Structural elucidation of fullerene dimers by high-resolution ion mobility measurements and trajectory calculation simulations

C-120, C-130, and C-140 cluster anions and cations produced by laser desorption of fullerene films have been examined by high-resolution ion mobility measurements and trajectory calculation simulations. The results for anions and cations are similar. Two peaks are present in the measured drift time distributions. They are attributed to fullerene cage isomers and dimers, The fullerene peak is broad, indicating the presence of a range of fullerene cages with different shapes. The measured mobilities of the dimers are in good agreement with mobilities determined by trajectory calculations for the [2 + 2] cycloadduct geometries of (C-60)(2), C-60. C-70, and (C-70)(2). Comparison of the mobilities calculated for the dimer geometries by the commonly used hard-sphere projection approximation with those determined by trajectory calculations with a realistic potential shows that the projection approximation significantly underestimates the inverse mobilities, The deviations, which are large enough to assign the observed features to the wrong geometries, result mainly from the complete neglect of the scattering process in the hard-sphere projection approximation.