XU Ya-wei, SHEN Li, DAI Chang-jian

(1. School of Science, Tianjin University of Technology, Tianjin 300384, China;2. Key Laboratory of Display Materials and Photoelectric Devices, Ministry of Education, Tianjin University of Technology， Tianjin 300384, China)*Corresponding Author, E-mail: daicj@126.com

Abstract: The spectra of odd-parity states of Eu atom are studied systematically with two-photon ionization detection technique. The two-photon ionization detection technique is utilized to obtain the spectra of odd-parity states with the two-step excitation: the wavelength of the first step laser is fixed, while that of the last-step laser is scanned around the region of 35 500-35 650 cm-1, 36 200-36 600 cm-1 and 46 650-46 950 cm-1, respectively. The characteristics of spectra in the energy region of 35 500-35 650 cm-1, 36 200-36 600 cm-1 and 46 650-46 950 cm-1 are investigated, demonstrating the special lifetime of highly excited states within those energy regions. In this paper, three different excitation pathways are used, not only the energy levels, line width and line shape are obtained, but also the q-reversal phenomenon is observed in the odd-parity states spectra. Besides, by analyzing the odd-parity states spectra of Eu atom, it is demonstrated that the possibility of 4f electrons are excited for the first time.

Key words: Eu atom; two-photon ionization; 4f electrons; q-reversal

1 Introduction

Studying the highly excited states of Eu atom has aroused extensive attention worldwide[1-2], as the impact of its half-filled 4f subshell is more challenging to both experimentalist and theorist. The investigation on its highly excited states can be used for not only testing new quantum theory but also for developing new energy programs, such as the laser isotope separation (LIS)[3-4]and the inertial confinement fusion (ICF)[5].

Unlike the Yb atom with a singleJ=0 ground state, the Eu atom has the ground states with many fine structures, leading to the much more complex spectra as the every final state of the multi-step excitation has multiple levels[6-7]. The complex spectra of Eu 4f76p1/2nland 4f76p3/2nlautoionizing states are reported, but the influence of the 4f electrons on them is not found[8-11]. For this reason, it is necessary to detect whether 4f electrons can be excited. The special lifetime of highly excited states in the energy region of 35 500-35 650 cm-1, 36 200-36 600 cm-1and 46 650-46 950 cm-1is persuasive evidences that 4f electrons have been stimulated.

Reproducing the spectra of odd-parity states of Eu atom with a multi-peaked fitting procedure will yield important information, such as energy levels, line width and line shape. Furthermore, theq-reversal phenomenon is observed in the odd-parity states spectra of Eu atom.

2 Experiments

2.1 Experimental Setup

As the experimental details have been presented previously[12], only a brief description will be given here. As shown in Fig.1, the experimental setup used here consists of three parts: a laser system, an atomic beam system, and a data acquisition system.

Fig.1 Experimental setup composed of a laser system, an atomic beam system, and a data acquisition system. The P in this figure represents a broadband half-wave plate combined with a linear polarizer.

The laser system includes a Nd∶YAG laser and two tunable dye lasers which are used for two-step excitation to the odd-parity states of Eu atom. The dye laser pulses, with 0.2 cm-1line width, have a typical energies of 0.5 mJ per pulse. To ensure the excitation sequence, the second laser pulse is delayed by 6-8 ns.

The atomic beam system includes a vacuum chamber, an atomic oven, and a temperature control instrument. The oven with the Eu metal is heated by a DC power supply whose temperature is monitored by a platinum-rhodium thermocouple and a temperature controller. At a temperature of 750 K, the atomic vapor is generated and collimated to produce the Eu atomic beam. To reduce the Doppler broadening, the atomic beam is crossed perpendicularly with the laser beams in the interaction region.

The signal acquisition and analysis system includes a MCP detector, a boxcar gate, and a digital pulse generator. After a time delay of 0.5 μs to the laser pulses, the ions from the highly excited states are extracted by the pulsed electric field of 50 V·cm-1to the MCP detector. The output of the MCP is fed into the boxcar and stored in a computer for further analysis.

