y. Liu , E. Romero-Romero , D. Garand , J.D. Lantis , K. Minamisono
b,d
, D.W. Stracener
T a
a
Physic Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA d Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA b c
A B S T R A C T
Three-step resonance ionization of atomic zirconium using Ti:Sapphire lasers is investigated for the first time. We have located eight new excited states between 41,160 and 41,824 cm−1 that could serve as the intermediate state for the second-step transition. Three-step ionization paths via two of the newly observed states have been studied and numerous high-lying and autoionizing levels are observed. Eight new Rydberg series of odd-parity are identified in the photoionization spectra. The convergence limits of these Rydberg series allow us to determine the first ionization potential of Zr to be 53,507.832(35)stat(20)sys cm−1 with an order of magnitude improvement in uncertainty over the previous measurements. In addition, our measurements for one of the selected three-step paths show that the transitions can be saturated with low to moderate laser powers.
1. Introduction Zirconium isotopes are of great interest to fundamental nuclear physics. They span a wide range of neutron numbers that includes some of the most interesting regions of neutron shell and sub-shell closures where dramatic nuclear structural changes, large deformations, and rapid shape transitions with increasing neutron numbers have been observed [1–3]. The shape evolutions in stable and short-lived Zr isotopes present challenging tests for various theoretical models and their study is important to the advance of our understanding of the nuclear structure [4–8]. Neutron-rich Zr isotopes are among the short-lived nuclei that are of critical importance to stewardship science [9–11]. The knowledge of specific neutron capture cross sections and nuclear reaction rates on those unstable nuclei is essential for accurate modeling of the neutron fluxes and neutron capture networks relevant to stockpile stewardship. Experimental studies of short-lived Zr radioisotopes require the production of pure isotopes of interest. Beams of Zr isotopes far from stability are experimentally difficult to obtain because of their highly refractory nature and their low production rates in nuclear reactions. Production methods that have been used include in-flight fission [12,13] and ion guide isotope separator on-line (IGISOL) [14,15]. In this study, we have investigated three-step resonance laser ionization of ☆
atomic Zr with the aim to develop efficient production means for Zr radioactive ion beams (RIB) at the facilities based on the isotope separator on-line (ISOL) production method [16]. Multi-step, multi-color resonance laser ionization of Zr atoms has been studied for laser isotope separation [17–22], resonance ionization mass spectrometry (RIMS) [23], and laser spectroscopy [24–26]. These previous studies have demonstrated two-step and three-step ionization schemes, but all required tunable dye lasers. All-solid-state tunable Ti:Sapphire lasers have been widely used for resonance ionization laser ion sources (RILIS) for RIB production at ISOL facilities. In fact, some of the RILISs employ exclusively Ti:Sapphire lasers [27–30]. The ionization schemes involving dye lasers usually require wavelengths not available from Ti:Sapphire lasers. Two-step ionization schemes for Zr exclusively with Ti:Sapphire lasers have been reported by Barzky et al. [31] and Tomita [32], which used frequency-doubled photons at 442.544 nm or 360–380 nm, respectively, for the second ionization step. For the Ti:Sapphire laser system at Oak Ridge National Laboratory (ORNL), these wavelengths are far away from the peak of the tuning curve and the corresponding laser power would be too low to saturate the last ionization step for high efficiency. Therefore, a motivation for this work is to find three-step resonance ionization schemes that can use more powerful fundamental output of the Ti:Sapphire lasers for the ionization step. In addition, no three-step,
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/ downloads/doe-public-access-plan). ⁎ Corresponding author. E-mail address: [email protected] (Y. Liu). https://doi.org/10.1016/j.sab.2019.105640 Received 9 April 2019; Received in revised form 17 June 2019 Available online 25 June 2019 0584-8547/ © 2019 Published by Elsevier B.V.
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
The lasers used a diffraction grating for wavelength selection and were continuously tunable between 720 nm and 960 nm. Laser wavelengths in the blue to UV regions were obtained by frequency doubling, tripling, and quadrupling. The three laser beams were collimated and merged into a single laser beam inside the laser room, which was then transported to IRIS2 and focused over a distance of > 10 m into the 3-mm cavity of the ion source. The typical efficiency of laser transportation into the cavity was measured to be 70–80% after the beams were merged. Accordingly, the full width at half-maximum (FWHM) of the laser beams in the laser-atom interaction region was estimated to be 2.4–3 mm. The Ti:Sapphire laser beams were pulsed at 10 kHz with a pulse width of about 30 ns. They were synchronized by adjusting the external trigger timing to the individual pump lasers, with time jitters of smaller than 5 ns between the synchronized lasers. The radiative lifetime of the Zr atomic states involved in the study was estimated to be on the order of 10 ns for the first excited state and at least tens of nanoseconds for the second excited states based on our observations that desynchronizing the laser pulses up to 10 ns of one another had little effect on the ionization efficiency. Photoionization spectra were obtained by scanning the wavelength of a selected laser while recording the ion current measured at the FC after the mass separator. The laser wavelength could be scanned in the direction of increasing photon energy (scan-up) or decreasing photon energy (scan-down). It has been observed that there was a scan-direction dependent shift that the spectral line centroids obtained by scan-up and by scan-down were slightly different with an average difference of about 0.04 cm−1. To mitigate this effect, each spectrum was measured at least twice by scan-up and scan-down, respectively. The line centroids were then determined as the average values of the scan-up and scan-down measurements and the experimental statistical uncertainty was calculated as the standard deviation of the measurements. This procedure was used for all the spectra measured in this study. The fundamental wavelengths of the three Ti:Sapphire lasers were monitored simultaneously using a calibrated wavelength meter (HighFinesse WS6–600) equipped with a 4-channel opto-mechanical switcher. The uncertainty in measuring the laser wavelengths was dominated by the absolute accuracy of the wavelength meter, which is 600 MHz or 0.02 cm−1 according to 3-sigma criterion. This uncertainty was taken as the systematic error of our measurement.
Fig. 1. Schematic drawing of the ion source assembly and laser ionization configuration.
three-color resonance ionization of Zr with exclusively Ti:Sapphire lasers has been reported to our knowledge. Thus, this study will allow us to access high-lying atomic levels and autoionizing (AI) and Rydberg levels in Zr that have not been previously studied. The results should provide new atomic spectroscopic data for Zr useful to fundamental and applied applications such as modeling atomic and nuclear structures, stockpile stewardship science, cosmochemistry [33], and geochemistry [34]. 2. Experimental The experimental setup has been reported previously [30,35], so the relevant components for this study are briefly described here. A RILIS consisting of a hot-cavity ion source and a system of three tunable Ti:Sapphire lasers [36] was used for three-step ionization of Zr. The ion source was installed on the high-voltage platform of the Injector for Radioactive Ion Species 2 (IRIS2) facility [30] at ORNL, while the laser system was located in a laser room next to IRIS2. As shown in Fig. 1, the hot-cavity ion source consisted of a 30-mm long Ta cylindrical cavity of 3-mm inner diameter and a closed-end sample tube of 8.5-mm inner diameter and about 100-mm long, also made of Ta. The cavity and the sample tube were heated resistively in series by an electrical current applied to the Heater Bus. A small Zr metal foil of about 2 mm × 10 mm × 0.025 mm was folded and heated in the sample tube, producing atomic vapor of Zr which effused into the cavity where Zr atoms were ionized by laser beams entering the cavity through the extraction electrode. The ions were extracted from the cavity, accelerated to 40 keV, and transported to a magnetic mass separator. Zr has five stable isotopes and all of them were ionized by the lasers. The ions of the most abundant isotope 90Zr were mass selected and measured with a Faraday cup (FC) after the mass separator. The ion beam intensity before the mass separator was also monitored with another FC. During the measurements, the ion source was heated with 350 A electrical current, at which the Zr sample temperature was estimated to be between 1700 and 1800 K, while the cavity was on the order of 2100 K based on previous temperature measurements [37]. We did not need to increase the temperatures because the Zr ion beam current was enough for our measurements. The three Ti:Sapphire lasers were pumped by three individual Qswitched Nd:YAG lasers at 532 nm at a pulse repetition rate of 10 kHz.
