Atoms, Molecules and Ions in Superfluid Helium

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Neutral atoms, molecules and ions in superfluid helium have been attracted a great interest because of their unique optical features and energies considerably different from those in gases, solids, and ordinary liquids. For example, the valence electron of neutral atoms pushes our surrounding helium owing to the Pauli exclusion force and make a bubble-like structure. They have high potentiality to be applied to fundamental physics, because of their slow movement and a long spin relaxation time. However, there existed a serious experimental difficulty to implant these particles, particularly neutral atoms, in liquid. To remove this difficulty, we developed at first the laser-sputtering methods and succeeded to implant many kinds of particles at high density in superfluid helium. We could succeeded to implant alkali atoms and molecules for the first time and to observe the excitation spectra and emission spectra. The shifts and broadening of observed spectra were found to be explained theoretically by using the spherical atomic bubble model. We found the existence of quadrupole type oscillation of the bubble shape, from the observed D_2 excitation spectra composed of splitted two components. In addition to laser spectroscopy, we made optical pumping experiment with a circularly polarized laser beam and neraly 100% polarization could be achieved for both electron and nuclear spins. The magnetic resonances and hyperfine resonances could also be observed by optical means and the g-values and hyperfine constants could be measured. Furthermore, we studied two dimentional polarized hydrogen trapped on the surface of superfluid helium, and we could obtained the number density very closed to the critical value for the Bose condensation.