An investigation of the properties of large krypton cluster ions (development of the P=3 and P=4 Mackay icosahedral shells)

Abstract

A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to study the unimolecular fragmentation patterns of krypton cluster ions, Kr+n, for n in the range 10–340. All cluster ions are observed to undergo the reaction Kr+n →Kr+n−1 + Kr; however, some clusters lose up to five atoms on the time scale of the experiment. The six naturally occurring isotopes of krypton complicates the cluster-ion mass spectra, with the number of ion peaks increasing rapidly with n for a given Kr+n. A computer model which successfully accounts for the influence of isotopes reveals that isotope enrichment of the cluster ions occurs. The relative intensities of the fragment ions exhibits pronounced intensity fluctuations, many of which coincide with similar observations on large argon cluster ions. It is proposed that particularly intense fragments, i.e., Kr+111 and Kr+120, arise from reactions which lead to the formation of kinetically favorable products. In contrast, intense ion peaks in the mass spectrum, i.e., Kr+147 and Kr+309, are attributed to the formation of thermodynamically favorable products. The latter coincide with shell closure in Mackay icosahedra.