RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Introduction of silver atoms into superﬂuid helium
using laser ablation method
T. Fujita,∗1,∗2 K. Imamura,∗1,∗3 D. Tominaga,∗1,∗4 T. Egami,∗1,∗4 W. Kobayashi ,∗1,∗4 Y. Nakamura,∗1,∗3M. Sanjo,∗1,∗4 T. Kobayashi,∗5 A. Takamine,∗1 T. Furukawa,∗1,∗6 H. Ueno,∗1 T. Shimoda,∗2 and Y. Matsuo∗1,∗4
The Optical Radioisotope atom Observation in Condensed Helium as Ion-catcher (OROCHI) utilizes superﬂuid helium (He II) as a stopping material ofenergetic ion beams of a few tens to hundreds ofMeV/nucleon. Injected ion beams are neutralized during the stopping process, and neutralized atoms are
conﬁned in a region of 1 to several mm2 over a second.
There He II also serves as a host matrix of laser spectroscopy of stopped atoms1) . Since He II is opticallytransparent in wide frequency region from ultraviolet(UV) to radio frequency (RF), it is possible to apply
laser-RF and laser-microwave (MW) double resonancemethods to atoms in He II to measure Zeeman andhyperﬁne splitting energies. From these splitting energies, values of nuclear spin and electromagnetic moment can be deduced.
The measured hyperﬁne splitting (HFS) energies
were slightly shifted from those in vacuum for alkali
atoms such as Rb and Cs owing to the eﬀect of surrounding helium atoms2) . However, the observed energy shift  E/E  1% was suﬃciently small to discuss nuclear structures. Moreover, the preliminary
results indicated that this method provides a highprecision measurement of a magnetic dipole hyperﬁnestructure constant to derive a hyperﬁne anomaly which
gives information on the distribution of nuclear charge
and magnetization. Recently, we started to apply themethod to group 11 atoms such as Au and Ag.3,4) Theobserved shift of the HFS energy for Au atoms waseven smaller than that for alkali atoms. The investigation of HFS of Ag atoms will lead to more detailed information because Ag has two stable isotopes,
which are necessary to determine a hyperﬁne anomaly
whereas Au has only one stable isotope.
In oﬄine experiments, we adopted a two-step lasersputtering technique to introduce atoms into He II(Fig. 1). As the ﬁrst step, a sample material placed 1
cm above the He II surface is ablated by either a thirdharmonic or a second-harmonic pulse of a neodymiumdoped yttrium aluminum garnet laser (wavelength:355 or 532 nm, repetition rate: 10 Hz, pulse width:
8 ns, pulse energy: ∼5 mJ). Clusters out of the sample material immerse into He II. Next, the clusters aredissociated by a femtosecond titanium-doped sapphirelaser (wavelength: 800 nm, repetition rate: 500 Hz,
∗1
∗2
∗3
∗4
∗5∗6

RIKEN Nishina Center
Department of Physics, Osaka University
Department of Physics, Meiji University
Department of Advanced Science, Hosei University
RIKEN Center for Advanced Photonics
Department of Physics, Tokyo Metropolitan University- 196 -pulse width: ∼120 fs, pulse energy: ∼200 μJ). The
method was successful in the previous experiments forRb, Cs and Au atoms. However, a suﬃcient numberof atoms were not introduced into He II in the caseof Ag. In this experiment, the ablation plasma produced by the ablation laser irradiation was observed,whereas the plasma produced by the dissociation laser
irradiation was not observed. We concluded almost no
introduction of Ag atoms in this condition because thisdissociation laser was not eﬀective for Ag.
One of the possible reasons for the diﬃculty in thedissociation by the dissociation laser could be the difference in absorption eﬃciencies between Ag particles and other species. The Ag sample has a highreﬂectance ratio in infrared and visible regions. To
achieve the measurement of HFS for Ag, we plan tochange the dissociation laser to a second-harmonic
pulse of the femtosecond laser (wavelength: 400 nm,repetition rate: 500 Hz, pulse width: ∼200 fs, pulse energy: ∼100 μJ) and a XeCl excimer laser pulse (wavelength: 308 nm, repetition rate: 10 Hz, pulse energy:∼10 mJ) because, generally, UV pulses dissociate particles more eﬃciently.

Fig. 1. Schematic of the two-step laser sputtering techniquein our experimental setup. For details, see the text.

References
1) X. F. Yang et al., Phys. Rev. A 90, 052516 (2014).
2) K. Imamura et al., RIKEN Accel. Prog. Rep. 48, 251(2015).
3) T. Fujita et al., RIKEN Accel. Prog. Rep. 47, 212(2014).
4) T. Furukawa et al., RIKEN Accel. Prog. Rep. 42, 194(2009).