On the origin of metastable decay in Ar+2

Metastable Ar+2 (Ar+2 →Ar++Ar) has been observed in a double-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)u state 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)u state. Detailed pressure dependent measurements show that collision-induced dissociation does not contribute to the observed Ar+ signal. The mean kinetic energy released to the Ar+ has been measured as 44 cm−1 in the center-of-mass frame, and calculations show that this value is consistent with the proposed mechanism.