Spectroscopic investigation of autoionizing Rydberg states of palladium accessible after odd-mass-selective laser excitation

Excitation of palladium to autoionizing Rydberg states are investigated in order to realize efficient ionization after odd-mass-selective excitation. Wavelength resolution is sufficient to distinguish two autoionizing Rydberg series: 4d9(2D3/2)np and 4d9(2D3/2)np converging to Pd II 4d9(2D3/2) as is expected from transition selection rules. Of all observed transitions, the largest autoionization yield is found to be through the 4d9(2D3/2)11p state at 69055.2 cm−1. From fitting analysis of the observed members of the 4d9(2D3/2)np series, we have obtained the energy of Pd II 4d9(2D3/2) to be 70780.38 ± 0.08 cm−1.

I t is of great and timely importance to address the environmental and economic consequences of accumulating quantities of high level radioactive waste produced from nuclear power plants. 1) Many of these fission products have relatively short half-lives of some decades, and simply storage for a period of time is an effective technique for inactivation. However, the presence of long-lived fission products (LLFP) having a very long lifetime (of order millions of years) necessitates development of a new technology. 2,3) Laser isotope separation had been applied to separate the radioactive uranium isotope 235 U from the natural mixture (with far more abundant 238 U) utilizing isotope shifts in the absorption spectrum. 4) However many LLFP have small isotope shifts, rendering the technique of frequency selective excitation and ionization unsuitable. 5) Additionally, in the case of non-naturally occurring LLFP, e.g., 107 Pd, laser isotope separation cannot be applied because no spectroscopic information is available. We are developing new separation technique based on palladium isotopes that uses a combination of two excitation processes: (1) Selective excitation of radioactive isotopes based on optical selection rules. Many LLFP have odd mass numbers and non-zero nuclear spins, e.g., 79 Se (I = 7=2), 93 Zr (I = 5=2), and 107 Pd (I = 5=2). 6) A selective excitation and ionization technique that does not require narrow-band tuning to small isotope shifts was proposed in the 1970s to separate odd-mass number radioactive isotopes from even-mass number non-radioactive isotopes. [7][8][9] Specifically, this method exploits the hyperfine structure of the electronic states derived from non-zero nuclear spin of odd-mass number isotopes.
(2) Excitation to autoionizing states to achieve efficient ionization. Resonant excitation to autoionizing states is a promising means to obtain efficient ionization after massselective excitation, where the yield is over an order of magnitude higher than excitation to the ionization continuum. 10) In the case of palladium (Pd), efficient ionization through resonant excitation to autoionizing Rydberg states after mass-selective excitation was proposed but has not to-date been demonstrated. 11) Through the investigation of autoionizing Rydberg states of palladium, we aim to find out the most efficient transition for autoionization after two-step odd-mass number selective excitation.
Investigation of the autoionizing Rydberg states of Pd I was initiated by theoretical studies 12,13) and followed by experimental studies. [14][15][16][17][18] Due to the spin-orbit interaction, palladium ion Pd II has two electronically ground states, namely 4d 9 ( 2 D 5=2 ) and 4d 9 ( 2 D 3=2 ), and the former is lower in energy than the latter. 14) We thus expect autoionizing Rydberg states of Pd I between those two ground states of Pd II.
From optical transition selection rules, direct photoexcitation from the ground state of Pd I is expected to give three autoionizing Rydberg series: 4d 9 ( 2 D 3=2 )np½1=2 o 1 , 4d 9 ( 2 D 3=2 )np½3=2 o 1 , and 4d 9 ( 2 D 3=2 )nf½3=2 o 1 designated in the J c K coupling scheme. [14][15][16][17] From photoionization measurements using synchrotron radiation, Karamatskos et al. observed a single autoionizing Rydberg series converging to the 4d 9 ( 2 D 3=2 ) state of Pd II. 14) However, the spectral bandwidth of the synchrotron radiation used in the experiment was too large to resolve the three possible Rydberg series. Callender et al. observed high-lying autoionizing Rydberg series by two-photon excitation through the intermediate 4d 9 ( 2 D 3=2 )5p state. 15) Two Rydberg series with quantum defects of ca. 0.0 and 0.5 were observed but no further analysis was presented. Baig et al. measured XUV absorption spectrum of Pd I and observed unresolved autoionizing Rydberg series, 4d 9 ( 2 D 3=2 )np½1=2 o 1 and 4d 9 ( 2 D 3=2 )np½3=2 o 1 (n = 8-17), with quantum defects of ca. 3.0. 16,17) Recently, Kron et al. reported efficient ionization of Pd through excitation to autoionizing Rydberg states via three different intermediate states, although the Rydberg series was not fully described and only one autoionizing state with n + = 9 of 4d 9 ( 2 D 3=2 )np or 4d 9 ( 2 D 3=2 )nf was tentatively assigned. 18) The two-step scheme used in this work for selective excitation of odd-mass isotopes of Pd I using circularly polarized laser pulses followed by a third laser to autoionizing Rydberg states is shown in Fig. 1. 9) As we have reported recently, the two circularly polarized lasers are equivalent to two linearly polarized orthogonally aligned laser pulses, which can reduce technical difficulties. 19) In the present study, we aim to investigate resonant transition to autoionizing Rydberg states via two intermediate states 4d 9 5p( 2 D 5=2 ) and 4d 9 5d( 2 D 5=2 ) by scanning the third laser frequency (ω 3 ). For the development of efficient odd-mass isotope separation process, it is of great importance determine which transition gives the largest ionization yield.
