Formation of aluminium clusters in helium nanodroplets

The addition of aluminium atoms to helium nanodroplets has been explored using electron impact mass spectrometry. A series of aluminium cluster ions, Al[subscript n][superscript +], were observed as the major products, which contrasts with a recent study where such cluster ions were not detected in any significant quantities (S.A. Krasnokutski, F. Huisken, J. Phys. Chem. A 115 (2011) 7120). The earlier finding was interpreted as evidence that Al atoms are separated by one or more layers of helium and therefore form a 3-dimensional ‘foam’ inside helium droplets. The current observations are not consistent with this suggestion and instead indicate that when multiple Al atoms are added to helium droplets they aggregate to form Al[subscript n] clusters inside the helium droplets.