3. Results and discussions 3.1. Ionization schemes Suitable transitions for the first-step excitation were studied using the Kurucz atomic line database [38]. Five candidates are listed in Table 1. The laser wavelengths for these transitions are available by frequency doubling. The first three transitions in the table start from the ground state a 3F2 or the first excited state a 3F3 at 570.41 cm−1, with λ1 between 382 and 387 nm, and require photons between 720 and 750 nm for the subsequent second and third steps to reach the ionization threshold, while the last two transitions with λ1 between 362 and 367 nm will use photons around 800 nm for the second and third steps. In comparison, the last two candidates were preferred for higher power
Table 1 Selected candidates for the first-step transition of three-step ionization schemes for Zr with following quantities: E0: energy of the lower level, E1: energy of the upper level, λ1: excitation laser wavelength, f: absorption oscillator strength expressed as log(gf) where g is the statistical weight of the lower level, and A21: atomic transition probability. Lower level
E0 (cm−1)
Upper level
E1 (cm−1)
λ1 (nm)
Log(gf)
A21 (s−1)
5s2 5s2 5s2 5s2 5s2
0 0 570.41 570.41 1240.84
5s5p x 3Do1 5s5p x 3Fo2 5s5p x 3Fo3 4d5s25p w 3Fo3 5s5p w 3Fo4
26,154.13 26,061.70 26,443.88 28,157.42 28,528.36
382.349 383.705 386.496 362.489 366.468
−0.86 −0.27 −0.24 −0.28 0.01
2.099E+7 4.866E+7 3.671E+7 3.806E+7 5.647E+7
a a a a a
3
F2 F2 3 F3 3 F3 3 F4 3
2
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
and temporally more stable laser pulses near 800 nm than at 720–750 nm. We finally selected the transition at λ1 = 366.468 nm, which has the largest absorption oscillator strength. A disadvantage for this transition is that the lower level is the a 3F4 state at 1240.84 cm−1 which is less populated than the 3F2 and 3F3 states. However, the hotcavity source is typically operated at high temperatures close to 2300 K. At 2300 K the 3F4 level is expected to be appreciably populated with about 29% of the Zr atoms, given by the Boltzmann distribution law, in comparison to 35% and 36% of the 3F3 and ground level populations, respectively. The selected first-step transition was measured experimentally by scanning the first laser (λ1) while other two lasers were fixed in wavelength for a resonant transition in the second step (see discussions next) and non-resonant ionization in the third step. The observed resonance peaks fit very well to a Gaussian profile with a central position of 27,287.511(51) cm−1 and FWHM of 0.53(2) cm−1. The uncertainty of the values was the standard deviation of the scan-up and scan-down measurements. Adding the energy of the a 3F4 level, we obtained E1 = 28,528.35(5) cm−1 for the first excited state 5s5p w 3F4o, in good agreement with the known value of 28,528.36 cm−1 [39]. Resonance transitions for the second step from the 3F4o level at E1 = 28,528.36 cm−1 were searched by scanning the second laser (λ2) between 744 and 836 nm, while the third laser wavelength (λ3) was fixed for non-resonant ionization. Eight resonance peaks were observed, as numbered in Fig. 2. The spectra are plotted as a function of E2 = E1 + hν2, where h is Planck's constant, ν2 is the second laser frequency, and E1 = 28,528.36 cm−1. The λ2 scan range corresponded to E2 = 40,490–41,969 cm−1 and the scan was repeated at three thirdphoton energies of 12,464.8, 12,614.6, and 12,625.7 cm−1, respectively. The E2 resonance centroids obtained with different third photon energies were found to be in excellent agreement. The observed peak intensity and linewidth may not reflect the relative transition strength because they could be affected by accidental coincidences where the
Table 2 Measured energies of the second excited states of even parity and the FWHM of the resonance transitions. The given uncertainties are the standard deviations of the scan-up and scan-down measurements, not including the systematic error of 0.02 cm−1. Peak #
E2 (cm−1)
FWHM (cm−1)
λ2 (nm)
1 2 3 4 5 6 7 8
41,162.17(4) 41,177.00(3) 41,179.44(3) 41,296.14(3) 41,314.00(2) 41,560.96(3) 41,694.12(3) 41,823.83(3)
0.37(8) 0.34(6) 0.34(9) 0.21(2) 0.27(2) 0.56(2) 0.48(2) 0.49(3)
791.53 790.60 790.45 783.22 782.13 767.31 759.55 752.14
third photon was on a resonance transition to an AI state. In fact, the three third-laser photons used here were later found to be on or near a λ3 resonance peak from some of the E2 states. Table 2 gives the mean energies of these E2 states, FWHM, and the excitation wavelengths for individual transitions. These even-parity states have not been previously reported, except that peak #3 at E2 = 41,179.44(3) cm−1 is very close to the 4d25s(4F) 5d e 5G4 level at 41,179.30 cm−1 in literature [39]. The 3F4 – 5G4 transition is semi-forbidden, but such intercombination transitions have previously been observed [25]. On the other hand, we did not observe the transitions to the 5G3 and 5G5 levels at 40,887.61 and 41,538.23 cm−1, respectively, which were also within the λ2 scan range. In addition, peak #3 was a relatively strong transition, as shown in Fig. 2(b). Hence, it is possible that this peak was not the 5G4 state. For dipole-allowed transitions, these new E2 states may have configurations of 4d25s6s and 4d25s5d and, for Russell-Saunders (LS) coupling, possible terms of 3D3, 3F3,4, and 3G3,4,5, but the assignment could not be determined by the spectra alone. At 2100 K, the doppler broadening was calculated to be approximately 2.8 GHz for the first-step transition and 1.3 GHz for the second and third transitions. The laser beam linewidth was on the order of 3–4 GHz for fundamental light and 4–5 GHz for frequency-doubled light, larger than the Doppler broadening. In Table 2, the FWHM of the measured spectral lines ranges from 6 to 15 GHz, significantly larger than the laser linewidths. As to be discussed later in section 3.4, the first- and second-step resonance transitions could be well saturated with the laser power used. Therefore, the linewidth of the spectral peaks was usually dominated by power broadening. Three-step ionization via the first two states in Table 2 has been investigated. As illustrated in Fig. 3, Zr atoms were excited by three lasers from the 5s2 3F4 state at 1240.84 cm−1 to the 5s5p 3Fo4 state at 28,528.36 cm−1, subsequently to levels at E2 = 41,162.17 cm−1 (λ2 = 791.95 nm) or 41,177.0 cm−1 (λ2 = 790.6 nm), and from there to ionization. The third laser was scanned to search for resonant transitions to high lying Rydberg and autoionizing states. The first excitation was obtained by frequency-doubled light and the second and third transitions were accomplished with the fundamental laser light between 720 and 960 nm. The selectivity of the two three-step excitation schemes was checked by blocking the individual lasers. It was found that approximately 97% of the laser ionized ions were produced by three-step, three-photon ionization, the other 3% were contributed by λ1 + λ2 photons (2%) or λ1 + λ3 photons (1%) alone.
3.2. Photoionization spectra Figs. 4 and 5 show the observed photoionization spectra from the two second intermediate states. The measured ion currents are plotted as a function of the total energy E3 of the three excitations, which is obtained by adding the third photon energy to the energy of the second excited state. The ion currents obtained with E2 = 41,177.0 cm−1
Fig. 2. Measured λ2 scan spectra (a) with eight resonance peaks observed and (b) containing the peaks #1 – #3, while the third photon energy was fixed to 12,614.6 cm−1. The spectrum in (b) was a separate scan under improved ion source operation conditions. 3
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
3.3. Analysis of Rydberg series The lower level of the third transition is expected to have 4d25s6s or 4d 5s5d configurations. Thus, the Rydberg series observed could be the 5snp or 5snf series of odd parity. To analyze the Rydberg spectra, the centroids and FWHMs of the resonances were determined by fitting to a line profile function. Most of the peaks are narrow and symmetric and can be fit to Gaussian profiles. Fano profiles are used for asymmetric autoionizing peaks. Again, the final line positions were given by the mean values of the scan-up and scan-down measurements and the standard deviation of the mean value as the experimental statistical uncertainty. The convergence limit of a Rydberg series is found by fitting the observed series members to the second-order Rydberg-Ritz formula 2
En = Elimit −
RM (n∗)2
n∗ = n − δ (n)
δ (n) = δ0 +
a (n − δ0 )2
(1) (2) (3)
where En is the measured centroid energy of the Rydberg peak of index n, Elimit is the series converging limit, RM = 109,736.646 cm−1 is the mass corrected Rydberg constant for 90Zr, n* is the effective quantum number, δ(n) is the quantum defect, δ0 is the asymptotic limiting value of the quantum defect, and a is a constant. The integer index n is chosen to be the nearest integer larger than n*. Three odd-parity Rydberg series were observed from the E2 = 41,177.0 cm−1 level, one series converging to the ground state (4F3/2) of Zr+, and two series converging to the first (4F5/2) and second (4F7/2) excited states of Zr+, respectively, as shown in Fig. 4. They are labeled as Series 1, 2 and 3 with the observed ranges of the index number n as given below.
Fig. 3. Two three-step ionization schemes investigated. The third laser wavelength (λ3) was scanned over energies across the first ionization potential (IP) of Zr.
(Fig. 4) were smaller than those from E2 = 41,162.17 cm−1 (Fig. 5), but the absolute intensities of the spectra depended on the experimental conditions, such as the ion source temperature, ion beam transmission, and laser power, etc., which were not the same when measuring the two spectra. The relative intensity of the spectrum lines is also a rough indication of their transition strengths, as it could be affected by unknown fluctuations in laser power, temporal and spatial overlapping between the laser pulses during the scan. In addition, the spectra have not been corrected for the third laser power dependence on wavelength throughout the scan range. There are many resonance peaks in each spectrum and most of the peaks could be assigned to Rydberg series of different convergence limits. Three relatively intense Rydberg series are indicated in each spectrum. We used the following general criteria to identify resonance peaks from background fluctuations: (1) A peak should be observed in repeating scan-up and scan-down spectra and at about the same energy positions. (2) Peak height is at least five times of the standard deviations of the baseline fluctuation. (3) There are more than one data points in the peak.
Series 1: converging to 4F3/2 of Zr+, n = 28–60. Series 2: converging to 4F5/2 of Zr+, n = 20–73. Series 3: converging to 4F7/2 of Zr+, n = 11–31. In the spectrum from the E2 = 41,162.17 cm−1 level (Fig. 5) five odd-parity Rydberg series were identified as listed below. Three were relatively strong with convergences being the 4F5/2, 4F7/2, and the third excited state 4F9/2 of Zr+, respectively. They are labeled as Series 4–6 and are indicated in Fig. 5. The other two were weaker series labeled as Series 7 and 8. Series 4: converging to 4F5/2 of Zr+, n = 15–76. Series 5: converging to 4F7/2 of Zr+, n = 10–64.
Fig. 4. Photoionization spectrum from the E2 = 41,177.0 cm−1 state. Comb lines indicate the identified members of three Rydberg series converging to the ground (4F3/2), first excited (4F5/2), and second excited (4F7/2) states of Zr II, respectively. Vertical dashed lines mark the positions of the series limits. 4
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Fig. 5. Photoionization spectrum from the E2 = 41,162.17 cm−1 state. Comb lines indicate the identified members of three Rydberg series converging to the first excited (4F5/2), second excited (4F7/2) and third excited (4F9/2) states of Zr II, respectively. Vertical dashed lines mark the positions of the series limits.
Series 6: converging to 4F9/2 of Zr+, n = 9–42. Series 7: converging to 4F5/2 of Zr+, n = 14–36. Series 8: converging to 4F7/2 of Zr+, n = 19–27. Some series members could not be unambiguously identified due to overlapping with other peaks and perturbations in the series, as to be discussed later. The observed Rydberg states of these series are summarized in Tables S1 – S5 in the Appendix. It is noticed that the series converging to the ground state 4F3/2 of + Zr was only excited from the E2 = 41,177.0 cm−1 state (Series 1) and was a very weak series, as shown in Fig. 4. Members of Series 1 in the range of n = 28–60 were observed, but the n = 40–42 peaks could not be determined due to the large perturbation around 53,440 cm−1 and n = 55 and 59 peaks were also not identified due to overlapping with much larger peaks. The series converging to the third excited state 4F9/2 of Zr+ was only observed from the 41,162.17 cm−1 level (Series 6) and was a relatively weak series as well (Fig. 5). Inter-channel interactions among the overlapping Rydberg series are noticeable in both spectra. In comparison, Series 4–6 from the 41,162.17 cm−1 level showed relatively large disturbances, as evidenced by substantial intensity irregularities in the series (Fig. 5). Clusters of peaks were observed when the lower members (n = 9–13) of the Series 6 interacted with other series. Fig. 6 displays the spectra
Fig. 7. Portion of the photoionization spectra containing the cluster of spectral lines at around 53,710 cm−1 and the peaks of Series 2–7 as indicated. ( ̶ • ̶) Spectrum from E2 = 41,177.0 cm−1 with members of Series 2 and 3, (—) spectrum from E2 = 41,162.17.0 cm−1 members of Series 4, 5, 6, and 7.
containing the cluster around 53,440 cm−1 where n = 9 member of Series 6 and n = 17 member of Series 2 or 4 met, and Fig. 7 shows the cluster centered at about 53,710 cm−1 that involved the n = 10 member of Series 6 and the n = 14 member of Series 3 or 5. Another relatively profound cluster was around 54,170 cm−1 (Fig. 5) corresponding to interactions with the n = 13 member of Series 6. As shown in Figs. 6 and 7, the clusters consist of complicated spectral structures that are different from the nearby unperturbed Rydberg members, from cluster to cluster, and from different starting lower levels. Hence, assignments for those fine structures are not possible. Such clustering features associated with the Series 6 have been observed by Page et al. [25] who reported clumps of overlapping spectra in three-step photoionization of Zr, even though they used different excitation transitions, including the clumps centered at 53,500, 53,710, and 54,170 cm−1. They attributed the formation of the spectral clumps to Rydberg-valence mixing and assigned the clump centers at 53,500, 53,710, and 54,170 cm−1 as the n* ≈ 9, 10, and 13 members of the Rydberg series converging to the third excited state 4F9/2 of Zr+, which is consistent with our observation. 3.3.1. Series converging to 4F5/2 of Zr+ Three Rydberg series were observed to converge to the first excited state 4F5/2 of Zr+: Series 2 excited from the 41,177.0 cm−1 state and Series 4 and 7 excited from the 41,162.17 cm−1 state. Perturbations
Fig. 6. Portion of the photoionization spectra showing peaks of Series 1 (circles) in the range n = 28–52 and the cluster of spectral lines at around 53,440 cm−1. The n = 40–42 members of Series 1 were disturbed by the cluster and could not be unambiguously identified. 5
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Fig. 9. Quantum defect modulo 1 versus the effective quantum number n* of Series 2, 4, 7 and the unidentified series suddenly appearing after n = 25 (Fig. 7). The δ +1 values of these series, except Series 2, are also displayed in order to show the merging between them. Solid line is the fit to Eq. (4).