The experimental setup is similar to that used in the previous study. 19) The outputs of three dye lasers (Lambda Physik FL-3002 × 2, Lumonics HD-500) pumped by an excimer laser (Lambda Physik Compex 110) have been used for excitation. Spectral bandwidth is 0.3 cm −1 , which sets the lower limit on energy level bandwidth measurements. Wavelength of the dye lasers was measured with a wavemeter (Coherent WaveMate Deluxe) at 0.001 nm wavelength resolution, calibrated against a neon optogalvanic cell (Hamamatsu Laser Galvatron L2783). Laser power was calibrated with a power meter (Coherent FieldMaster gs with an LM-P2 detector) and monitored with a nanosecond photodetector (Newport 1621) during frequency scan.
Resonant excitation to the intermediate states are confirmed by optimization of the emission intensity detected with a photomultiplier tube (Hamamatsu R955) through a monochromator (Jasco CT-25C).
Ions produced after autoionization are accelerated with a delayed high voltage pulse applied to three-plate electrodes and detected with a two-stack microchannel plate (Hamamatsu F4655-11).
Commercially available palladium (Rare Metallic) placed in a vacuum chamber was evaporated by heating to around 1700 K in a Knudsen cell (Kitano Seiki KMD-Cell), and the temperature monitored with a W 5% Re-W 26% Re thermocouple.
Excitation to three autoionizing Rydberg series are expected via the 4d 9 [ 2 D 5=2 ]5d[1=2] 0 intermediate state of Pd I: 4d 9 ( 2 D 3=2 )np½1=2 o 1 , 4d 9 ( 2 D 3=2 )np½3=2 o 1 , and 4d 9 ( 2 D 3=2 )nf½3=2 o 1 . They are exactly the same series with those observed by direct excitation from the 4d 10 ground state 14,17) as both transitions are of the d-electron. Figure 2 is the entire ion yield spectrum obtained by scanning the third laser frequency ω 3 and Fig. 3 is an expanded trace of the high energy part. As shown in Figs. 2 and 3, two Rydberg series I and II are observed. Their state energies are summarized in Table I. From a practical standpoint of the present study, we find the transition to the autoionizing 4d 9 ( 2 D 3=2 )11p½1=2 o 1 state at 69055.2 cm −1 is the best candidate to achieve efficient ionization of Pd I after two-step selective excitation of odd-mass number isotopes.
The result does not follow the general trend of decreasing oscillator strength for increasing n Rydberg state, as was observed by Karamatskos et al. in which transition to the lowest autoionizing Rydberg state of n = 9 is the most intense. 14,20) This is because the initial state of the transition is different; in the case of photoionization of Pd I using synchrotron radiation reported by Karamatskos et al., transitions to the autoionizing Rydberg state is from the ground state of Pd I (4d 10 ), whereas in the present study, the transition is from the 4d 9 5d state (i.e., after two-step massselective excitation). A similar n-dependence of the transition intensity of Pd I to the result presented here was reported by Ishikawa after three-step excitation to the Rydberg states converging to the lower ground state of Pd II. 21) Between the two Rydberg series observed, transitions to the Rydberg series I is dominant in signal intensity and they are assigned to 4d 9 ( 2 D 3=2 )np½1=2 o 1 on the basis of both the assignment given by Baig et al. for n = 9-17 17) and the assignment for the lower states, n = 5 and 6. 22) As many more states are observed for this series compared to the other two, we perform a global fitting analysis on this series to obtain the upper ionization energy of Pd II as follows. 23) The state energies of the Rydberg states E(n) can be described approximately by the extended Ritz formula as with a truncated quantum defect δ(n) to be where IP is the ionization energy of Pd I, R Pd is the masscorrected Rydberg constant of Pd I (109736.75 cm −1 ), and both δ 0 and δ 2 are the energy-dependent quantum defects. By least squares fitting using IP, δ 0 and δ 2 as fitting parameters, we have obtained IP of 70780.38 ± 0.08 cm −1 with δ 0 = 3.0255 ± 0.0013 and δ 2 = −0.039 ± 0.051. The IP obtained here has a lower uncertainty and is within the range of values reported in literature (70780 ± 1 and 70779.8 ± 0.8 cm −1 ). 15,17) The assignment of Rydberg series I and II to 4d 9 ( 2 D 3=2 )np½1=2 o 1 and 4d 9 ( 2 D 3=2 )np½3=2 o 1 , respectively, is based on the energies reported for the lower states, n = 5 and 6. 20) We observe fine-structure splitting of levels n = 10, 15, 16, 17, and 18; possibly due to the presence of a local perturbation. However, no definite candidate of the perturbing state can be presented here because of the lack of spectroscopic data on the two-electron excited states in this energy region. 22) From transition rules, we also expect 4d 9 ( 2 D 3=2 )nf½3=2 o 1 , however, we do not observe any members of this series. Similarly, this series was not observed in other studies, possibly due to low signal intensity. 14,17) Of the two observed Rydberg series converging to the upper ionization limit of palladium after odd-mass number selective excitation, resonant excitation to the 4d 9 ( 2 D 3=2 )11p½1=2 o 1 state gives the largest ion yield. This result is a significant aid in the development of an efficient separation process of the LLFP of palladium 107 Pd from nuclear waste.
As we have specified, this study was carried out on the basis of the selective excitation scheme for odd-mass number isotopes of palladium proposed by Chen. 9) However, during our investigation another mass selective excitation scheme was found utilizing intermediate states of Pd I with a 6d 9 ( 2 D 3=2 ) ion core, showing a significantly increased efficiency of ionization via autoionizing Rydberg states. Results using this new selective excitation scheme will be reported in the near future.