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Acta Physica Sinica

Investigation of odd-parity excited states of europium atoms by resonant photoionization spectroscopy

Xiao Ying, Dai Chang-Jian, Zhao Hong-Ying, Qin Wen-Jie, 显示材料与光电器件教育部重点实验室，天津 300191；天津理工大学理学院，天津 300384

Published Jan 1, 2009• 5 citations
Publisher

Abstract

In this paper, two different detection methods, photoionization and autoionization, are combined to study the spectrum of highly excited states of Eu atom. On the one hand, the bound Eu spectrum with odd-parity in the region of 43927—45010 cm-1 has been measured. The spectral information, such as position and intensity, of 32 transitions can be deduced from the spectra with calibration and the error estimation. The information of excitation process and the selection rules enables us to determine their total angular momenta. Comparison with the related literature show that 12 of the states are newly discovered. On the other hand, we design an effective method to identify different types of highly excited states. The spectra of odd-parity states in the same region are measured by both photoionization and autoionization methods, from which we may separate the Rydberg states from the valence states. of the 32 transitions only 14 can be identified as Rydberg states, and the others are valence states. Furthermore, the effective quantum numbers of the Rydberg states are also discussed in detail with the quantum theory.

DOI
10.7498/aps.58.3071

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In this paper, two different detection methods, photoionization and autoionization, are combined to study the spectrum of highly excited states of Eu atom. On the one hand, the bound Eu spectrum with odd-parity in the region of 43927—45010 cm-1 has been measured. The spectral information, such as position and intensity, of 32 transitions can be deduced from the spectra with calibration and the error estimation. The information of excitation process and the selection rules enables us to determine their total angular momenta. Comparison with the related literature show that 12 of the states are newly discovered. On the other hand, we design an effective method to identify different types of highly excited states. The spectra of odd-parity states in the same region are measured by both photoionization and autoionization methods, from which we may separate the Rydberg states from the valence states. of the 32 transitions only 14 can be identified as Rydberg states, and the others are valence states. Furthermore, the effective quantum numbers of the Rydberg states are also discussed in detail with the quantum theory.
Even-parity highly excited states of an europium (Eu) atom have been investigated by employing the three-colour stepwise resonant excitation technique. Seventy-eight even-parity states of the Eu atom have been detected in the energy region between 43 500 and 45 200 cm −1 with two different excitation schemes. With a detailed analysis of the experimental data, this work not only confirms or improves the previous results by determining their level energy, J assignment, but also reports 21 new states of the Eu atom with a unique J assignment. In addition, the information about relative line intensities of these 78 related transitions are also provided for the first time.
The spectra of 4f76p1/2(J = 3 or 4)8s, 4f76p1/2(J = 3 or 4)6d and 4f75dnl autoionizing states of the europium atom are investigated using the resonance-ionization spectroscopy method via the same intermediate state, 4f76s(9S4o)8s 8S7/2. The profound interaction among different Rydberg series is observed in this work. The fingerprints of the 4f75d(7D)n′l′ series are identified from the complex structures of autoionization spectra of the 4f76p1/2(J = 3 or 4)8s and 4f76p1/2(J = 3 or 4)6d states. The information of these autoionizing states, such as their level energy, line width, is also provided.
An improved method for the angular momentum measurement of autoionizing levels of complex atoms based on photoionization polarization spectroscopy is reported. In this method, a variation of the ion yield is measured as a function of the combination of the polarizations of the lasers used to excite the autoionizing level through a ladder of intermediate resonant states. Here, eight combinations of different polarizations in three lasers are used, which include not only circular but also linear polarizations. It has been confirmed that the angular momentum of the autoionizing level can be determined uniquely by relative ion-yield measurements for any combination of angular momenta of the levels of the excitation ladders by choosing appropriate intensities of the lasers.