51 could be described by a modification of the quantum defect given by [41,42]
δ ′ (n) = δ (n) −
1 Γ/2 ⎞ arctan ⎛⎜ ⎟ π E n ⎝ − Ep ⎠
(4)
where Ep is the energy and Γ is the spectral width of the perturbing state. The fitted quantum defect is also shown in Fig. 9, which gives Ep = 53,780.83(17) cm−1 or np* = 51.3(1).
Fig. 8. Expanded spectra showing the fine structures of low n members of Series 2, 4, and 7. ( ̶ • ̶) Spectrum from E2 = 41,177.0 cm−1 with members of Series 2, (—) spectrum from E2 = 41,162.17.0 cm−1 with members of Series 4 and 7.
3.3.2. Series converging to 4F7/2 of Zr+ Three Rydberg series converging to the second excited state 4F7/2 of + Zr were also observed: Series 3 excited from the 41,177.0 cm−1 level and Series 5 and 8 excited from the 41,162.17 cm−1 level. The Rydberg levels of Series 3 have slightly lower centroid energies than those of Series 5, as shown in Fig. 10. Also shown in Fig. 10 are the peaks of the weak Series 8. The lower members (n = 10–17) of Series 5 are also found to have different fine structures than those of higher members. Fig. 11 shows the calculated δ vs n* for Series 3 and 5 and 8. Perturbations are noticeable in Series 5 at n* ≈ 16–17 and n* ≈ 27–28, by unknown interlopers, and at n* ≈ 33–34 by the n = 13 member of
modified the series from the 41,162.17 cm−1 state. For example, a few intense members of an unidentified series suddenly appeared after the n = 25 peak of Series 4 (Fig. 7) and soon merged with Series 4 before the cluster at 53,710 cm−1. Also shown in Fig. 7 are members of the Series 7, which were typically very weak but became the dominant peaks after the cluster. The n = 20–35 peaks of Series 2 and 4 were observed at about the same energies, indicating that they could be the same Rydberg levels excited from two lower levels. However, after the cluster, Series 7 merged with Series 4 at n > 35 and the resulting peaks shifted to slightly higher centroid energies than the peaks of Series 2 (Fig. 7). That is, the n > 35 peaks of Series 4 were different Rydberg levels than those of Series 2. For n < 20, both Series 2 and 4 showed fine structures different than higher members and different between the two series as well (Fig. 8). For each fine structure group, the peaks that appeared in both series at about the same energy are selected as the series members. However, it is possible that these low members (n < 20) may not be correctly interpreted. Fig. 9 shows the quantum defects of Series 2, 4 and 7 as well as the unidentified series peaks as a function of n*. The effective quantum number n* is calculated from Eq. (1) and the quantum defect is δ = n – n* according to Eq. (2). Since n is the nearest integer larger than n*, δ is in fact the modulo 1 (mod 1) value of the real quantum defect. The average δ value is about 0.1 for Series 2 and 4 and about 0.98 for Series 7. The data show the presence of perturbations around n* ≈ 35–35, 39, and 51 in Series 4 and the merge of the unidentified series and Series 7 with Series 4. The perturbation at n* ≈ 35–36 could be due to the n = 10 member of Series 6, at n* ≈ 39 is due to an unknown interloper, and at n* ≈ 51 is due to the n = 15 member of Series 5. The data could be analyzed using the multichannel quantum defect theory (MQDT) [40], but it is out of the scope of this study. On the other hand, a large fraction of the Rydberg levels is not perturbed so we can use the singlechannel quantum defect analysis. The relatively large deviation at n* ≈
Fig. 10. Photoionization spectra containing members of Series 3, 5 and 8. ( ̶ • ̶) Spectrum from E2 = 41,177.0 cm−1 with members of Series 3, (—) spectrum from E2 = 41,162.17.0 cm−1 with members of Series 5, 6, and 8. 6
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Fig. 11. Quantum defect modulo 1 versus the effective quantum number n* of Series 3, 5, and 8.
Fig. 12. Rydberg-Ritz fit of Series 2 with n = 20–48 and 51–73 members. (Top) Measured energies (dots) of the Rydberg states and calculated energies (solid line) using the fitted parameters. (Bottom) Fit residuals with experimental statistical uncertainties as the error bars.
Series 6 at around 54,170 cm−1. Similar to Series 7, Series 8 has an average δ of about 0.97, different than that of Series 3 and 5. The observed Rydberg series are expected to have 4d25s(4F)np or 4d25s(4F)nf configurations. The different quantum defect (mod 1) values suggest that Series 1–6 have the same configuration which may be different than that of Series 7–8. However, specific assignments could not be made on the base of the quantum defects alone. 3.3.3. Series limits and ionization potential of Zr Series 1–6 are fitted to the Rydberg-Ritz formula of Eqs. (1)–(3) to determine the convergence limit and δ0 value for each series. For Series 1, the fit uncertainty for the parameter a is quite large, and it is found that this series is best fit to a constant δ(n) = δ0. The results are summarized in Table 3. By subtracting the known energy of the excited states of Zr+ with respect to the ground state of Zr+ from the best-fit convergence limits, we obtained the IP value for Zr from each series as given in the last column of the table. We did not attempt to fit Series 7 because it merged into Series 4. Series 8 was too short to fit. Not all the assigned members of the series are used in the fits. As an example, Fig. 12 shows the Rydberg-Ritz fit for Series 2 with only n = 20–73 peaks and the residuals of the fit, which gave the best fit result. Similarly, the n = 10–17 and n = 27 members of Series 5 are not included in fitting. For Series 4, the perturbation at n* ≈ 51 (Fig. 9) is taken into account by combining Eqs. (3) and (4) for the Rydberg-Ritz fit. The resulting fit residuals are compared with those without Eq. (4) in Fig. 13. The changes in the best-fit values for Elimit and δ0 after considering the perturbation are found to be very small. Therefore, we have ignored the other perturbations in all series for the determination of series convergence. The IP values derived from Series 1–5 are very close, but the value from Series 6 is about 0.2 cm−1 smaller. The reason is unknown and needs further investigation. We used the results of Series 1–5 to
Fig. 13. Residuals of the Rydberg-Ritz fit for Series 4, (a) without and (b) with the use of Eq. (4) for the perturbation at n* ≈ 51.
determine the final value for the IP of Zr. The average of the five individual data gives IP = 53,507.832(35)stat(20)sys cm−1, including one standard deviation of statistical uncertainty and the systematic uncertainty of 0.02 cm−1. This IP value agrees with the recent result by Hasegawa et al. [26], but our uncertainty is an order of magnitude lower than the previous measurement.
Table 3 Results of Rydberg-Ritz fit for series 1–6. The given uncertainties are the standard deviation of the fit. Last column is the value of IP derived from the series limits. Series
Limit level
Elimit (fit) (cm−1)
δ0
a
IP (cm−1)
1 2 3 4 5 6
a4F3/2 a4F5/2 a4F7/2 a4F5/2 a4F7/2 a4F9/2
53,507.812 (22) 53,822.507(4) 54,271.249(92) 53,822.517(11) 54,271.298(24) 54,830.571(69)
0.093(5) 0.1115(17) 0.1109(45) 0.0841(24) 0.1194(69) 0.1032(28)
−7.83(75) −2.36(55) 5.51(60) −14.6(24) −0.42(25)
53,507.812(22) 53,507.837(4) 53,507.809(92) 53,507.847(11) 53,507.858(24) 53,507.661(69)
7
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
power density in the interaction region was on the order of 0.12, 1.6, and 7 W/cm2 for the first, second, and third excitations, respectively. 4. Summary Three-step resonant ionization of atomic Zr using Ti:Sapphire lasers has been demonstrated. For the first step, we have selected the transition from the 5s2 a 3F4 state at 1240.84 cm−1 to the 5s5p w 3F4o excited state at 28,528.36 cm−1, which has a large absorption oscillator strength. For the second step, eight new excited states were observed. Two of them were used to explore resonance transitions for the third ionization step. We have observed > 350 resonance ionization and autoionizing transitions. Eight new Rydberg series of odd-parity and about 280 Rydberg states in total have been identified to converge to the ground and the first, second, and third excited states of singlycharged Zr ion. These series are expected to have 4d25s(4F)np or 4d25s (4F)nf configurations based on their convergence levels. The convergence limits and limiting quantum defects for six of the series are obtained by Rydberg-Ritz fitting. The analysis allowed us to determine the first ionization potential of Zr to be 53,507.832(35)stat(20)sys cm−1 with an order of magnitude improvement in uncertainty over the previous measurements. Saturation measurements for one of the three-step transition ladders showed that the selected three steps can be easily saturated with low to moderate laser powers. More spectroscopy studies using other intermediate states in Table 2 and evaluation of the ionization efficiencies of different three-step schemes will be the subject of future work. Acknowledgements This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics; in part by National Nuclear Security Admission, Grant No. DE-NA0002924; the National Science Foundation, Grants No. PHY-15-65546, and this research used resources of the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory, which was a DOE Office of Science User Facility.
Fig. 14. Saturation curves of the first, second, and third excitations via E2 = 41,162.17 cm−1 level. Dots represent the measured data and lines are fits to Eq. (5).
