Ⅲ-1. Atomic & Solid State Physics (Ion)

RIKEN Accel. Prog. Rep. 53

Baseline correction system for precision measurement of the hyperfine
structure of Rb atoms in superfluid helium
M. Doi,∗1,∗2 K. Imamura,∗2 A. Takamine,∗2 Y. Takeuchi,∗1,∗2 T. Yamamoto,∗1,∗2 H. Ueno,∗2 and Y. Matsuo∗1,∗2
We have been developing a laser spectroscopic method
for the atoms injected into superﬂuid helium (He II)with the objective of studying the nuclear structure ofunstable nuclei with low production yields and short
lifetimes. In this method, laser-induced ﬂuorescence(LIF) is observed by applying the laser-radiofrequency(RF)/microwave (MW) double resonance method toatoms in He II to measure Zeeman splitting and hyperﬁne structure (HFS). It enables us to determine thenuclear magnetic moments and nuclear spins that reﬂect
nuclear structure.
One of the eﬀective ways to introduce atoms to He II
is a two-step laser sputtering method. In a previous experiment, we successfully measured the hyperﬁne structure of 133 Cs atoms in He II with an uncertainty of 10−5
via the laser-microwave double resonance method.1) In
order to discuss the hyperﬁne anomaly of atoms in HeII, it is necessary to measure hyperﬁne structure splittings for at least two isotopes. We attempted to show
the feasibility of deducing hyperﬁne splittings with anaccuracy and a precision of 10−6 for Rb, which has twostable isotopes. However, it has been diﬃcult to realize
measurement with a suﬃciently high precision owing to
the ﬂuctuation of the number of observed photons when
we use the two-step laser sputtering method to supplyatoms to He II. To overcome this diﬃculty, a baseline
correction system to cancel the ﬂuctuation of the number of atoms was developed.2)
The baseline correction system consists of an electrooptic modulator (EOM) to switch the polarization of theexcitation laser quickly and a multichannel scaler (MCS)
equipped with multi-channel inputs to count photons forcircular and linear polarizations. It has been successfully demonstrated by observing the LIF count diﬀerence
between spin-polarized and unpolarized atoms that theFig. 1. (a) Switching of the polarization of the excitation
laser by EOM. (b) Schematic of the experimental setup
in a cryostat.
∗1∗2

Department of Advanced Sciences, Hosei UniversityRIKEN Nishina Center

Fig. 2. Laser RF double-resonance spectrum of 85, 87 Rb obtained using the baseline correction system.

baseline correction system works properly.2)
Next, we applied this system to a double-resonanceexperiment. With circularly polarized laser light irradiation, we expect to observe ﬂuorescence from atoms
only when the RF/MW frequency is resonant with theZeeman splitting or hyperﬁne structure while scanning
the RF/MW frequency. On the other hand, when thelaser light is linearly polarized, all the atoms are excited;
thus, the observed number of photons was proportional
to the number of atoms.
We performed an RF double-resonance experiment for
Rb atoms injected into He II using the baseline correction system. Rb atoms were irradiated with a 120-mW
Ti:Sa laser (wavelength: 780 nm, laser diameter: 2 mm)of either circular or linear polarization. Figure 2 shows
the spectrum obtained when we swept the RF frequency.
We observed two ﬂuorescence peaks. The peaks on theleft and right correspond to resonance for the Zeeman
splitting of 85 Rb and 87 Rb, respectively. The center frequencies of both peaks were consistent with the resonance frequencies estimated from our experimental condition. This result indicates that the baseline correction
system works well in the double-resonance experiment
as well as the previous work.3)
In the future, the present research will be extended bymeasuring the hyperﬁne structure of Rb with the baseline correction system to evaluate the hyperﬁne anomalyof Rb isotopes in He II, which requires a MW doubleresonance experiment with a long sweep time and, hence,an increased ﬂuctuation in the number of atoms.
References

1) K. Imamura et al., Hyperfine Interact. 230, 73 (2015).
2) M. Sanjo et al., RIKEN Accel. Prog. Rep. 52, 153 (2019).
3) T. Fujita et al., RIKEN Accel. Prog. Rep. 47, 212 (2014).

- 142 -