Annual Review 2000

51RESEARCH ACTIVITIES II
Department of Molecular Structure
II-A Laser Cooling and Trapping of Metastable HeliumAtoms
In the past two decades, extensive developments have occurred in the laser cooling and trapping of neutral
atoms, with many workers reporting the application of these techniques to such diverse atomic species as alkaliatoms, alkali earth atoms, and rare gas atoms. Among these, the helium atom is unique on account of its small mass,
simple energy level structure, and easy availability in two isotopic forms (3He and 4He) of differing quantum
statistics. For this reason, we have been studying the laser cooling and trapping of helium atoms.
II-A-1 Magneto-Optical Trap of Metastable
Helium-3 Atoms
KUMAKURA, Mitsutaka; MORITA, Norio[Appl. Phys. B: Lasers Opt. 70, 555 (2000)]
A magneto-optical trap (MOT) of metastable 3Heatoms has been demonstrated for the first time. Some105 atoms have successfully been confined in a region
with a diameter of ~0.4 mm at a temperature of ~0.5mK; the atomic number density is estimated to be ~109/cm3 at the trap center. These characteristics of the 3He
MOT are almost comparable to those of the 4He MOT
so far demonstrated by many workers. Monitoring thefluorescence from the MOT, the trap loss rate has also
been measured and discussed. Since 3He is a uniquefermionic atom on account of its small mass and simple
energy level structure, we can expect that such a 3He
MOT will be useful as a fundamental tool for futurestudies on the physics of fermions at ultralow
temperatures.

II-B Spectroscopic Studies on Atoms and Ions in Liquid
HeliumAtoms and ions in liquid helium are known to reside in bubble-like cavities due to the Pauli repulsive force
between electrons. Physical properties of these exotic surroundings are determined by the potential energy of theimpurity-Hen system, the surface tension energy of the liquid helium, and the pressure-volume work. Spectroscopic
studies of such impurity atoms and ions in liquid helium are expected not only to give information on the structureand dynamics of the bubbles but also to contribute to the study on the property of superfluid liquid helium.
II-B-1 Theoretical Studies on the Spectra ofYb+ Ions in Liquid Helium
MORIWAKI, Yoshiki; MORITA, Norio

Consequently, it has been found that the blue shifts arewell understood with this bubble model, and also thatthe dynamic Jahn-Teller effect due to the quadrupole
vibration of the bubble plays an important role for thedouble-peaked profile of the D2 excitation spectrum.

[Eur. Phys. J. D 13, 11 (2001)]
In our previous experimental studies on Yb + inliquid helium, we found that its spectra have two
characteristic properties: (1) The 4f146s2S1/2–6p2P1/2(D1) excitation spectrum is much broadened and blueshifted compared with the spectrum of free Yb+ ions,while the emission spectrum of the same transition has
relatively small spectral width and shift compared withthe excitation spectrum. (2) The excitation spectrum of
the 4f146s2S1/2–6p2P3/2 (D2) transition is doubly peaked.
To explain these properties, we have carried out
theoretical calculations on the basis of a vibrating
bubble model, in which the bubble surface is assumed tovibrate in the spherical (breathing), dipolar andquadrupolar modes. These calculations are essentiallybased on adiabatic potential curves of an Yb+–He pair,which have been obtained from our complete-activespace self-consistent field (CASSCF) and multireference configuration-interaction (MRCI) calculations.

II-B-2 Measurements of Fine Structure
Changing Cross Sections of Ca + and Sr + in
Collisions with He Atoms
MORIWAKI, Yoshiki; MATSUO, Yukari 1 ;MORITA, Norio(1RIKEN)
In our previous spectroscopic study of Yb+ in liquid
helium, we observed an emission from its 4f146p 2P1/2
fine structure level not only when this level was directlyexcited but also when another fine structure level(4f146p 2P3/2) was excited. This suggested the presenceof a fast inter-multiplet transition from 2P3/2 to 2P1/2 dueto the interaction between Yb+ and He atoms. From ourestimation, which was based on the comparison between
the emission intensities measured when the 2P1/2 and2P2
3/2 levels were excited, we found that the P3/2 →2P
1/2 transition should be extremely faster than expected

52RESEARCH ACTIVITIES II Department of Molecular Structure

for two-body collisions of other atoms and ions with He.
This enhancement of the transition may have arisen
from some many-body effect, such as the quadrupole
vibration of the helium bubble surface. However we
could not discuss it any more, because there were nodata on the fine structure changing rate in two-bodycollisions, which was necessary to exactly estimate themany-body effect. This fact motivated us to study finestructure changing rates of alkali-earth ions in two-bodycollisions with He.
We have started with measurements on Ca+ and Sr+
to test our experimental apparatus. These ions have been
produced by laser ablation of pure metal samples.
Detecting laser induced fluorescence, we have measured
cross sections of collision induced transitions between
fine structure levels in the 4p2PJ state of Ca+ and the5p2PJ state of Sr+ due to collisions with He atoms atroom temperature (298 K). The cross sections obtained
are σ(Ca+: 4p2P3/2 → 4p2P1/2) = 1.17 ± 0.05 Å2, σ(Ca+:4p 2P 1/2 → 4p 2P 3/2) = (7.92 ± 0.44) × 10 –1 Å 2, andσ(Sr+: 5p2P3/2 → 5p2P1/2) = (1.44 ± 0.10) × 10–2 Å2.
These cross sections are much smaller than those ofneutral K and Rb atoms, which have the same electron
configurations as Ca+ and Sr+, respectively. This may
probably be because stronger spin-orbit couplings in the2PJ states of Ca+ and Sr+ prevent their electron spinsfrom flip-flopping all the more. On the other hand, it is
known that the cross sections of alkali atoms areroughly proportional to e–C∆E (where ∆E is the finestructure splitting and C is a constant). This is seen fromthe fact that the cross sections for alkali atoms arealmost on a straight line in Figure 1. However, thepresent cross sections for Ca + and Sr + significantly
deviate upward from this line, as seen in Figure 1; thatis, these cross sections are much larger than expectedfrom only the sizes of the fine structure splittings. This
may probably be due to a difference between theinteractions in ion-He and atom-He pairs.

Figure 1. Fine structure changing cross sections so far
measured for various alkali atoms and alkali earth ions incollisions with He atoms, as a function of their fine structure
splittings; ● shows the one for the np 2 P 3/2 → np 2 P 1/2
transition of each alkali atom (for Na, K, Rb and Cs, n = 3, 4,
5 and 6, respectively, and the collision temperature T = 397,368, 340 and 311 K, respectively) (by Krause), ◆ Mg+ 3p2P3/2→ 3p2P1/2 at 1600 K (by Brust), ▲ Ca+ 3d2D5/2 → 3d2D3/2 at
10000 K (by Knoop et al.), and ● the present data.