PRESSURE SHIFT OF ATOMIC RESONANCE LINES IN LIQUID AND SOLID HELIUM

We show that for atoms trapped in a liquid He matrix the excitation an d emission spectra can be quantitatively described by the bubble model which treats the matrix as an incompressible continuous medium. Compa rison with experimental results shows that the model also holds for a solid matrix. The model is only valid in the case where the attractive He-He interaction dominates the attractive part of the foreign atom-h elium interaction. Its failure in the case of implanted Na atoms is di scussed.