Formation and decomposition of high-mass metal–oxygen cluster ions in the fast atom bombardment of lanthanide salts

Abstract

Fast atom bombardment of lanthanide nitrates in involatile organic matrices gave rise to series of lanthanide–oxygen cluster monocations of general formula [(LnO)_xO_y]⁺ containing up to 40 metal atoms, e.g.[(LaO)₃₃O₁₉]⁺,[(HoO)₂₇O₁₄]⁺ and [(PrO)₄₁O₂₀]⁺. The behaviour within each series depends on the metal atom, but odd–even alternation effects in the ion abundances are universal, with some even-x members missing altogether. The stoichiometry of the clusters also depends on the metal atom, with virtually all series displaying regions of particular stoichiometry, the ratio y:x increasing sharply at particular x values. These well defined regions are particularly numerous for lanthanides (Eu and Sm) featuring two relatively stable oxidation states. There also appear to be certain ‘islands of stability’ where a particular value of x is associated with exceptional stability. Collision-induced decomposition studies of a variety of the cluster ions revealed loss of LnO LnO₂ or Ln₂O₃ moieties, with relative abundances of daughter ions dependent on the pressure of the collision gas; odd–even alternation effects were again observed. Consideration of the detailed stoichiometry of the cluster ions suggests remarkable parallels with the phase behaviour of the lanthanide oxides in the solid state.