An experimental study of the formation and reactivity of ionic hydrogen clusters: The first observation and characterization of the even clusters H+4, H+6, H+8, and H+10

A specially designed coaxial drift tube type ion source has been utilized to generate ionic hydrogen clusters. The cluster spectrum observed is dominated by odd clusters (H+5, H+7, H+9, and H+11 ) in line with past studies on hydrogen ion clusters. However, for the first time even hydrogen cluster ions are formed in a high pressure ion source, with relative intensities (H+6≫H+8>H+10>H+4) . The observation of a relatively intense H+6 peak (H+6/H+5 =0.05) was unexpected. The peak at nominal mass 6 was confirmed to be H+6 (and not H4D+) by high resolution studies. A number of possible formation mechanisms for H+6 are discussed. The most likely mechanism is reaction of odd hydrogen cluster ions (H+3, H+5, H+7,... ) with metastable electronically excited odd hydrogen cluster neutrals (H*3, H*5, H*7,...) . The H*3 species has been experimentally characterized by other research groups (Herzberg; Kuppermann) and is probably formed by electron recombination with H+3 (v≠0), H+5 or H+7 in our source. The H+6 ion is observed to lose both H atom and H2 by metastable decomposition in a ratio of H/H2≂4. The kinetic energy distribution for H loss is broad and bimodal with an average release of 38 meV and a maximum release of 140 meV. Collision induced decomposition studies are utilized to characterize both the even and odd clusters. A 50/50 mixture of H2 and D2 yielded a large number of isotopically mixed clusters that were identified by high resolution mass spectrometry.