QUADRUPOLAR DEFORMATIONS OF ATOMIC BUBBLES IN SOLID HE-4

We have studied the magnetic properties of Cs atoms trapped in cubic a nd hexagonal solid He-4 matrices. Observations in the cubic phase supp ort the assumption of a highly symmetrical trapping site (spherical bu bble), while zero-field magnetic resonance spectra and magnetic dipole -forbidden transitions observed in the hexagonal phase are interpreted in terms of a quadrupolar distortion of the atomic bubbles. From the perturbed hyperfine interaction in the hexagonal phase we infer a bubb le deformation of 6%-7%.