Mass Spectrometric Observation of Doubly Charged Alkaline-Earth Argon Ions.

Doubly charged diatomic ions MAr2+ where M=Mg, Ca, Sr or Ba have been observed by mass spectrometry with an inductively coupled plasma ion source. Abundance ratios are quite high, 0.1% for MgAr2+, 0.4% for CaAr2+, 0.2% for SrAr2+ and 0.1% for BaAr2+ relative to the corresponding doubly charged atomic ions M2+. It is assumed that these molecular ions are formed through reactions of the doubly charged metal ions with neutral argon atoms within the ion source. Bond dissociation energies (D-0) were calculated and agree well with previously published values. The abundance ratios MAr+/M+ and MAr2+/M2+ generally follow the predicted bond dissociation energies with the exception of MgAr2+. Mg2+ should form the strongest bond with Ar [D-0 (MgAr2+)=124 to 130kJmol(-1)] but its relative abundance is similar to that of the weakest bound BaAr2+ (D-0=34 to 42kJmol(-1)). The relative abundances of the various MAr2+ ions are higher than those expected from an argon plasma at T=6000K, indicating that collisions during ion extraction reduce the abundance of the MAr2+ ions relative to the composition in the source. The corresponding singly charged MAr+ ions are also observed but occur at about three orders of magnitude lower intensity than MAr2+.