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

RIKEN Accel. Prog. Rep. 53

Simultaneous cluster dissociation and optical pumping of Ag insuperfluid helium using a single UV laser
T. Yamamoto,∗1,∗2 K. Imamura,∗1 A. Takamine,∗1 W. Kobayashi,∗2 Y. Takeuchi,∗1,∗2 M. Doi,∗1,∗2 H. Ueno,∗1 andY. Matsuo∗1,∗2We have been developing a laser spectroscopy method
for atoms introduced into superﬂuid helium (He II).
We generate spin polarization by the optically pumping
the atoms and perform laser-radiofrequency and lasermicrowave (MW) double resonance. This method enables us to determine the nuclear spins and electromagnetic moments through measurements of Zeeman splitting and hyperﬁne structure (HFS) splitting. Moreover,because HFSs in He II shift slightly from the value invacuum, we are studying such an interaction between
He II and atoms.
We have successfully measured HFSs of alkali atoms
such as stable 85, 87 Rb and 133 Cs in He II and observed
small diﬀerences from those in vacuum.1,2) Currently, weare attempting to apply this method to group 11 atoms
to verify whether a similar diﬀerence appears in atoms
other than alkali metal elements. The HFS transition of
the stable isotope 197 Au atom in He II was observed using this technique.3) To discuss the diﬀerential hyperﬁneanomalies between isotopes, it is necessary to measure
the HFSs of at least two isotopes. Silver is a good candidate with two stable isotopes, 107 Ag and 109 Ag, thenatural abundance ratio of which is almost 1:1.
In oﬄine experiments, atoms have been introduced
into He II using a two-step laser sputtering method,
namely, by laser ablation and laser dissociation. As theﬁrst step, a metal sample placed above the He II surface
is ablated by the second-harmonic pulse of a Nd: YAGlaser (wavelength: 532 nm, repetition rate: 10 Hz, pulse
width: 8 ns, pulse energy: ∼5 mJ). Only clusters generated by the ablation process can be immersed in HeII. Subsequently, atoms are supplied by the dissociation
process for the spectroscopy of atoms in He II. Althougha femtosecond Ti:Sa laser (wavelength: 800 nm, repetition rate: 500 Hz, pulse width: ∼120 fs, pulse energy:∼200 µJ) has been eﬃcient in the dissociation for various atomic species, we found this is not the case for Agclusters.4) Inspired by a report that Ag clusters in He II
droplets show absorption in the range of 330–360 nm instead of 800 nm,5) our group applied the third-harmonic
pulse of Nd:YAG laser (wavelength: 355 nm, repetition
rate: 20 Hz, pulse width: 5 ns, pulse energy: 8 mJ) tothe dissociation process. The dissociation of Ag clusters
was clearly conﬁrmed by the generation of spin polarization of Ag atoms in He II through optical pumping.6)
For further study of Ag atoms using the laser-RF/MW
double resonance method, it is indispensable to increase
laser induced ﬂuorescence (LIF) photon counts.
In particular, increasing the power of the pumping∗1∗2

RIKEN Nishina Center
Department of Advanced Sciences, Hosei UniversityFig. 1. Variation of LIF intensity with a magnetic field.

laser is promising because the LIF intensity is proportional to the pumping laser power for atoms in superﬂuid helium.7) This approach should also be eﬀective if
the pumping laser simultaneously function as the dissociation laser.
Herein, we report the introduction of the forth harmonics of a new DPSS laser (wavelength: 335 nm, repetition rate: 20 kHz, pulse width: ∼20 ns, pulse energy:
5 µJ). The wavelength of the laser matches the wavelength for both Ag cluster dissociation and Ag atomic
excitation in He II. Since the output power of the laser
is greater than 100 mW on average, it is expected thatthe laser pulses can dissociate Ag clusters.
Figure 1 shows the result of increase and decrease in
LIF intensity when a magnetic ﬁeld (0–11.5 G) and an
RF ﬁeld (10.0 MHz) were applied to Ag atoms in He
II. When a suﬃcient magnetic ﬁeld is applied, spin polarization is achieved, and the LIF intensity decreases.
In addition, when a constant RF is applied, the LIF intensity increases at the magnetic ﬁeld that provides theZeeman splitting energy corresponding to the resonant
RF. We conﬁrmed that the single laser can eﬃciently
perform as both the dissociation and pumping laser for
Ag atoms. In the next step, we will measure the HFSsplitting of Ag atoms in He II by using this laser system.
References

1) T. Furukawa, Doctral thesis, Osaka University (2007).
2) K. Imamura et al., Hyperfine Interact. 230, 73 (2014).
3) T. Fujita et al., RIKEN Accel. Prog. Rep. 47, 212 (2014).
4) T. Fujita et al., RIKEN Accel. Prog. Rep. 50, 196 (2014).
5) E. Loginov et al., Phys. Rev. Lett. 106, 233401 (2011).
6) W. Kobayashi et al., RIKEN Accel. Prog. Rep. 52, 19 (2019).
7) K. Imamura et al., Appl. Phys. Express. 12, 016502 (2019).

- 141 -