Submersion of potassium clusters in helium nanodroplets

Small alkali clusters do not submerge in liquid helium nanodroplets but instead survive predominantly in high spin states that reside on the surface of the nanodroplet. However, a recent theoretical prediction by Stark and Kresin [Phys. Rev. B 81, 085401 (2010)], based on a classical description of the energetics of bubble formation for a fully submerged alkali cluster, suggests that the alkali clusters can submerge on energetic grounds when they exceed a critical size. Following recent work on sodium clusters, where ion yield data from electron impact mass spectrometry was used to obtain the first experimental evidence for alkali cluster submersion, we report here on similar experiments for potassium clusters. Evidence is presented for full cluster submersion at n > 80 for K n clusters, which is in good agreement with the recent theoretical prediction. In an additional observation, we report "magic number" sizes for both K-n(+) and K-n(2+) ions derived from helium droplets, which are found to be consistent with the jellium model.