Formation and ionization energies of small chlorine-doped lithium clusters by thermal ionization mass spectrometry

RATIONALE Theoretical calculations have shown that the first ionization energy of clusters of the type LinCl (n ≥2), with more than eight valent electrons, is lower than that of alkali metal atoms; hence they are named superalkali. Superalkali clusters can mimic the chemical behavior of alkali metals and may be used as building blocks of new cluster-assembled materials. There is currently no reliable experimental proof of this kind of clusters and such proof is required. METHODS The LinCl (n = 2–6) clusters were produced by a thermal ionization source of modified design, and mass selected by a magnetic-sector mass spectrometer. The modification pertains to the replacement of the side filaments by a cylinder in the triple-filament thermal ionization source. The sample, which is LiCl salt, was pressed into a ring and placed on the inner wall of the cylinder. RESULTS It was observed that the ions of clusters with an even number of lithium atoms (Li2Cl+, Li4Cl+, Li6Cl+) are more stable than the odd-numbered ones (Li5Cl+, Li3Cl+). The ionization energies were determined to be 3.98 ± 0.25 eV for Li2Cl, 4.10 ± 0.25 eV for Li3Cl, 3.90 ± 0.25 eV for Li4Cl, 4.01 ± 0.25 eV for Li5Cl, and 4.09 ± 0.25 eV for Li6Cl. The presence of a halogen atom reduces the ionization energy of the metal clusters. CONCLUSIONS The thermal ionization source of modified design presents a suitable simple way to obtaining and measuring the ionization energies of very small lithium monochloride clusters. Clusters LinCl, n = 4 to 6, were detected for the first time. Copyright © 2012 John Wiley & Sons, Ltd.