3.4. Saturation measurement
Appendix A. Supplemental data
The laser power used in the present study was 100–150 mW frequency-doubled light for the first excitation and 1–2 W fundamental light for the second and third excitations. The corresponding laser power density at the interaction region in the hot-cavity was on the order of 1.5–2 W/cm2 for the first laser and 15–28 W/cm2 for the second and third lasers. We measured the laser power needed to saturate each excitation step for the ionization scheme using the E2 = 41,162.17 cm−1 intermediate level since this scheme seemed to result in higher ion currents. For the saturation measurement, the third step was the transition to an unidentified AI level at E3 = 53,600.14 cm−1 that gave the largest ion current (Fig. 5). The Zr ion current was measured as a function of the laser power for each excitation and the data were then fitted to the formula
I = I0 + A
P P + Psat
References [1] J. Ebert, R.A. Meyer, K. Sistemich (Eds.), Nuclear Structure of the Zirconium Region, Proceedings of the International Workshop, Bad Honnef, Fed. Rep. of Germany, April 24–28, 1988, Springer-Verlag, Berlin, 1988. [2] S.J. Zheng, F.R. Xu, S.F. Shen, H.L. Liu, R. Wyss, Y.P. Yan, Shape coexistence and triaxiality in nuclei near 80Zr, Phy. Rev. C 90 (2014) 064309. [3] F. Browne, A.M. Bruce, T. Sumikama, I. Nishizuka, S. Nishimura, et al., Lifetime measurements of the first 2+ states in 104,106Zr: evolution of ground-state deformations, Phys. Lett. B 750 (2015) 448–452. [4] S. Ansari, J.-M. Régis, J. Jolie, N. Saed-Samii, N. Warr, et al., Experimental study of the lifetime and phase transition in neutron-rich 98,100,102Zr, Phy. Rev. C 96 (2017) 054323. [5] E.T. Gregor, M. Scheck, R. Chapman, L.P. Gaffney, J. Keatings, et al., Shell evolution of stable N = 50–56 Zr and Mo nuclei with respect to low-lying octupole excitations, Eur. Phys. J. A 53 (2017) 50. [6] T. Togashi, Y. Tsunoda, T. Otsuka, N. Shimizu, Quantum phase transition in the shape of Zr isotopes, Phys. Rev. Lett. 117 (2016) 172502. [7] H. Mei, J. Xiang, J.M. Yao, Z.P. Li, J. Meng, Rapid structural change in low-lying states of neutron-rich Sr and Zr isotopes, Phy. Rev. C 85 (2012) 034321. [8] K. Sieja, F. Nowacki, K. Langanke, G. Martínez-Pinedo, Shell model description of zirconium isotopes, Phy. Rev. C 79 (2009) 064310. [9] L. Ahle, L.A. Bernstein, M. Hausmann, D.J. Vieira, Science based stockpile stewardship and RIA, Proceedings of Sixth International Meeting on Nuclear Applications of Accelerator Technology, American Nuclear Society, 2003, pp. 286–291. [10] M.N. Kreisler, National nuclear security and other applications of rare isotopes, Proceedings of Particle Accelerator Conference (PAC07), Albuquerque, New Mexico, 2007, pp. 124–126 June 25–29. [11] E. Hartouni, Role of radioactive beams in stockpile stewardship science, 240th American Chemical Society Fall National Meeting, 2010 August 22–26. (Boston,
(5)
where I is measured ion current, P is measured laser power, Psat is the saturation laser power, I0 is the background ion current, and A is a constant. The fitting results are presented in Fig. 14. From the fittings we obtained Psat = 11.4(11), 150(11), and 630(75) mW for the first, second, and third excitations, respectively. This means that the selected three-step transitions can be saturated with laser powers that are easily available. It should be pointed out that the laser power was measured in the laser room before the laser was sent into IRIS2. From that point, typically 70–80% of the laser power was injected into the ion source. That is, the actual saturation power Psat for the excitation steps was about 70–80% of the above fitted values. The corresponding saturation 8
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Massachusetts). [12] T. Sumikama, K. Yoshinaga, H. Watanabe, S. Nishimura, Y. Miyashita, et al., Structural evolution in the neutron-rich nuclei 106Zr and 108Zr, Phys. Rev. Lett. 106 (2011) 202501. [13] T. Sumikama, Decay spectroscopy of neutron-rich nuclei in the vicinity of 110Zr at RIBF, Acta Phys. Pol. B 44 (2013) 319–325. [14] P. Campbell, J. Billowesa, B. Chealb, K.T. Flanagana, D.H. Forest, et al., Laser spectroscopy of radioactive Ti, Zr and Hf isotopes and isomers at the JYFL laserIGISOL facility, Spectrochim. Acta B 58 (2003) 1069–1076. [15] H.L. Thayer, J. Billowes, P. Campbell, P. Dendooven, K.T. Flanagan, et al., Collinear laser spectroscopy of radioisotopes of zirconium, J. Phys. G 29 (2003) 2247–2262. [16] J.R. Beene, D.W. Bardayan, A. Galindo-Uribarri, C.J. Gross, K.L. Jones, J.F. Liang, W. Nazarewicz, D.W. Stracener, B.A. Tatum, R.L. Varner, ISOL science at the Holifield Radioactive Ion Beam Facility, J. Phys. G 38 (2011) 024002. [17] P.A. Hackett, H.D. Morrison, O.L. Bourne, B. Simard, D.M. Rayner, Pulsed singlemode laser ionization of hyperfine levels of zirconium-91, J. Opt. Soc. Am. B 5 (1988) 2409–2416. [18] L.W. Green, G.A. McRae, P.A. Rochefort, Selective resonant ionization of zirconium isotopes using intermediate-state alignment, Phy. Rev. A 47 (1993) 4946–4954. [19] H. Niki, T. Kuroyanagi, Y. Horiuchi, S. Tokita, Laser isotope separation of zirconium for nuclear transmutation process, J. Nucl. Sci. Technol. 6 (Suppl) (2008) 101–104. [20] H. Niki, T. Kuroyanagi, K. Motozu, Isotopic selectivity in two-color multiphoton ionization spectroscopy of zirconium, Jpn. J. Appl. Phys. 48 (2009) 116510. [21] H. Niki, Two-color ionization spectroscopy and laser isotope separation of zirconium for efficient transmutation of long-lived fission products, J. Korean Phys. Soc. 56 (2010) 190–194. [22] T. Fujiwara, T. Kobayashi, K. Midorikawa, Selective resonance photoionization of odd mass zirconium isotopes towards efficient separation of radioactive waste, Sci. Rep. 9 (2019) 1754. [23] D.R. Spiegel, W.F. Calaway, G.A. Curlee, A.M. Davis, R.S. Lewis, M.J. Pellín, D.M. Gruen, R.N. Clayton, Three-color and 1 + 1 resonance ionization mass spectrometry of zirconium sputtered from refractory carbides, Anal. Chem. 66 (1994) 2647–2655. [24] P.A. Hackett, M.R. Humphries, S.A. Mitchell, D.M. Rayner, The first ionization potential of zirconium atoms determined by two laser, field ionization spectroscopy of high lying Rydberg series, J. Chem. Phys. 85 (1986) 3194–3197. [25] R.H. Page, S.C. Dropinski, E.F. Worden, J.A.D. Stockdale, Progress in zirconium resonance ionization spectroscopy, Proc. SPIE Vol. 1859, Laser Isotope Separation, 1993, pp. 49–63. [26] S. Hasegawa, D. Nagamoto, Two-color laser resonance ionization spectroscopy of zirconium atoms, J. Phys. Soc. Jpn. 86 (2017) 104302. [27] J. Lassen, P. Bricault, M. Dombsky, J.P. Lavoie, M. Gillner, T. Gottwald,
[28] [29] [30]
[31]
[32]
[33]
[34]
[35]
[36] [37] [38] [39]
[40] [41]
[42]
9
F. Hellbusch, A. Teigelhöfer, A. Voss, K.D.A. Wendt, Laser ion source operation at the TRIUMF radioactive ion beam facility, AIP Conf. Proc. 1104 (2009) 9–15. C. Geppert, Laser systems for on-line laser ion sources, Nucl. Instr. Meth. B 266 (2008) 4354–4361. N. Lecesne, Laser ion sources for radioactive beams, Rev. Sci. Instrum. 83 (2012) 02A916. Y. Liu, C.U. Jost, A.J. Mendez II, D.W. Stracener, C.L. Williams, C.J. Gross, R.K. Grzywacz, M. Madurga, K. Miernik, D. Miller, S. Padgett, S.V. Paulauskas, K.P. Rykaczewski, M. Wolinska-Cichocka, On-line commissioning of the HRIBF resonant ionization laser ion sourc, Nucl. Instr. Meth. B 298 (2013) 5–12. J.G. Barzyk, M.R. Savina, A.M. Davis, R. Gallino, F. Gyngard, S. Amari, E. Zinner, M.J. Pellin, R.S. Lewis, R.N. Clayton, Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC, Meteorit. Planet. Sci. 42 (7/8) (2007) 1103–1119. H. Tomita, Latest developments on the narrow bandwidth laser systems for high resolution resonance ionization spectroscopy, LA3NET Laser Ion Source Workshop, 2016 24–25 October. (Paris, France). T. Stephan, R. Trappitsch, A. Davis, M. Pellin, D. Rost, M. Savina, R. Yokochi, N. Liu, CHILI – the Chicago Instrument for Laser Ionization – a new tool forisotope measurements in cosmochemistry, Int. J. Mass Spectrom. 407 (2016) 1–15. E.C. Inglisa, J.B. Creecha, Z. Denga, F. Moyniera, High-precision zirconium stable isotope measurements of geological reference materials as measured by doublespike MC-ICPMS, Chem. Geol. 493 (2018) 544–552. T. Kron, Y. Liu, S. Richter, F. Schneider, K. Wendt, High efficiency resonance ionization of palladium with Ti:sapphire lasers, J. Phys. B Atomic Mol. Phys. 48 (2016) 185003. Y. Liu, D.W. Stracener, A resonant ionization laser ion source at ORNL, Nucl. Instr. Meth. B 376 (2016) 68–72. Yuan Liu, ORNL developments in laser ion sources for radioactive ion beam production, Hyperfine Interact. 227 (2014) 85–99. Atomic Line Data (R.L. Kurucz and B. Bell) Kurucz CD-ROM No. 23, Smithsonian Astrophysical Observatory, Cambridge, MA, 1995. A. Kramida, Yu. Ralchenko, J. Reader, NIST ASD Team, NIST Atomic Spectra Database (ver. 5.6.1), [Online], Available https://physics.nist.gov/asd 2019, May 10 National Institute of Standards and Technology, Gaithersburg, MD, 2018, https://doi.org/10.18434/T4W30F. M.J. Seaton, Quantum defect theory, Rep. Prog. Phys. 46 (1983) 167–257. J. Roßnagel, S. Raeder, A. Hakimi, R. Ferrer, N. Trautmann, K. Wendt, Determination of the first ionization potential of actinium, Phys. Rev. A 85 (2012) 012525. U. Fano, Effects of configuration interaction on intensities and phase shifts, Phys. Rev. 124 (1961) 1866–1878.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Three-step resonance ionization of zirconium with Ti:Sapphire lasers☆ a,⁎
a
b
b,c
Y. Liu , E. Romero-Romero , D. Garand , J.D. Lantis , K. Minamisono
b,d
, D.W. Stracener
T a
a
Physic Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA d Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA b c
A B S T R A C T
Three-step resonance ionization of atomic zirconium using Ti:Sapphire lasers is investigated for the first time. We have located eight new excited states between 41,160 and 41,824 cm−1 that could serve as the intermediate state for the second-step transition. Three-step ionization paths via two of the newly observed states have been studied and numerous high-lying and autoionizing levels are observed. Eight new Rydberg series of odd-parity are identified in the photoionization spectra. The convergence limits of these Rydberg series allow us to determine the first ionization potential of Zr to be 53,507.832(35)stat(20)sys cm−1 with an order of magnitude improvement in uncertainty over the previous measurements. In addition, our measurements for one of the selected three-step paths show that the transitions can be saturated with low to moderate laser powers.
