Metastable decay of Ar n + involving single monomer evaporation and the loss of peculiar numbers of monomers

Abstract

Production and stability of Ar n +* ions (withn up to 420) formed by electron impact ionization of a supersonic Ar cluster beam were investigated with a double focussing sector field mass spectrometer. The present study confirms previous magic number determinations up to the 4th icosahedral shell. A systematic study of metastable dissociations (monomer evaporation, magic number evaporation) for singly charged cluster ions as a function of cluster size, internal excitation energy and time elapsed since ion formation gives new insight into the ionization process and subsequent reactions of the ions formed. At a well-defined threshold energy ofca. 28 eV, the magic number loss mechanism occurs simultaneously with the well known single monomer evaporation process which proceeds at all energies. The new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.