Decorating (C₆₀) ⁺, n = 1-3, with CO₂ at low temperatures: Sterically enhanced physisorption

Multiple attachment of CO₂ to the monomer, dimer and trimer cations of C₆₀ has been observed in the mass spectra of He nanodroplets sequentially doped with C₆₀ and CO₂ and exposed to electron ionization at 50 eV. Remarkable anomalies were seen in the ion yield for CO₂ coverage for (C₆₀)₂⁺(CO₂)₈ and (C₆₀)₃⁺(CO₂)_1,2. These provide insight into the influence of steric properties on the nature of physisorption. The enhanced stabilities of (C₆₀)₂⁺(CO₂)₈ and (C₆₀)₃⁺(CO₂)_1,2 are attributed to physisorption inside the "groove" of the dimer and the two "dimples" in the trimer cations of C₆₀. Molecular dynamics simulations provide a qualitative assessment of the observed physisorption and a useful visualization of structural aspects.