1. Introduction Zirconium isotopes are of great interest to fundamental nuclear physics. They span a wide range of neutron numbers that includes some of the most interesting regions of neutron shell and sub-shell closures where dramatic nuclear structural changes, large deformations, and rapid shape transitions with increasing neutron numbers have been observed [1–3]. The shape evolutions in stable and short-lived Zr isotopes present challenging tests for various theoretical models and their study is important to the advance of our understanding of the nuclear structure [4–8]. Neutron-rich Zr isotopes are among the short-lived nuclei that are of critical importance to stewardship science [9–11]. The knowledge of specific neutron capture cross sections and nuclear reaction rates on those unstable nuclei is essential for accurate modeling of the neutron fluxes and neutron capture networks relevant to stockpile stewardship. Experimental studies of short-lived Zr radioisotopes require the production of pure isotopes of interest. Beams of Zr isotopes far from stability are experimentally difficult to obtain because of their highly refractory nature and their low production rates in nuclear reactions. Production methods that have been used include in-flight fission [12,13] and ion guide isotope separator on-line (IGISOL) [14,15]. In this study, we have investigated three-step resonance laser ionization of ☆
atomic Zr with the aim to develop efficient production means for Zr radioactive ion beams (RIB) at the facilities based on the isotope separator on-line (ISOL) production method [16]. Multi-step, multi-color resonance laser ionization of Zr atoms has been studied for laser isotope separation [17–22], resonance ionization mass spectrometry (RIMS) [23], and laser spectroscopy [24–26]. These previous studies have demonstrated two-step and three-step ionization schemes, but all required tunable dye lasers. All-solid-state tunable Ti:Sapphire lasers have been widely used for resonance ionization laser ion sources (RILIS) for RIB production at ISOL facilities. In fact, some of the RILISs employ exclusively Ti:Sapphire lasers [27–30]. The ionization schemes involving dye lasers usually require wavelengths not available from Ti:Sapphire lasers. Two-step ionization schemes for Zr exclusively with Ti:Sapphire lasers have been reported by Barzky et al. [31] and Tomita [32], which used frequency-doubled photons at 442.544 nm or 360–380 nm, respectively, for the second ionization step. For the Ti:Sapphire laser system at Oak Ridge National Laboratory (ORNL), these wavelengths are far away from the peak of the tuning curve and the corresponding laser power would be too low to saturate the last ionization step for high efficiency. Therefore, a motivation for this work is to find three-step resonance ionization schemes that can use more powerful fundamental output of the Ti:Sapphire lasers for the ionization step. In addition, no three-step,
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/ downloads/doe-public-access-plan). ⁎ Corresponding author. E-mail address: [email protected] (Y. Liu). https://doi.org/10.1016/j.sab.2019.105640 Received 9 April 2019; Received in revised form 17 June 2019 Available online 25 June 2019 0584-8547/ © 2019 Published by Elsevier B.V.
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
The lasers used a diffraction grating for wavelength selection and were continuously tunable between 720 nm and 960 nm. Laser wavelengths in the blue to UV regions were obtained by frequency doubling, tripling, and quadrupling. The three laser beams were collimated and merged into a single laser beam inside the laser room, which was then transported to IRIS2 and focused over a distance of > 10 m into the 3-mm cavity of the ion source. The typical efficiency of laser transportation into the cavity was measured to be 70–80% after the beams were merged. Accordingly, the full width at half-maximum (FWHM) of the laser beams in the laser-atom interaction region was estimated to be 2.4–3 mm. The Ti:Sapphire laser beams were pulsed at 10 kHz with a pulse width of about 30 ns. They were synchronized by adjusting the external trigger timing to the individual pump lasers, with time jitters of smaller than 5 ns between the synchronized lasers. The radiative lifetime of the Zr atomic states involved in the study was estimated to be on the order of 10 ns for the first excited state and at least tens of nanoseconds for the second excited states based on our observations that desynchronizing the laser pulses up to 10 ns of one another had little effect on the ionization efficiency. Photoionization spectra were obtained by scanning the wavelength of a selected laser while recording the ion current measured at the FC after the mass separator. The laser wavelength could be scanned in the direction of increasing photon energy (scan-up) or decreasing photon energy (scan-down). It has been observed that there was a scan-direction dependent shift that the spectral line centroids obtained by scan-up and by scan-down were slightly different with an average difference of about 0.04 cm−1. To mitigate this effect, each spectrum was measured at least twice by scan-up and scan-down, respectively. The line centroids were then determined as the average values of the scan-up and scan-down measurements and the experimental statistical uncertainty was calculated as the standard deviation of the measurements. This procedure was used for all the spectra measured in this study. The fundamental wavelengths of the three Ti:Sapphire lasers were monitored simultaneously using a calibrated wavelength meter (HighFinesse WS6–600) equipped with a 4-channel opto-mechanical switcher. The uncertainty in measuring the laser wavelengths was dominated by the absolute accuracy of the wavelength meter, which is 600 MHz or 0.02 cm−1 according to 3-sigma criterion. This uncertainty was taken as the systematic error of our measurement.
Fig. 1. Schematic drawing of the ion source assembly and laser ionization configuration.
three-color resonance ionization of Zr with exclusively Ti:Sapphire lasers has been reported to our knowledge. Thus, this study will allow us to access high-lying atomic levels and autoionizing (AI) and Rydberg levels in Zr that have not been previously studied. The results should provide new atomic spectroscopic data for Zr useful to fundamental and applied applications such as modeling atomic and nuclear structures, stockpile stewardship science, cosmochemistry [33], and geochemistry [34]. 2. Experimental The experimental setup has been reported previously [30,35], so the relevant components for this study are briefly described here. A RILIS consisting of a hot-cavity ion source and a system of three tunable Ti:Sapphire lasers [36] was used for three-step ionization of Zr. The ion source was installed on the high-voltage platform of the Injector for Radioactive Ion Species 2 (IRIS2) facility [30] at ORNL, while the laser system was located in a laser room next to IRIS2. As shown in Fig. 1, the hot-cavity ion source consisted of a 30-mm long Ta cylindrical cavity of 3-mm inner diameter and a closed-end sample tube of 8.5-mm inner diameter and about 100-mm long, also made of Ta. The cavity and the sample tube were heated resistively in series by an electrical current applied to the Heater Bus. A small Zr metal foil of about 2 mm × 10 mm × 0.025 mm was folded and heated in the sample tube, producing atomic vapor of Zr which effused into the cavity where Zr atoms were ionized by laser beams entering the cavity through the extraction electrode. The ions were extracted from the cavity, accelerated to 40 keV, and transported to a magnetic mass separator. Zr has five stable isotopes and all of them were ionized by the lasers. The ions of the most abundant isotope 90Zr were mass selected and measured with a Faraday cup (FC) after the mass separator. The ion beam intensity before the mass separator was also monitored with another FC. During the measurements, the ion source was heated with 350 A electrical current, at which the Zr sample temperature was estimated to be between 1700 and 1800 K, while the cavity was on the order of 2100 K based on previous temperature measurements [37]. We did not need to increase the temperatures because the Zr ion beam current was enough for our measurements. The three Ti:Sapphire lasers were pumped by three individual Qswitched Nd:YAG lasers at 532 nm at a pulse repetition rate of 10 kHz.
