Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 52 (2019)

Attempt to measure relaxation time of atomic bubble surrounding
alkaline atoms in superfluid helium
Y. Takeuchi,∗1,∗2 M. Sanjo,∗1,∗2 W. Kobayashi,∗1,∗2 H. Kuramochi,∗3 A. Takamine,∗2 Y. Matsuo,∗1,∗2 Y. Zempo,∗4T. Tahara,∗3 and H. Ueno∗2
Our research group has been developing a new typeof nuclear laser spectroscopy method named OROCHI,
which is based on laser spectroscopy of atoms in superﬂuid helium (He II). In this method, the presence of theinteraction between injected atoms and surrounding helium atoms plays an important role to prepare a uniquespectroscopic environment. In this study, we focus on
the structure of the atoms surrounded by He.
When the atoms are introduced into He II, the surrounding helium atoms are pushed out by the exchange(Pauli) repulsion force.1) The resulting vacuum region iscalled “atomic bubble.” For the electronic ground stateof the introduced atom, the bubble typically has a radiusof 5 ∼ 10 Å. When the shape of the electron orbit of theatom changes owing to a state transition such as excitation and absorption, it is considered that atomic bubble
is also deformed in He II as the shape of the orbital deforms.
Figure 1 shows the deformation process of the atomicbubble. Because the Franck-Condon principle holds truein the process of absorbing and releasing photons, the radius of the bubble is kept constant before and after thetransitions. Therefore, after these short time transitions,the bubble is aﬀected by the electron orbit of the introduced atom and deforms (relaxes) over a certain time.
It is known that the wavelengths of atoms in He II shift
between absorption and emission owing to this process.2)
It is estimated that it takes an order of a few pico seconds to deform the bubble but so far the relaxation time
has not been measured in the time domain. To clarify
the dynamics of the atomic bubble system, we will combine the laser spectroscopic technique in He II and theultrafast laser spectroscopic technique.3)
In this research, we decided to start with Rb as the target atom of this experiment because its characteristics inHe II is well studied.2,4) We plan to perform experiments
using a detection system with a high time resolution for
the relaxation time in the excited state.

Fig. 2. Energy calculated value of Rb atomic bubble system.
(18 helium, Hybrid density functional APF with the basis
set Def2TZVPP was used.)

The experiment will be conducted in an oﬄine environment using laser ablation for the introduction of atoms.4)
In the cryostat used for oﬄine experiments, the atoms aresupplied by laser ablation and laser dissociation into He
II in a quartz cell installed in the upper part of the helium
tank. Then, a pumping laser irradiates to Rb atoms in
the observation region and ﬂuorescence is observed using
the Time-Correlated Single Photon Counting (TCSPC)
system. We plan to obtain the time dependence of theintensity of ﬂuorescence using this system that records
the time diﬀerence between the irradiation of the excitation laser signal and the detection of the ﬂuorescentphoton.5) The time resolution of the system is approximately 80 ps. It is considered that emission also occurs
during the deformation process of the bubble to the energy minimum of the excited state. Therefore, if we canobserve the intensity changes of ﬂuorescence up to several
tens of ns, which is the spontaneous emission lifetime, wecan estimate the time required for bubble deformation.
Currently, performance evaluation of picosecond pulsed
Ti: sapphire laser for excitation is ongoing.
Additionally, in parallel with the preparation of the experiment, we calculated atomic energy levels using Gaussian 09 to estimate the amount of wavelength change dueto the relaxation of atomic bubbles. The calculation of
the energy of the atomic bubble formed by 18 heliumatoms resulted in the conﬁrmation of the broadening of
the bubble radius and shifts in the atomic transition energy (Fig. 2). Besides, we will derive the inﬂuence dueto the increase of the number of He atoms for bubble
formation and simulate the case for other atoms such asCs.
References

Fig. 1. Deformation cycle of an atomic bubble.
∗1
∗2
∗3
∗4

Department of Advanced Sciences, Hosei University
RIKEN Nishina Center
Molecular Spectroscopy Laboratory, RIKEN
Department of Digital Media, Hosei University

1) H. Bauer et al., Phys. Lett. A 146, 134 (1990).
2) Y. Takahashi et al., Phys. Rev. Lett. 71, 1035 (1993).
3) D. Mandal et al., Chem. Phys. Lett. 359, 77 (2002).
4) T. Furukawa et al., Hyperfine Interact 196, 191 (2010).
5) W. Becker, Advanced Time-Correlated Single Photon
Counting Techniques, (Springer, Berlin, Heidelberg, 2005).
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