Spectroscopic studies of neutral otoms in and on superfluid helium

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A neutral atom implanted into liquid helium produces a bubble-like structure of helium, pushing out the surrounding helium due to the Pauli exclusion principle, This model of impurity atom in a self-produced bubble is called atomic bubble model. It is now known that this model explains well the general features of observed spectra. In the present work, we have focused particularly on the phenomena which cannot be explained by this model. For the ground state alkali atom, we observed the splitting of the exciting D2 spectra, which were found to be due to quadrupole oscillations caused by zero-point fluctuation. For the excited alkali atom, some new infrared spectra were observed, which were shifted considerably from the D lines. Comparing with calculated adiabatic potentials, we found that these lines infrared lines were from the molecules or clusters with several helium atoms sticking to the excited alkali atom. The Tm atoms implanted into liquid and solid helium were also studied. The optical transitions between the ground and the lower-lying states are due to those of the inner core electrons. There exist two outer electrons, which are expected to shield surrounding helium. We could succeed to observe several transitions of Tm atoms, the inner core transitions being extremely sharp due to this shielding. We succeeded to observe a weak magnetic dipole transition between two fine-structure levels and to measure precisely the lifetime of the upper metastable state. We have also studied gaseous alkali atoms interaction with the surface of liquid helium. The binding energy on the surface is very low and hence the interaction is mostly repulsive, Using the liquid helium film coated on the cell walls as an atomic reflector, we could realize, for the first time, a long-lived alkali vapor cell at a liquid helium temperature. Almost perfect spinpolarization of alkali atoms in the cell could be attained by using optical pumping techniques.