3. Results and discussions 3.1. Ionization schemes Suitable transitions for the first-step excitation were studied using the Kurucz atomic line database [38]. Five candidates are listed in Table 1. The laser wavelengths for these transitions are available by frequency doubling. The first three transitions in the table start from the ground state a 3F2 or the first excited state a 3F3 at 570.41 cm−1, with λ1 between 382 and 387 nm, and require photons between 720 and 750 nm for the subsequent second and third steps to reach the ionization threshold, while the last two transitions with λ1 between 362 and 367 nm will use photons around 800 nm for the second and third steps. In comparison, the last two candidates were preferred for higher power
Table 1 Selected candidates for the first-step transition of three-step ionization schemes for Zr with following quantities: E0: energy of the lower level, E1: energy of the upper level, λ1: excitation laser wavelength, f: absorption oscillator strength expressed as log(gf) where g is the statistical weight of the lower level, and A21: atomic transition probability. Lower level
E0 (cm−1)
Upper level
E1 (cm−1)
λ1 (nm)
Log(gf)
A21 (s−1)
5s2 5s2 5s2 5s2 5s2
0 0 570.41 570.41 1240.84
5s5p x 3Do1 5s5p x 3Fo2 5s5p x 3Fo3 4d5s25p w 3Fo3 5s5p w 3Fo4
26,154.13 26,061.70 26,443.88 28,157.42 28,528.36
382.349 383.705 386.496 362.489 366.468
−0.86 −0.27 −0.24 −0.28 0.01
2.099E+7 4.866E+7 3.671E+7 3.806E+7 5.647E+7
a a a a a
3
F2 F2 3 F3 3 F3 3 F4 3
2
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
and temporally more stable laser pulses near 800 nm than at 720–750 nm. We finally selected the transition at λ1 = 366.468 nm, which has the largest absorption oscillator strength. A disadvantage for this transition is that the lower level is the a 3F4 state at 1240.84 cm−1 which is less populated than the 3F2 and 3F3 states. However, the hotcavity source is typically operated at high temperatures close to 2300 K. At 2300 K the 3F4 level is expected to be appreciably populated with about 29% of the Zr atoms, given by the Boltzmann distribution law, in comparison to 35% and 36% of the 3F3 and ground level populations, respectively. The selected first-step transition was measured experimentally by scanning the first laser (λ1) while other two lasers were fixed in wavelength for a resonant transition in the second step (see discussions next) and non-resonant ionization in the third step. The observed resonance peaks fit very well to a Gaussian profile with a central position of 27,287.511(51) cm−1 and FWHM of 0.53(2) cm−1. The uncertainty of the values was the standard deviation of the scan-up and scan-down measurements. Adding the energy of the a 3F4 level, we obtained E1 = 28,528.35(5) cm−1 for the first excited state 5s5p w 3F4o, in good agreement with the known value of 28,528.36 cm−1 [39]. Resonance transitions for the second step from the 3F4o level at E1 = 28,528.36 cm−1 were searched by scanning the second laser (λ2) between 744 and 836 nm, while the third laser wavelength (λ3) was fixed for non-resonant ionization. Eight resonance peaks were observed, as numbered in Fig. 2. The spectra are plotted as a function of E2 = E1 + hν2, where h is Planck's constant, ν2 is the second laser frequency, and E1 = 28,528.36 cm−1. The λ2 scan range corresponded to E2 = 40,490–41,969 cm−1 and the scan was repeated at three thirdphoton energies of 12,464.8, 12,614.6, and 12,625.7 cm−1, respectively. The E2 resonance centroids obtained with different third photon energies were found to be in excellent agreement. The observed peak intensity and linewidth may not reflect the relative transition strength because they could be affected by accidental coincidences where the
Table 2 Measured energies of the second excited states of even parity and the FWHM of the resonance transitions. The given uncertainties are the standard deviations of the scan-up and scan-down measurements, not including the systematic error of 0.02 cm−1. Peak #
E2 (cm−1)
FWHM (cm−1)
λ2 (nm)
1 2 3 4 5 6 7 8
41,162.17(4) 41,177.00(3) 41,179.44(3) 41,296.14(3) 41,314.00(2) 41,560.96(3) 41,694.12(3) 41,823.83(3)
0.37(8) 0.34(6) 0.34(9) 0.21(2) 0.27(2) 0.56(2) 0.48(2) 0.49(3)
791.53 790.60 790.45 783.22 782.13 767.31 759.55 752.14
third photon was on a resonance transition to an AI state. In fact, the three third-laser photons used here were later found to be on or near a λ3 resonance peak from some of the E2 states. Table 2 gives the mean energies of these E2 states, FWHM, and the excitation wavelengths for individual transitions. These even-parity states have not been previously reported, except that peak #3 at E2 = 41,179.44(3) cm−1 is very close to the 4d25s(4F) 5d e 5G4 level at 41,179.30 cm−1 in literature [39]. The 3F4 – 5G4 transition is semi-forbidden, but such intercombination transitions have previously been observed [25]. On the other hand, we did not observe the transitions to the 5G3 and 5G5 levels at 40,887.61 and 41,538.23 cm−1, respectively, which were also within the λ2 scan range. In addition, peak #3 was a relatively strong transition, as shown in Fig. 2(b). Hence, it is possible that this peak was not the 5G4 state. For dipole-allowed transitions, these new E2 states may have configurations of 4d25s6s and 4d25s5d and, for Russell-Saunders (LS) coupling, possible terms of 3D3, 3F3,4, and 3G3,4,5, but the assignment could not be determined by the spectra alone. At 2100 K, the doppler broadening was calculated to be approximately 2.8 GHz for the first-step transition and 1.3 GHz for the second and third transitions. The laser beam linewidth was on the order of 3–4 GHz for fundamental light and 4–5 GHz for frequency-doubled light, larger than the Doppler broadening. In Table 2, the FWHM of the measured spectral lines ranges from 6 to 15 GHz, significantly larger than the laser linewidths. As to be discussed later in section 3.4, the first- and second-step resonance transitions could be well saturated with the laser power used. Therefore, the linewidth of the spectral peaks was usually dominated by power broadening. Three-step ionization via the first two states in Table 2 has been investigated. As illustrated in Fig. 3, Zr atoms were excited by three lasers from the 5s2 3F4 state at 1240.84 cm−1 to the 5s5p 3Fo4 state at 28,528.36 cm−1, subsequently to levels at E2 = 41,162.17 cm−1 (λ2 = 791.95 nm) or 41,177.0 cm−1 (λ2 = 790.6 nm), and from there to ionization. The third laser was scanned to search for resonant transitions to high lying Rydberg and autoionizing states. The first excitation was obtained by frequency-doubled light and the second and third transitions were accomplished with the fundamental laser light between 720 and 960 nm. The selectivity of the two three-step excitation schemes was checked by blocking the individual lasers. It was found that approximately 97% of the laser ionized ions were produced by three-step, three-photon ionization, the other 3% were contributed by λ1 + λ2 photons (2%) or λ1 + λ3 photons (1%) alone.
3.2. Photoionization spectra Figs. 4 and 5 show the observed photoionization spectra from the two second intermediate states. The measured ion currents are plotted as a function of the total energy E3 of the three excitations, which is obtained by adding the third photon energy to the energy of the second excited state. The ion currents obtained with E2 = 41,177.0 cm−1
Fig. 2. Measured λ2 scan spectra (a) with eight resonance peaks observed and (b) containing the peaks #1 – #3, while the third photon energy was fixed to 12,614.6 cm−1. The spectrum in (b) was a separate scan under improved ion source operation conditions. 3
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
3.3. Analysis of Rydberg series The lower level of the third transition is expected to have 4d25s6s or 4d 5s5d configurations. Thus, the Rydberg series observed could be the 5snp or 5snf series of odd parity. To analyze the Rydberg spectra, the centroids and FWHMs of the resonances were determined by fitting to a line profile function. Most of the peaks are narrow and symmetric and can be fit to Gaussian profiles. Fano profiles are used for asymmetric autoionizing peaks. Again, the final line positions were given by the mean values of the scan-up and scan-down measurements and the standard deviation of the mean value as the experimental statistical uncertainty. The convergence limit of a Rydberg series is found by fitting the observed series members to the second-order Rydberg-Ritz formula 2
En = Elimit −
RM (n∗)2
n∗ = n − δ (n)
δ (n) = δ0 +
a (n − δ0 )2
(1) (2) (3)
where En is the measured centroid energy of the Rydberg peak of index n, Elimit is the series converging limit, RM = 109,736.646 cm−1 is the mass corrected Rydberg constant for 90Zr, n* is the effective quantum number, δ(n) is the quantum defect, δ0 is the asymptotic limiting value of the quantum defect, and a is a constant. The integer index n is chosen to be the nearest integer larger than n*. Three odd-parity Rydberg series were observed from the E2 = 41,177.0 cm−1 level, one series converging to the ground state (4F3/2) of Zr+, and two series converging to the first (4F5/2) and second (4F7/2) excited states of Zr+, respectively, as shown in Fig. 4. They are labeled as Series 1, 2 and 3 with the observed ranges of the index number n as given below.
Fig. 3. Two three-step ionization schemes investigated. The third laser wavelength (λ3) was scanned over energies across the first ionization potential (IP) of Zr.
(Fig. 4) were smaller than those from E2 = 41,162.17 cm−1 (Fig. 5), but the absolute intensities of the spectra depended on the experimental conditions, such as the ion source temperature, ion beam transmission, and laser power, etc., which were not the same when measuring the two spectra. The relative intensity of the spectrum lines is also a rough indication of their transition strengths, as it could be affected by unknown fluctuations in laser power, temporal and spatial overlapping between the laser pulses during the scan. In addition, the spectra have not been corrected for the third laser power dependence on wavelength throughout the scan range. There are many resonance peaks in each spectrum and most of the peaks could be assigned to Rydberg series of different convergence limits. Three relatively intense Rydberg series are indicated in each spectrum. We used the following general criteria to identify resonance peaks from background fluctuations: (1) A peak should be observed in repeating scan-up and scan-down spectra and at about the same energy positions. (2) Peak height is at least five times of the standard deviations of the baseline fluctuation. (3) There are more than one data points in the peak.
Series 1: converging to 4F3/2 of Zr+, n = 28–60. Series 2: converging to 4F5/2 of Zr+, n = 20–73. Series 3: converging to 4F7/2 of Zr+, n = 11–31. In the spectrum from the E2 = 41,162.17 cm−1 level (Fig. 5) five odd-parity Rydberg series were identified as listed below. Three were relatively strong with convergences being the 4F5/2, 4F7/2, and the third excited state 4F9/2 of Zr+, respectively. They are labeled as Series 4–6 and are indicated in Fig. 5. The other two were weaker series labeled as Series 7 and 8. Series 4: converging to 4F5/2 of Zr+, n = 15–76. Series 5: converging to 4F7/2 of Zr+, n = 10–64.
Fig. 4. Photoionization spectrum from the E2 = 41,177.0 cm−1 state. Comb lines indicate the identified members of three Rydberg series converging to the ground (4F3/2), first excited (4F5/2), and second excited (4F7/2) states of Zr II, respectively. Vertical dashed lines mark the positions of the series limits. 4
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Fig. 5. Photoionization spectrum from the E2 = 41,162.17 cm−1 state. Comb lines indicate the identified members of three Rydberg series converging to the first excited (4F5/2), second excited (4F7/2) and third excited (4F9/2) states of Zr II, respectively. Vertical dashed lines mark the positions of the series limits.
Series 6: converging to 4F9/2 of Zr+, n = 9–42. Series 7: converging to 4F5/2 of Zr+, n = 14–36. Series 8: converging to 4F7/2 of Zr+, n = 19–27. Some series members could not be unambiguously identified due to overlapping with other peaks and perturbations in the series, as to be discussed later. The observed Rydberg states of these series are summarized in Tables S1 – S5 in the Appendix. It is noticed that the series converging to the ground state 4F3/2 of + Zr was only excited from the E2 = 41,177.0 cm−1 state (Series 1) and was a very weak series, as shown in Fig. 4. Members of Series 1 in the range of n = 28–60 were observed, but the n = 40–42 peaks could not be determined due to the large perturbation around 53,440 cm−1 and n = 55 and 59 peaks were also not identified due to overlapping with much larger peaks. The series converging to the third excited state 4F9/2 of Zr+ was only observed from the 41,162.17 cm−1 level (Series 6) and was a relatively weak series as well (Fig. 5). Inter-channel interactions among the overlapping Rydberg series are noticeable in both spectra. In comparison, Series 4–6 from the 41,162.17 cm−1 level showed relatively large disturbances, as evidenced by substantial intensity irregularities in the series (Fig. 5). Clusters of peaks were observed when the lower members (n = 9–13) of the Series 6 interacted with other series. Fig. 6 displays the spectra
Fig. 7. Portion of the photoionization spectra containing the cluster of spectral lines at around 53,710 cm−1 and the peaks of Series 2–7 as indicated. ( ̶ • ̶) Spectrum from E2 = 41,177.0 cm−1 with members of Series 2 and 3, (—) spectrum from E2 = 41,162.17.0 cm−1 members of Series 4, 5, 6, and 7.
containing the cluster around 53,440 cm−1 where n = 9 member of Series 6 and n = 17 member of Series 2 or 4 met, and Fig. 7 shows the cluster centered at about 53,710 cm−1 that involved the n = 10 member of Series 6 and the n = 14 member of Series 3 or 5. Another relatively profound cluster was around 54,170 cm−1 (Fig. 5) corresponding to interactions with the n = 13 member of Series 6. As shown in Figs. 6 and 7, the clusters consist of complicated spectral structures that are different from the nearby unperturbed Rydberg members, from cluster to cluster, and from different starting lower levels. Hence, assignments for those fine structures are not possible. Such clustering features associated with the Series 6 have been observed by Page et al. [25] who reported clumps of overlapping spectra in three-step photoionization of Zr, even though they used different excitation transitions, including the clumps centered at 53,500, 53,710, and 54,170 cm−1. They attributed the formation of the spectral clumps to Rydberg-valence mixing and assigned the clump centers at 53,500, 53,710, and 54,170 cm−1 as the n* ≈ 9, 10, and 13 members of the Rydberg series converging to the third excited state 4F9/2 of Zr+, which is consistent with our observation. 3.3.1. Series converging to 4F5/2 of Zr+ Three Rydberg series were observed to converge to the first excited state 4F5/2 of Zr+: Series 2 excited from the 41,177.0 cm−1 state and Series 4 and 7 excited from the 41,162.17 cm−1 state. Perturbations
Fig. 6. Portion of the photoionization spectra showing peaks of Series 1 (circles) in the range n = 28–52 and the cluster of spectral lines at around 53,440 cm−1. The n = 40–42 members of Series 1 were disturbed by the cluster and could not be unambiguously identified. 5
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Fig. 9. Quantum defect modulo 1 versus the effective quantum number n* of Series 2, 4, 7 and the unidentified series suddenly appearing after n = 25 (Fig. 7). The δ +1 values of these series, except Series 2, are also displayed in order to show the merging between them. Solid line is the fit to Eq. (4).
