Electronic excitations in liquid helium: The evolution from small clusters to large droplets

The absorption coefficient of liquid He droplets is reported in the energy range from the first electronic excitation up to the ionization limit. It is obtained from total fluorescence yield curves. The size of He droplets is varied between approximately 10⁶ atoms/particle to small clusters containing on the order of 50 atoms. The character of the excited states is discussed in view of different theoretical approaches, i.e., molecular excitations, Frenkel and Wannier type excitons. It turned out that the Frenkel and Wannier model being very successful for the description of excited states in insulators has severe shortcomings in the case of liquid He. Furthermore, the implications for the structural peculiarities of liquid He are addressed.