The study of autoionizing states of the samarium atom

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Abstract

The investigation of the samarium (Sm) atom autoionizing states, belonging to the 4f6 6pnl and 4f5 5d6snl (l=0, 2) configurations, has been carried out by using a three-step isolated-core-excitation (ICE) scheme. Fifty-seven new autoionizing states of the Sm atom have been observed in the energy region between 60,700 and 66,500 cm−1. These autoionizing states are classified into five autoionization series in the light of different ionization limits. In most cases, the transition profiles of these autoionizing states are nearly symmetric, from which the peak positions and widths can be easily obtained by Lorentzian-profile-fitting procedure. Some autoionizing states have multi-peak profiles whose spectroscopic properties are analyzed in order to understand the complicated structures.

Introduction

Investigations of the atomic autoionizing states can play an important role in laser isotope separation, development of new lasers, laser without population inversion and the role of autoionizing states in coherent control [1], [2], [3], [4], [5], [6], in addition to its application in plasmas, astrophysics, atomic physics. Since the development of tunable dye lasers and multi-step laser excitation scheme, the autoionizing states of alkaline-earth-metal atoms have been studied widely in the last two decades [7], [8], [9], [10]. Recently, the investigation of the autoionizing states of rare-earth elements has also gained interest [11], [12], [13], [14], [15].

The spectra of rare-earth atoms are usually very complex due to the fact that the 4f shell is only partially filled for these atoms except for Yb. Among the rare-earth atoms, the Yb atom has been studied the most due to the fact that the 4f shell is completely filled [16], [17], [18]. With only two s-electrons outside the closed shell, the spectrum of the Yb atom is basically an alkaline-earth-like spectrum. However, in the case of the Sm atom, with an open shell consisting of six 4f electrons, the spectrum contains transitions of the electrons from both the outer 6s2 shell and the inner 4f6 shell. In addition, seven fine structures of the same spectral term, 4f6 6s2 7FJ, J=0–6, are the lowest states of the Sm atom, which can be thermally populated by the temperature (800 K) producing the Sm atomic beam. The thermal populations at the operation temperature in accordance with the Boltzman distribution are the following: 21% (7F0), 37.8% (7F1), 25.5% (7F2), 10.9% (7F3), 3.6% (7F4), 1% (7F5), and 0.2% (7F6). It is the thermal population that makes the spectrum of the Sm atom even difficult, and prohibits one from obtaining the autoionization spectra by conventional spectral methods.

With the development of experimental methods, there have been several studies devoted to the Sm atom autoionizing states during the last two decades. However, most of the existing studies mainly focus on the series that converging to the 4f6 6s+ and 4f6 5d+ ionization limits, such as even-parity bound and autoionizing series converging to the ionization limits 4f6 (7F) 6s+ [8F1/2], 4f6 (7F) 6s+ [8F3/2] and 4f6 (7F) 6s+ [8F5/2] [19]; odd-parity autoionizing states located in the energy region of 45,794–48,046 cm−1 and 48,800–51,200 cm−1 belonging to the 4f6 5dnp configurations [20], [21].

Based on the recent study of the autoionizing states belonging to the 4f6 6pnl and 4f5 5d6snl (l=0, 2) configurations in the lower energy region between 57,990 and 61,030 cm−1[22], the present work will explore them further in the region between 60,700 and 66,500 cm−1. This investigation will enrich the spectral data of the Sm atom in higher energy region, and uncover the new properties of autoionizing states of the Sm atom. Extending the energy from the lower to the higher region is expected to introduce some difficulties and challenges. Firstly, all three dye lasers, instead of two dye lasers in the previous work, must be pumped by the third harmonic generation (THG) of the same Nd:YAG pulsed laser with wavelength of 355 nm; secondly, since the density of bound Rydberg states increase rapidly with energy, the better laser linewidth is required; thirdly, the admixture among different series become severe, leading to a transition profile with more complicated structures.

In this work, the autoionizing states of the Sm atom, converging to the 4f6 6p+ and 4f5 5d6s+ ionization limits, are investigated using the isolated-core excitation (ICE) scheme [23]. As a powerful technique, the ICE has been very successful in the study of the autoionizing states of alkaline-earth atoms [10], [24], [25], [26], [27]. Several major advantages of the ICE scheme are obvious: firstly, only low laser powers are required because all the transitions are single-electron transition; secondly, it leads to a simplified autoionization spectrum with a symmetrical profile whose peak and width can be obtained easily, as will be demonstrated in the Section 3. In contrast, in the case without the ICE scheme, an autoionization spectrum usually shows a Fano profile with lots of interference effects [28], where much effort has to be made to have the information on level energy and autoionizing rate.

In the following text, an introduction of the experimental approach is given in Section 2; the experimental results and discussion are presented in Section 3, and Section 4 gives main conclusions.

Section snippets

Experimental

The experimental setup, described in detail previously [22], is comprised of three parts: a laser system, an atomic beam and ion-detection system, and a data acquisition system. All the lasers used here are manufactured by Quanta System: a Giant-770 model Nd:YAG pulsed laser with pulse duration 6–8 ns running at 20 Hz; three D-100 model dye lasers with line-width of 0.1 cm−1. The atomic beam is produced in a vacuum chamber, within which a micro-channel plate (MCP) is used as a detector for ions.

Results and discussion

Before obtaining the spectra of autoionizing states, we have determined the positions of the bound 4f6 6snl Rydberg states in order to locate the wavelength of the second laser at each desire states. Similar to the previous experiment [30], in this work, the Rydberg states of the Sm atom are detected by autoionizing detection method instead of direct photoionization method. In addition, like the ground spectral term of the Sm atom, seven fine structures of the same spectral term, 4f6 6s+, J

Conclusions

Using the ICE scheme and three-step resonance ionization spectroscopy, the spectra of 4f6 6pnl and 4f5 5d6snl autoionizing states of the Sm atom are studied. 57 new autoionizing states have been obtained in the energy region between 60,700 and 66,500 cm−1. These autoionizing states are classified into five autoionization series converging to different ionization limits. The transition profiles excited by ICE scheme show a high degree of symmetry, from which the level energies and widths can be

References (30)

  • R.S. Dygdala et al.

    Investigation of highly excited states of calcium by three-photon ionization

    Eur. Phys. J. D

    (2004)
  • M. Yaseen et al.

    Two-color three-photon resonant excitation spectrum of strontium in the autoionization region

    Eur. Phys. J. D

    (2002)
  • S.B. Li et al.

    The 6p3/2ns (J=1,2) autoionizing states of barium

    J. Phys. B: At. Mol. Opt. Phys.

    (2001)
  • D.L. Donohue et al.

    Resonance ionization photoelectron spectroscopy of lanthanide elements

    J. Chem. Phys.

    (1986)
  • E.P. Vidoliva-Angelova et al.

    Laser resonance ionization spectroscopy of the cerium atom

    J. Phys. B: At. Mol. Opt. Phys.

    (1997)
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    Project supported by the National Natural Science Foundation of China (Grant Nos. 10574098, 10674102) and the Natural Science Foundation of Tianjin (Grant No. 05YFJMJC05200).

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