51 could be described by a modification of the quantum defect given by [41,42]
δ ′ (n) = δ (n) −
1 Γ/2 ⎞ arctan ⎛⎜ ⎟ π E n ⎝ − Ep ⎠
(4)
where Ep is the energy and Γ is the spectral width of the perturbing state. The fitted quantum defect is also shown in Fig. 9, which gives Ep = 53,780.83(17) cm−1 or np* = 51.3(1).
Fig. 8. Expanded spectra showing the fine structures of low n members of Series 2, 4, and 7. ( ̶ • ̶) Spectrum from E2 = 41,177.0 cm−1 with members of Series 2, (—) spectrum from E2 = 41,162.17.0 cm−1 with members of Series 4 and 7.
3.3.2. Series converging to 4F7/2 of Zr+ Three Rydberg series converging to the second excited state 4F7/2 of + Zr were also observed: Series 3 excited from the 41,177.0 cm−1 level and Series 5 and 8 excited from the 41,162.17 cm−1 level. The Rydberg levels of Series 3 have slightly lower centroid energies than those of Series 5, as shown in Fig. 10. Also shown in Fig. 10 are the peaks of the weak Series 8. The lower members (n = 10–17) of Series 5 are also found to have different fine structures than those of higher members. Fig. 11 shows the calculated δ vs n* for Series 3 and 5 and 8. Perturbations are noticeable in Series 5 at n* ≈ 16–17 and n* ≈ 27–28, by unknown interlopers, and at n* ≈ 33–34 by the n = 13 member of
modified the series from the 41,162.17 cm−1 state. For example, a few intense members of an unidentified series suddenly appeared after the n = 25 peak of Series 4 (Fig. 7) and soon merged with Series 4 before the cluster at 53,710 cm−1. Also shown in Fig. 7 are members of the Series 7, which were typically very weak but became the dominant peaks after the cluster. The n = 20–35 peaks of Series 2 and 4 were observed at about the same energies, indicating that they could be the same Rydberg levels excited from two lower levels. However, after the cluster, Series 7 merged with Series 4 at n > 35 and the resulting peaks shifted to slightly higher centroid energies than the peaks of Series 2 (Fig. 7). That is, the n > 35 peaks of Series 4 were different Rydberg levels than those of Series 2. For n < 20, both Series 2 and 4 showed fine structures different than higher members and different between the two series as well (Fig. 8). For each fine structure group, the peaks that appeared in both series at about the same energy are selected as the series members. However, it is possible that these low members (n < 20) may not be correctly interpreted. Fig. 9 shows the quantum defects of Series 2, 4 and 7 as well as the unidentified series peaks as a function of n*. The effective quantum number n* is calculated from Eq. (1) and the quantum defect is δ = n – n* according to Eq. (2). Since n is the nearest integer larger than n*, δ is in fact the modulo 1 (mod 1) value of the real quantum defect. The average δ value is about 0.1 for Series 2 and 4 and about 0.98 for Series 7. The data show the presence of perturbations around n* ≈ 35–35, 39, and 51 in Series 4 and the merge of the unidentified series and Series 7 with Series 4. The perturbation at n* ≈ 35–36 could be due to the n = 10 member of Series 6, at n* ≈ 39 is due to an unknown interloper, and at n* ≈ 51 is due to the n = 15 member of Series 5. The data could be analyzed using the multichannel quantum defect theory (MQDT) [40], but it is out of the scope of this study. On the other hand, a large fraction of the Rydberg levels is not perturbed so we can use the singlechannel quantum defect analysis. The relatively large deviation at n* ≈
Fig. 10. Photoionization spectra containing members of Series 3, 5 and 8. ( ̶ • ̶) Spectrum from E2 = 41,177.0 cm−1 with members of Series 3, (—) spectrum from E2 = 41,162.17.0 cm−1 with members of Series 5, 6, and 8. 6
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Fig. 11. Quantum defect modulo 1 versus the effective quantum number n* of Series 3, 5, and 8.
Fig. 12. Rydberg-Ritz fit of Series 2 with n = 20–48 and 51–73 members. (Top) Measured energies (dots) of the Rydberg states and calculated energies (solid line) using the fitted parameters. (Bottom) Fit residuals with experimental statistical uncertainties as the error bars.
Series 6 at around 54,170 cm−1. Similar to Series 7, Series 8 has an average δ of about 0.97, different than that of Series 3 and 5. The observed Rydberg series are expected to have 4d25s(4F)np or 4d25s(4F)nf configurations. The different quantum defect (mod 1) values suggest that Series 1–6 have the same configuration which may be different than that of Series 7–8. However, specific assignments could not be made on the base of the quantum defects alone. 3.3.3. Series limits and ionization potential of Zr Series 1–6 are fitted to the Rydberg-Ritz formula of Eqs. (1)–(3) to determine the convergence limit and δ0 value for each series. For Series 1, the fit uncertainty for the parameter a is quite large, and it is found that this series is best fit to a constant δ(n) = δ0. The results are summarized in Table 3. By subtracting the known energy of the excited states of Zr+ with respect to the ground state of Zr+ from the best-fit convergence limits, we obtained the IP value for Zr from each series as given in the last column of the table. We did not attempt to fit Series 7 because it merged into Series 4. Series 8 was too short to fit. Not all the assigned members of the series are used in the fits. As an example, Fig. 12 shows the Rydberg-Ritz fit for Series 2 with only n = 20–73 peaks and the residuals of the fit, which gave the best fit result. Similarly, the n = 10–17 and n = 27 members of Series 5 are not included in fitting. For Series 4, the perturbation at n* ≈ 51 (Fig. 9) is taken into account by combining Eqs. (3) and (4) for the Rydberg-Ritz fit. The resulting fit residuals are compared with those without Eq. (4) in Fig. 13. The changes in the best-fit values for Elimit and δ0 after considering the perturbation are found to be very small. Therefore, we have ignored the other perturbations in all series for the determination of series convergence. The IP values derived from Series 1–5 are very close, but the value from Series 6 is about 0.2 cm−1 smaller. The reason is unknown and needs further investigation. We used the results of Series 1–5 to
Fig. 13. Residuals of the Rydberg-Ritz fit for Series 4, (a) without and (b) with the use of Eq. (4) for the perturbation at n* ≈ 51.
determine the final value for the IP of Zr. The average of the five individual data gives IP = 53,507.832(35)stat(20)sys cm−1, including one standard deviation of statistical uncertainty and the systematic uncertainty of 0.02 cm−1. This IP value agrees with the recent result by Hasegawa et al. [26], but our uncertainty is an order of magnitude lower than the previous measurement.
Table 3 Results of Rydberg-Ritz fit for series 1–6. The given uncertainties are the standard deviation of the fit. Last column is the value of IP derived from the series limits. Series
Limit level
Elimit (fit) (cm−1)
δ0
a
IP (cm−1)
1 2 3 4 5 6
a4F3/2 a4F5/2 a4F7/2 a4F5/2 a4F7/2 a4F9/2
53,507.812 (22) 53,822.507(4) 54,271.249(92) 53,822.517(11) 54,271.298(24) 54,830.571(69)
0.093(5) 0.1115(17) 0.1109(45) 0.0841(24) 0.1194(69) 0.1032(28)
−7.83(75) −2.36(55) 5.51(60) −14.6(24) −0.42(25)
53,507.812(22) 53,507.837(4) 53,507.809(92) 53,507.847(11) 53,507.858(24) 53,507.661(69)
7
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
power density in the interaction region was on the order of 0.12, 1.6, and 7 W/cm2 for the first, second, and third excitations, respectively. 4. Summary Three-step resonant ionization of atomic Zr using Ti:Sapphire lasers has been demonstrated. For the first step, we have selected the transition from the 5s2 a 3F4 state at 1240.84 cm−1 to the 5s5p w 3F4o excited state at 28,528.36 cm−1, which has a large absorption oscillator strength. For the second step, eight new excited states were observed. Two of them were used to explore resonance transitions for the third ionization step. We have observed > 350 resonance ionization and autoionizing transitions. Eight new Rydberg series of odd-parity and about 280 Rydberg states in total have been identified to converge to the ground and the first, second, and third excited states of singlycharged Zr ion. These series are expected to have 4d25s(4F)np or 4d25s (4F)nf configurations based on their convergence levels. The convergence limits and limiting quantum defects for six of the series are obtained by Rydberg-Ritz fitting. The analysis allowed us to determine the first ionization potential of Zr to be 53,507.832(35)stat(20)sys cm−1 with an order of magnitude improvement in uncertainty over the previous measurements. Saturation measurements for one of the three-step transition ladders showed that the selected three steps can be easily saturated with low to moderate laser powers. More spectroscopy studies using other intermediate states in Table 2 and evaluation of the ionization efficiencies of different three-step schemes will be the subject of future work. Acknowledgements This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics; in part by National Nuclear Security Admission, Grant No. DE-NA0002924; the National Science Foundation, Grants No. PHY-15-65546, and this research used resources of the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory, which was a DOE Office of Science User Facility.
Fig. 14. Saturation curves of the first, second, and third excitations via E2 = 41,162.17 cm−1 level. Dots represent the measured data and lines are fits to Eq. (5).
