  The stability of two-component clusters consisting of light (Na or K) and
heavy (Rb or Cs) alkali atoms formed on helium nanodroplets is studied by
femtosecond laser ionization in combination with mass spectrometry.
Characteristic stability patterns reflecting electron shell-closures are
observed in dependence of the total number of atoms contained in the mixed
clusters. Faster decay of the stability of mixed clusters compared to the pure
light ones as a function of size indicates a destabilizing effect of heavy
alkali atoms on light alkali clusters, presumably due to second order
spin-orbit interaction.
