We present new results on the properties of Celsium and Rubidium atoms trapped in crystalline helium matrices. We have extended the measurements of optical excitation spectra and excited state lifetimes, previously performed in superfluid helium, to solid helium. Cs atoms in the hcp phase of 4He exhibit several distinctive features, such as dipole-forbidden transitions and zero-field magnetic resonance lines. We interpret these features as being due to static quadrupolar deformations of the local trapping sites (atomic bubbles). The degree of deformation of the bubbles can be obtained by comparing hyperfine coupling constants in the cubic and hexagonal phases. We have investigated in detail optical pumping of Cs in solid helium, and have gained new insights into the pumping mechanism. We have also succeeded, for the first time, in detecting Rb and Rb in solid helium, by applying an absorption technique, based on driven spin precession.
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