3.4. Saturation measurement
Appendix A. Supplemental data
The laser power used in the present study was 100–150 mW frequency-doubled light for the first excitation and 1–2 W fundamental light for the second and third excitations. The corresponding laser power density at the interaction region in the hot-cavity was on the order of 1.5–2 W/cm2 for the first laser and 15–28 W/cm2 for the second and third lasers. We measured the laser power needed to saturate each excitation step for the ionization scheme using the E2 = 41,162.17 cm−1 intermediate level since this scheme seemed to result in higher ion currents. For the saturation measurement, the third step was the transition to an unidentified AI level at E3 = 53,600.14 cm−1 that gave the largest ion current (Fig. 5). The Zr ion current was measured as a function of the laser power for each excitation and the data were then fitted to the formula
I = I0 + A
P P + Psat
References [1] J. Ebert, R.A. Meyer, K. Sistemich (Eds.), Nuclear Structure of the Zirconium Region, Proceedings of the International Workshop, Bad Honnef, Fed. Rep. of Germany, April 24–28, 1988, Springer-Verlag, Berlin, 1988. [2] S.J. Zheng, F.R. Xu, S.F. Shen, H.L. Liu, R. Wyss, Y.P. Yan, Shape coexistence and triaxiality in nuclei near 80Zr, Phy. Rev. C 90 (2014) 064309. [3] F. Browne, A.M. Bruce, T. Sumikama, I. Nishizuka, S. Nishimura, et al., Lifetime measurements of the first 2+ states in 104,106Zr: evolution of ground-state deformations, Phys. Lett. B 750 (2015) 448–452. [4] S. Ansari, J.-M. Régis, J. Jolie, N. Saed-Samii, N. Warr, et al., Experimental study of the lifetime and phase transition in neutron-rich 98,100,102Zr, Phy. Rev. C 96 (2017) 054323. [5] E.T. Gregor, M. Scheck, R. Chapman, L.P. Gaffney, J. Keatings, et al., Shell evolution of stable N = 50–56 Zr and Mo nuclei with respect to low-lying octupole excitations, Eur. Phys. J. A 53 (2017) 50. [6] T. Togashi, Y. Tsunoda, T. Otsuka, N. Shimizu, Quantum phase transition in the shape of Zr isotopes, Phys. Rev. Lett. 117 (2016) 172502. [7] H. Mei, J. Xiang, J.M. Yao, Z.P. Li, J. Meng, Rapid structural change in low-lying states of neutron-rich Sr and Zr isotopes, Phy. Rev. C 85 (2012) 034321. [8] K. Sieja, F. Nowacki, K. Langanke, G. Martínez-Pinedo, Shell model description of zirconium isotopes, Phy. Rev. C 79 (2009) 064310. [9] L. Ahle, L.A. Bernstein, M. Hausmann, D.J. Vieira, Science based stockpile stewardship and RIA, Proceedings of Sixth International Meeting on Nuclear Applications of Accelerator Technology, American Nuclear Society, 2003, pp. 286–291. [10] M.N. Kreisler, National nuclear security and other applications of rare isotopes, Proceedings of Particle Accelerator Conference (PAC07), Albuquerque, New Mexico, 2007, pp. 124–126 June 25–29. [11] E. Hartouni, Role of radioactive beams in stockpile stewardship science, 240th American Chemical Society Fall National Meeting, 2010 August 22–26. (Boston,
(5)
where I is measured ion current, P is measured laser power, Psat is the saturation laser power, I0 is the background ion current, and A is a constant. The fitting results are presented in Fig. 14. From the fittings we obtained Psat = 11.4(11), 150(11), and 630(75) mW for the first, second, and third excitations, respectively. This means that the selected three-step transitions can be saturated with laser powers that are easily available. It should be pointed out that the laser power was measured in the laser room before the laser was sent into IRIS2. From that point, typically 70–80% of the laser power was injected into the ion source. That is, the actual saturation power Psat for the excitation steps was about 70–80% of the above fitted values. The corresponding saturation 8
Spectrochimica Acta Part B 158 (2019) 105640
Y. Liu, et al.
Massachusetts). [12] T. Sumikama, K. Yoshinaga, H. Watanabe, S. Nishimura, Y. Miyashita, et al., Structural evolution in the neutron-rich nuclei 106Zr and 108Zr, Phys. Rev. Lett. 106 (2011) 202501. [13] T. Sumikama, Decay spectroscopy of neutron-rich nuclei in the vicinity of 110Zr at RIBF, Acta Phys. Pol. B 44 (2013) 319–325. [14] P. Campbell, J. Billowesa, B. Chealb, K.T. Flanagana, D.H. Forest, et al., Laser spectroscopy of radioactive Ti, Zr and Hf isotopes and isomers at the JYFL laserIGISOL facility, Spectrochim. Acta B 58 (2003) 1069–1076. [15] H.L. Thayer, J. Billowes, P. Campbell, P. Dendooven, K.T. Flanagan, et al., Collinear laser spectroscopy of radioisotopes of zirconium, J. Phys. G 29 (2003) 2247–2262. [16] J.R. Beene, D.W. Bardayan, A. Galindo-Uribarri, C.J. Gross, K.L. Jones, J.F. Liang, W. Nazarewicz, D.W. Stracener, B.A. Tatum, R.L. Varner, ISOL science at the Holifield Radioactive Ion Beam Facility, J. Phys. G 38 (2011) 024002. [17] P.A. Hackett, H.D. Morrison, O.L. Bourne, B. Simard, D.M. Rayner, Pulsed singlemode laser ionization of hyperfine levels of zirconium-91, J. Opt. Soc. Am. B 5 (1988) 2409–2416. [18] L.W. Green, G.A. McRae, P.A. Rochefort, Selective resonant ionization of zirconium isotopes using intermediate-state alignment, Phy. Rev. A 47 (1993) 4946–4954. [19] H. Niki, T. Kuroyanagi, Y. Horiuchi, S. Tokita, Laser isotope separation of zirconium for nuclear transmutation process, J. Nucl. Sci. Technol. 6 (Suppl) (2008) 101–104. [20] H. Niki, T. Kuroyanagi, K. Motozu, Isotopic selectivity in two-color multiphoton ionization spectroscopy of zirconium, Jpn. J. Appl. Phys. 48 (2009) 116510. [21] H. Niki, Two-color ionization spectroscopy and laser isotope separation of zirconium for efficient transmutation of long-lived fission products, J. Korean Phys. Soc. 56 (2010) 190–194. [22] T. Fujiwara, T. Kobayashi, K. Midorikawa, Selective resonance photoionization of odd mass zirconium isotopes towards efficient separation of radioactive waste, Sci. Rep. 9 (2019) 1754. [23] D.R. Spiegel, W.F. Calaway, G.A. Curlee, A.M. Davis, R.S. Lewis, M.J. Pellín, D.M. Gruen, R.N. Clayton, Three-color and 1 + 1 resonance ionization mass spectrometry of zirconium sputtered from refractory carbides, Anal. Chem. 66 (1994) 2647–2655. [24] P.A. Hackett, M.R. Humphries, S.A. Mitchell, D.M. Rayner, The first ionization potential of zirconium atoms determined by two laser, field ionization spectroscopy of high lying Rydberg series, J. Chem. Phys. 85 (1986) 3194–3197. [25] R.H. Page, S.C. Dropinski, E.F. Worden, J.A.D. Stockdale, Progress in zirconium resonance ionization spectroscopy, Proc. SPIE Vol. 1859, Laser Isotope Separation, 1993, pp. 49–63. [26] S. Hasegawa, D. Nagamoto, Two-color laser resonance ionization spectroscopy of zirconium atoms, J. Phys. Soc. Jpn. 86 (2017) 104302. [27] J. Lassen, P. Bricault, M. Dombsky, J.P. Lavoie, M. Gillner, T. Gottwald,
[28] [29] [30]
[31]
[32]
[33]
[34]
[35]
[36] [37] [38] [39]
[40] [41]
[42]
9
F. Hellbusch, A. Teigelhöfer, A. Voss, K.D.A. Wendt, Laser ion source operation at the TRIUMF radioactive ion beam facility, AIP Conf. Proc. 1104 (2009) 9–15. C. Geppert, Laser systems for on-line laser ion sources, Nucl. Instr. Meth. B 266 (2008) 4354–4361. N. Lecesne, Laser ion sources for radioactive beams, Rev. Sci. Instrum. 83 (2012) 02A916. Y. Liu, C.U. Jost, A.J. Mendez II, D.W. Stracener, C.L. Williams, C.J. Gross, R.K. Grzywacz, M. Madurga, K. Miernik, D. Miller, S. Padgett, S.V. Paulauskas, K.P. Rykaczewski, M. Wolinska-Cichocka, On-line commissioning of the HRIBF resonant ionization laser ion sourc, Nucl. Instr. Meth. B 298 (2013) 5–12. J.G. Barzyk, M.R. Savina, A.M. Davis, R. Gallino, F. Gyngard, S. Amari, E. Zinner, M.J. Pellin, R.S. Lewis, R.N. Clayton, Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC, Meteorit. Planet. Sci. 42 (7/8) (2007) 1103–1119. H. Tomita, Latest developments on the narrow bandwidth laser systems for high resolution resonance ionization spectroscopy, LA3NET Laser Ion Source Workshop, 2016 24–25 October. (Paris, France). T. Stephan, R. Trappitsch, A. Davis, M. Pellin, D. Rost, M. Savina, R. Yokochi, N. Liu, CHILI – the Chicago Instrument for Laser Ionization – a new tool forisotope measurements in cosmochemistry, Int. J. Mass Spectrom. 407 (2016) 1–15. E.C. Inglisa, J.B. Creecha, Z. Denga, F. Moyniera, High-precision zirconium stable isotope measurements of geological reference materials as measured by doublespike MC-ICPMS, Chem. Geol. 493 (2018) 544–552. T. Kron, Y. Liu, S. Richter, F. Schneider, K. Wendt, High efficiency resonance ionization of palladium with Ti:sapphire lasers, J. Phys. B Atomic Mol. Phys. 48 (2016) 185003. Y. Liu, D.W. Stracener, A resonant ionization laser ion source at ORNL, Nucl. Instr. Meth. B 376 (2016) 68–72. Yuan Liu, ORNL developments in laser ion sources for radioactive ion beam production, Hyperfine Interact. 227 (2014) 85–99. Atomic Line Data (R.L. Kurucz and B. Bell) Kurucz CD-ROM No. 23, Smithsonian Astrophysical Observatory, Cambridge, MA, 1995. A. Kramida, Yu. Ralchenko, J. Reader, NIST ASD Team, NIST Atomic Spectra Database (ver. 5.6.1), [Online], Available https://physics.nist.gov/asd 2019, May 10 National Institute of Standards and Technology, Gaithersburg, MD, 2018, https://doi.org/10.18434/T4W30F. M.J. Seaton, Quantum defect theory, Rep. Prog. Phys. 46 (1983) 167–257. J. Roßnagel, S. Raeder, A. Hakimi, R. Ferrer, N. Trautmann, K. Wendt, Determination of the first ionization potential of actinium, Phys. Rev. A 85 (2012) 012525. U. Fano, Effects of configuration interaction on intensities and phase shifts, Phys. Rev. 124 (1961) 1866–1878.