ELECTRONIC EXCITATIONS IN LIQUID-HELIUM - THE EVOLUTION FROM SMALL CLUSTERS TO LARGE DROPLETS

The absorption coefficient of liquid He droplets is reported in the en ergy range from the first electronic excitation up to the ionization l imit. It is obtained from total fluorescence yield curves. The size of He droplets is varied between approximately 10(6) atoms/particle to s mall clusters containing on the order of 50 atoms. The character of th e excited states is discussed in view of different theoretical approac hes, i.e., molecular excitation, Frenkel and Wannier type excitons. Th e Frenkel and Wannier model, which is very successful for the descript ion of excited states in insulators, has severe shortcomings in the ca se of liquid He. Furthermore, the implications for the structural pecu liarities of liquid He are addressed.