The even-parity spectra of strontium and barium are very complex due to strong perturbation by several doubly excited configurations and this also increases discrepancies between theory and experiment in accurate level assignment and energy level and atomic structure data, particularly for the spectra of highly excited atoms and their behaviour in external fields and collisional processes which are of fundamental importance in the emerging applications in physics of cold atoms, quantum computing, precision atomic clocks and in the crossed-field problem. This thesis presents results from an experimental study to improve and extend the previously published observations in the two-photon spectra of the high-lying Rydberg states of SrI and Bal placed under a variety of test conditions such as external fields, collisions etc. Orthogonal excitation of a novel atomic jet in a purpose-built heat-pipe setup using a Xenon Chloride excimer laser pumped tunable narrow bandwidth dye laser and high sensitivity space-charge ionization in thermionic diode with complimentary optical detection was used for measurements. Presented are the two-photon spectra covering the dye laser wavelength (air) range of 460 - 425 nm for strontium and 495 - 467 nm for barium by which the J = 0,2 even-parity Rydberg series and autoionizing resonances just above the ionization limits labeled as; (4d2+5p2) 1D2 in strontium and 5d5f2p1 in barium were observed. Original new energy level values are presented with substantial extension to the data in the literature for even-parity Rydberg series: Sr I: 5sns 1S0 (12 < n < 46), 5snd 3D2 (9 < n < 47) and 5snd 1D2 (9 < n < 82) Ba I: 6sns 1S0 (13 < n < 68), 6snd 3D2 (11 < n < 46) and 6snd 1D2 (11 < n < 88) New observation of certain anomalies including a Stark shift reversal effect and zero-field thermionic diode detection with remarkable sensitivity preserving all spectral features of Rydberg states is also presented. Controlled configuration mixing by exploiting electric field and collisions has allowed new observations for the two-photon forbidden odd-parity Rydberg series of SrI: 5s2 1S0 -> 5snp 1P1o (32 < n < 46) and 5s2 1S0 -> 5snf 1F30 (29 < n < 44) Extensive study of collision involving Rydberg atoms with neutral rare gas atoms has allowed to obtain new data for shift and broadening rates for Sr I series: 5sns 1S0 and 5snd 1,3D2 in the perturbed region which clearly exhibit Ramsauer effect. Collisional evolution of a remarkably broad resonance, labeled as 4d2 1G4, also exhibiting Ramsauer effect, is presented besides new level data for strontium 4d2 3PJ states accessed by two distinct processes- direct two-photon excitation and single-photon sequential excitation. Furthermore, the thesis presents the results of a novel experimental technique involving time-resolved gated pulsed field detection technique (GPT) employed for the first time to examine the time decay of Rydberg atoms yielding their effective lifetimes.
The ionization of 6sns1S06sns^{1}S_{0}6sns1S0​ Rydberg states of 138Ba^{138}\mathrm{Ba}138Ba has been investigated by high-resolution electron spectroscopy. The Rydberg states were excited by two cw dye lasers via the 6s6p1P16s6p^{1}P_{1}6s6p1P1​ level. The electron spectra show rich structure between 0.025 and 3 eV, largely attributable to collisional (Penning) ionization, photoionization, and autoionizing transitions. The first process might be interpreted as superelastic scattering of the Rydberg electron by optically pumped Ba(6s6p1P1)\mathrm{Ba}(6s6p^{1}P_{1})Ba(6s6p1P1​) or Ba(6s5d1D2)\mathrm{Ba}(6s5d^{1}D_{2})Ba(6s5d1D2​) atoms.
The fine structure of high-angular-momentum n=9n=9n=9 and 10 Rydberg states of barium has been measured precisely, using the resonant excitation Stark ionization spectroscopy method. Optical transitions corresponding to $(n,{n}^{\ensuremath{'}})=(10,30)$, (9,17), and (9,20) were induced with a Doppler-tuned CO2{\mathrm{CO}}_{2}CO2​ laser, determining the fine-structure energies corresponding to all n=9n=9n=9 and 10 levels with $L\ensuremath{\geqslant}6$. The pattern of these fine-structure energies conforms closely with an effective potential model, by comparison with which the dipole and quadrupole polarizabilities of Ba+{\mathrm{Ba}}^{+}Ba+ can be determined. Combining our data with earlier measurements made it possible to deduce, in addition, the portion of ${\ensuremath{\alpha}}_{2}$ due to the lowest excited DDD state of Ba+{\mathrm{Ba}}^{+}Ba+. Our best estimates of these three properties are ${\ensuremath{\alpha}}_{1}=124.30(16){a}_{0}^{3}$, ${\ensuremath{\alpha}}_{2}=2462(361){a}_{0}^{5}$, and ${\ensuremath{\alpha}}_{2}^{0}=1828(88){a}_{0}^{5}$.