2.2 Experimental Principle

In order to find more evidences to prove the influence of 4f electron, three excitation schemes are applied in this paper.

Scheme Ⅰ:

E=35 500-35 650 cm-1[J=3/2,5/2,7/2],

Scheme Ⅱ:

E=36 200-36 600 cm-1[J=7/2,9/2,11/2]，

Scheme Ⅲ:

E=46 650-46 950 cm-1[J=5/2,7/2,9/2].

In scheme Ⅰ, the first laser with the wavelength of 564.7 nm excites the Eu atom from the ground state 4f76s28S7/2to the 4f76s6p6P5/2state; The second laser, whose wavelength is scanned from 557.6-569.5 nm to obtain the spectrum of odd-parity states. Similarly, if excitation schemes Ⅱ and Ⅲ are used, the wavelengths of the first laser are fixed at 686.6 nm and 462.8 nm, which excites the Eu atom from the ground state 4f76s2 8S7/2to the intermediate states 4f76s6p10P9/2and 4f76s6p10P7/2, respectively. The wavelength of the second step laser is varied from 440-460 nm and 394-402 nm to obtain the spectra of odd-parity states, respectively.

To ensure the accuracy of experimental data, the following work need to be done. (1)Scan the same spectrum multiple times to reduce random errors. (2)The standard wavelength of the hollow cathode lamp is used to perform a systematic wavelength calibration on each wavelength band of the dye laser to ensure the spectral frequency accuracy and thus reduce the systematic error. The uncertainty of the present experiment is estimated to be better than ±0.2 cm-1.

3 Results and Discussion

This section will show the spectra of the odd- parity states obtained by the three excitation schemes and their fitting results. Because the bound states do not have a linear structure, so they are not fitted. The spectrum obtained with scheme Ⅱ is first shown, then the spectra obtained with scheme Ⅰ and scheme Ⅲ are shown, sequentially.

With the scheme Ⅱ, the spectrum of the odd-parity states can be obtained as shown in Fig.2. A detailed interpretation to spectrum is fulfilled by reproducing its line shapes with a multi-peaked fitting procedure. The abscissa is the sum of photon energies of the two lasers in cm-1; the ordinate is the signal intensity.

Fig.2 Spectrum of two-electron excited states(solid line) of Eu atom, together with the fitting result(dashed line), in which the first laser with the wavelength of 686.6 nm excites the Eu atom from the ground state 4f76s28S7/2to the 4f76s6p10P9/2state and the second laser whose wavelength is scanned from 440 nm to 460 nm to obtain the spectrum of odd-parity states.

As shown in Fig.2, the line shapes of the peaks are asymmetric, which can be described by Fano profiles. The Fano-type line shape is caused by the quantum interference effect of the two-channel transition, which is from the initial state to the continuous state and the discrete excited state, respectively. According to the Fano’s theory[13], the line shape can be written in the following form:

in the formula

whereErrepresents the energy level,Γrepresents the line width andqis called the shape index.

The experimental values of energy level, line width andq-factor listed in Tab.1 are obtained by fitting the date to the three Fano profiles.

Tab.1Energylevel(E),linewidth(Γ)andq-factoroftwo-electronexcitedstateofEuatom

Peak numberE/cm-1Γ/cm-1q136 338.68.1-0.1236 502.78.50.1336 549.82.4-0.1

As shown in Fig.2 and Tab.1, theq-reversal phenomenon is observed. Clearly,qchanges sign twice in the spectrum. The change in symmetry orq-reversal effect occurs in Rydberg series of autoionization lines when a broad intruder is presented, while it also occurs when a relatively sharp autoionization line is ‘tuned’through a broad resonance. It is not conditional that theq-reversal phenomenon occurs as long as there is an interference, because it does not occur when the coupling enhanced[14].

