Metastable Ar+2(Ar+2rarr;Ar++Ar) has been observed in a doublehyphen;focusing mass spectrometer from ions created by 70 eV electron bombardment of an Ar cluster beam. New ground and excited state potential energy curves have been calculated for Ar+2, and these have been used to show that metastability is due to radiative decay from the II(1/2)ustate of the ion. It is shown that vertical (FC) ionization from neutral Ar2, with a vibrational temperature of approximately 30 K, results in a significant fraction of the ions occupying the II(1/2)ustate. Detailed pressure dependent measurements show that collisionhyphen;induced dissociation does not contribute to the observed Ar+signal. The mean kinetic energy released to the Ar+has been measured as 44 cmminus;1in the centerhyphen;ofhyphen;mass frame, and calculations show that this value is consistent with the proposed mechanism.
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