On the stability of cationic complexes of neon with helium – solving an experimental discrepancy

Abstract

Helium nanodroplets are doped with neon and ionized by electrons. The size-dependence of the ion abundance of He_nNe_x⁺, identified in high-resolution mass spectra, is deduced for complexes containing up to seven neon atoms and dozens of helium atoms. Particularly stable ions are inferred from anomalies in the abundance distributions. Two pronounced anomalies at n = 11 and 13 in the He_nNe⁺ series confirm drift-tube data reported by Kojima et al. [T. M. Kojima et al., Z. Phys. D, 1992, 22, 645]. The discrepancy with previously published spectra of neon-doped helium droplets, which did not reveal any abundance anomalies [T. Ruchti et al., J. Chem. Phys., 1998, 109, 10679–10687; C. A. Brindle et al., J. Chem. Phys., 2005, 123, 064312], is most likely due to limited mass resolution, which precluded unambiguous analysis of contributions from different ions with identical nominal mass. However, calculated dissociation energies of He_nNe⁺ reported so far do not correlate with the present data, possibly because of challenges in correctly treating the linear, asymmetric [He–Ne–He]⁺ ionic core in He_nNe⁺. Anomalies identified in the distributions of He_nNe_x⁺ for x > 1, including prominent ones at He₁₂Ne₂⁺ and He₁₄Ne₂⁺, may help to better understand solvation of Ne⁺ and Ne_x⁺ in helium.