II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Observation of Rb D1 fluorescence in superfluid helium using
picosecond time-resolved detection
Y. Takeuchi,∗1,∗2 H. Kuramochi,∗3 K. Ishii,∗4 K. Imamura,∗2 A. Takamine,∗2 M. Doi,∗1,∗2 T. Yamamoto,∗1,∗2Y. Matsuo,∗1,∗2 Y. Zempo,∗5 T. Tahara,∗4 and H. Ueno∗2Our research group is developing a laser spectroscopy
technique (OROCHI) for atoms in superﬂuid helium (HeII). When atoms are introduced into He II, the surrounding helium atoms are pushed out by the exchange (Pauli)
repulsion force.1) The resulting vacuum region is called
an atomic bubble. Because the shape of the electron
orbit of an impurity atom is deformed according to its
energy state, the atomic bubble is also deformed following the shape change of the atomic orbit. According
to the Franck-Condon principle, the time required forthe atomic transition is 10−15 s, which is much shorter
than the time required for the bubble deformation therefore, it is considered that the bubble is deformed after
the transition. The wavelengths of atomic transitions
for both absorption and emission in He II are shiftedfrom those in vacuum owing to this deformation cycle.2)
The emission wavelength is considered to change in correspondence with the degree of bubble deformation.
It is estimated that the time required for the bubble
deformation is of the order of a few picoseconds, but
so far, the relaxation time has not been measured in
the time domain in bulk He II. In this study, we aim
to determine the relaxation time through time-resolved
emission measurements at diﬀerent wavelengths.
The OROCHI group is conducting a research projectin collaboration with the Molecular spectroscopy laboratory to observe the relaxation time of Rb atomic bubbles in He II.3) Recently, we evaluated the performanceof the time-correlated single photon counting (TCSPC)
detection system that will be used for photo-detectionin the relaxation time measurement. Observations wereperformed using an avalanche photodiode (APD) andtime amplitude converter (TAC) as detectors. So far,our research group has used PMT for laser-induced ﬂuorescence (LIF) photon counting of Rb atoms. This measurement will be the ﬁrst LIF observation by the TCSPCsystem.

Fig. 1. Experimental setup of LIF detection.
∗1
∗2
∗3
∗4
∗5Department of Advanced Sciences, Hosei University
RIKEN Nishina Center
Institute for Molecular Science
Molecular Spectroscopy Laboratory, RIKEN
Department of Computer and Information Science, Hosei University

Fig. 2. Observed photon intensity as a function of delay time.

We ﬁrst conﬁrmed the ﬂuorescence detection of theD1 line of the Rb atom (absorption center wavelengthin He II: 778.0 nm). We used a picosecond modelocked Ti:sapphire laser (laser power: 130 mW, repetition rate: 80 MHz, pulse width: 1.6 ps, center wavelength: 778.2 nm) as an excitation laser, and we observed
LIF using the APD and acquired data using a TCSPC
module (SPCM, Becker & Hickl GmbH) (Fig. 1). The
Rb atoms were supplied by performing laser ablation on
the RbCl sample installed above the observation areaand laser dissociation to RbCl clusters in He II. The
monochromator wavelength was set at 793 nm, which isthe center of the emission line.
Figure 2 shows the result of the observation. Thedata plotted here are the observed photon intensities as afunction of delay time. In this ﬁgure, the blue triangle isthe total detected light when all lasers are turned on, andthe orange dot is the scattered light when only the excitation laser is turned on. The fact that the peaks appear
at the same position of the horizontal axis indicates thatthe excitation laser is scattered inside the cryostat andis slightly detected, although the monochromator wavelength is 15 nm longer than that of the excitation laser.
The background also contains ambient light from sources
other than the laser in the environment. The blue curve
has a longer tail. This tail indicates the decay due tothe lifetime of the Rb D1 emission. While the excitation
pulse interval was 12.5 ns, the ﬂuorescence lifetime was
27 ns therefore, atoms are excited by the next laser pulse
before the emission vanishes and LIF photons are piledup. Therefore, the result agreed with the expectationthat the spectrum would have the appearance of a sawtooth wave with repeated cycles of decay and rise due toexcitation on top of the piled-up signal.
With the current data, the background scattered lightis not small, and the setup should be adjusted so that
more ﬂuorescence light can be obtained. We are planningto make such improvements and move to the observationof weaker emission on the short-wavelength side.
References

1) B. Tabbert et al., J. Low Temp. Phys. 109, 653 (1997).
2) Y. Takahashi et al., Phys. Rev. Lett. 71, 1035 (1993).
3) Y. Takeuchi et al., RIKEN Accel. Prog. Rep. 53, 130 (2019).

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