Structural Characterization of Unprecedented Al₁₄O^– and Al₁₅O₂^–: Photoelectron Spectroscopy and Density Functional Calculations

New aluminum oxide clusters Al₁₄O^– and Al₁₅O₂^– were observed unprecedentedly in the gas-phase reaction of Al_n^– and O₂. Photoelectron spectroscopic measurements and density functional calculations indicated that Al₁₄O^– and Al₁₅O₂^– are composed of an icosahedral Al₁₃ moiety bonded by one and two OAl unit(s), respectively. The preferential formation of Al₁₄O^– and Al₁₅O₂^– is explained in terms of the high stability associated with the Al₁₃ moiety and efficient collisional trapping as intermediates of oxidative etching reactions.