Stark effect, the shift and splitting of atomic spectral lines by an external electric field, was discovered in 1913. The tensor and scalar polarizabilities have become interesting and heightened properties in recent years due to several applications, such as the development of next-generation optical atomic clocks, optical cooling and trapping schemes, the study of long-range interactions, and atomic transition rate determinations . And the research could supply information on the study of electric dipole moment (EDM) of electrons  . The study of parity nonconservation (PNC) in atoms even may yield a clue to “new physics”  . In the other hand, theoretical calculations of electric dipole polarizabilities have achieved a remarkable development; low-lying levels of Ba have been accurately calculated with ab initio calculation . The two-valence atom Si2+ calculated by configuration interaction + all-order method also shows a good agreement with experimental results . Properties of energies, lifetimes, hyperfine constants, multipole polarizabilities, and blackbody radiation shift in 137Ba II have been correctly calculated with relativistic many-body calculation . Calculations for highly excited states are expected. Thus precise experimental data on highly excited states will provide a further test of theoretical calculation.
However, experimental polarizability data of two-electron atoms at highly excited states were rarely reported. This is due to the difficulties of generating high electric field and performing high resolution spectroscopy at highly excited states. In this paper, we report high-resolution Stark spectroscopy for Ba highly excited states using the diode laser technique together with a collimated atomic beam.
2. Experimental Procedure
The experimental setup is similar with that used in previous studies of K, Rb, and Yb -. The metastable level 6s5d3D2 was populated by a discharge burning in barium vapor directly in front of the oven hole. A high- resolution atomic beam spectroscopy was performed to measure atomic Stark spectra, with a voltage applier capable of generating strong and static voltage up to 26.0 kV corresponding to a field strength of 43.4 kV/cm. A commercial tunable diode laser (TEC520) with an output power of 80 mW covered the wavelength range of 725 - 736 nm. The line width of the laser was smaller than 1 MHz. A confocal Fabry-Perot interferometer (FPI) with a free spectral range of 300 MHz was used to determine relative frequency. A fluorescence induced by laser beam was measured with a photomultiplier.
3. Results and Analysis
In the absence of the hyperfine interaction, the Stark shift can be written as :
where J is the quantum number of the total electronic angular momentum and is the projection of J on the direction of the electric field. E is electric field strength. and are traditionally called the scalar and tensor polarizabilities, respectively. It is obvious that the number of peaks by the Stark splitting is depending on the . The Stark shift and splitting are proportional to the square of the electric field, thus we can get tensor and scalar polarizabilities from measured.
Stark spectra were observed for the 6s5d3D2 − 5d6p3F3 transition at 728.0 nm. The Figure 1 shows the measured Stark spectrum at 43.4 kV/cm together with that at 0 field for the 728.0 nm transition. The strongest peak is corresponding to 138Ba with the 71.7% abundance. No hyperfine structure shows in this isotope owing to nuclear spins 0. The spectrum is found to shift to right by the electric field, i.e., the larger transition frequency, and 4 spectral lines are clearly observed. The background is due to the scattering of the laser beam from the electrodes. The transition was precisely measured at the range of electric field from 16.7 to 43.4 kV/cm. A good linear relationship between the shift or splitting and the square of electric field was confirmed.
The results obtained are given in Table 1. The scalar polarizability of the transition from 6s5d3D2 to 5d6p3F3 at 728.0 nm and the tensor polarizability of the 3F3 level have been determined for the first time. Uncertainties of
Figure 1. Observed Stark spectra of the 6s5d3D2 − 5d6p3F3 transition at 728.0 nm in Ba.
Table 1. Determined scalar and tensor polarizabilities for Ba.
results are mainly from the uncertainty of the distance between the two electrodes, and the uncertainty of peak fit.
The Stark effect of highly excited states in Ba has been investigated using the diode laser. The diode laser with its lower cost, together with the atomic beam, has been proved to be a powerful technique to perform high resolution atomic spectroscopy. The fundamental atomic data such as the electric polarizabilities can be derived from spectroscopic measurements. In this paper, the scalar polarizability of the 6s5d3D2 − 5d6p3F3 transition in Ba and the tensor polarizability of the 3F3 level have been determined for the first time and such kinds of atomic data at highly excited states will provide a critical challenge to theoretical calculations.
The author would like to thank Mr. T. Neya and Mr. Y. Yamaguchi for their contributions to this experiment.
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