Due to the two-electron excited states are excited from the 4f76s6p10P9/2state, there are many possible electronic configuration for this states. If the 6p electron is excited, the electronic configurations of the states are 4f76sεs or 4f76dεd; moreover, if the 4f electron is excited, the electronic configurations of the states are 4f66s6pεd or 4f66s6pεg. The state of peaks 1-3 must not be 4f76sεs or 4f76dεd, because their line-widths are large, so 4f electrons must be excited by the second step laser to the states of 4f66s6pεd and 4f66s6pεg. Theq-reversal effect occurs because the 4f66s6pεd or 4f66s6pεg states is equivalent to an interference.

Let’s take a look at the spectrum of another energy region. In the energy region of 35 500-35 650 cm-1, there are still two peaks in the right of Fig.3 whose line-width is much larger than that of the bound state. Besides, the change in symmetry orq-reversal effect also occurs.

The experimental values of energy level, line width andq-factor are obtained by fitting the date to the two Fano profiles that are listed in Tab.2.

Fig.3 Spectrum of two-electron excited state(solid line) of Eu atom, together with the fitting result(dashed line), in which the first laser with the wavelength of 564.7 nm excites the Eu atom from the ground state 4f76s28S7/2to the 4f76s6p6P5/2state and the second laser whose wavelength is scanned from 557.6 nm to 569.5 nm to obtain the spectrum of odd-parity states.

Tab.2Energylevel(E),linewidth(Γ)andq-factoroftwo-electronexcitedstateofEuatom

Peak numberE/cm-1Γ/cm-1q1'35 576.41.6-3.02'35 586.96.8-0.1

Compared with the literature, the experimental results are in good agreement with the literature reports in the energy regions of 35 500-35 650 cm-1and 36 200-36 600 cm-1. In those energy regions, all atomic states reported in the literature are confirmed in this paper. However, in the energy region of 35 250-35 390 cm-1, certain atomic states reported in the literature have not been detected in the experiment. The reasons are the following aspects: (1)The energy region is at the edge of the range that dye exits laser and the intensity of the laser is weak, resulting in the signal intensity of some photoionization lines weakly. As being overwhelmed by noise, it cannot be detected. (2)The atomic number density on the high excited state to be measured is too small, resulting in submerged by noise.

Fig.4 shows the spectrum of autoionizing states in the 46 600-46 900 cm-1energy region. Since this energy region exceeds the first ionization limit, the broad peak structures in this energy region are autoionization peaks. According to the above analysis, if the 6p electron is excited, the electronic configurations of the states are 4f76sεs or 4f76dεd; moreover, if the 4f electron is excited, the electronic configurations of the states are 4f66s6pεd or 4f66s6pεg.

Fig.4 Spectrum of two-electron excited state(solid line) of Eu atom, together with the fitting result(dashed line), in which the first laser with the wavelength of 462.8 nm excites the Eu atom from the ground state 4f76s28S7/2to the 4f76s6p8P7/2state and the second laser whose wavelength is scanned from 394 nm to 402 nm to obtain the spectrum of odd-parity states.

The experimental values of energy level, line width andq-factor are obtained by fitting the date to the six Fano profiles that are listed in Tab.3.

As shown in Tab.3, the corresponding energy level, line width andqfactor of autoionization peaks are listed. As the line widths of autoionizing states observed in Tab.3 are irregular, it indicates that they do not belong to the same autoionization series, therefore the electronic configuration for this autoionizing states mentioned above can be accepted. The peak 2″ and the peak 4″ in Fig.4 are symmetric and nearly similar to Lorentzian profiles and theirq-factors are also greater than 7, which consistent with the Fano’s theory.

Tab.3Energylevel(E),linewidth(Γ)andq-factoroftwo-electronexcitedstateofEuatom

Peak numberE/cm-1Γ/cm-1q1″46 674.20.7-0.12″46 708.113.0-0.23″46 780.01.8-0.44″46 810.17.87.05″46 862.30.30.26″46 872.12.5-0.1

4 Conclusion

By analyzing the three spectra we observed, the evidence that 4f electrons can be excited is found, providing data for testing new quantum theory. Theq-reversal effect is found when there is an interference in Rydberg series of autoionization lines. However, due to the complexity spectra of the Eu atom, the problem of spectral attribution of the atomic states is still not completely resolved. It remains to be further studied to fully determine their atomic states and electronic configurations.