Protonated rare gas clusters have previously been shown to display markably different structures than their pure, cationic counterparts. Here we have performed high resolution mass spectrometry measurements of protonated and pristine clusters of He containing up to 50 atoms. We identify notable differences between the magic numbers present in the two types of clusters, but in contrast to heavier rare gas clusters, neither the protonated nor pure clusters exhibit signs of icosahedral symmetries. These findings are discussed in light of results from heavier rare gases and previous theoretical work on protonated helium.
Keywords: helium clusters; helium droplets; mass spectrometry; noble gases; protonation.