Atoms, Molecules and Ions in Superfluid Helium

超流氦中的中性原子、分子和离子由于其独特的光学性质和能量与气体、固体和普通液体中的中性原子、分子和离子有很大的不同而引起人们的极大兴趣。例如，中性原子的价电子由于泡利排斥力而推动我们周围的氦，并形成气泡状结构。由于它们的慢运动和长的自旋弛豫时间，在基础物理中具有很高的应用潜力。然而，在液体中植入这些粒子，特别是中性原子，存在严重的实验困难。为了克服这个困难，我们首先发展了激光溅射的方法，并成功地在超流氦中注入了高密度的多种粒子。首次成功地实现了碱金属原子和分子的注入，并观察到了激发光谱和发射光谱。用球形原子泡模型从理论上解释了观测光谱的移动和展宽。我们从观测到的由分裂的两个分量组成的D_2激发谱中发现了气泡形状的四极型振荡。除了激光光谱学之外，我们还用圆偏振激光束进行了光泵浦实验，对电子和核自旋一般都能达到100%的偏振。磁共振和超精细共振也可以通过光学手段观察到，g值和超精细常数可以测量。此外，我们还研究了二维极化氢在超流氦表面的囚禁，得到了非常接近玻色凝聚临界值的数密度。

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.