Ion Pickup of Large, Surface-Adsorbed Molecules: A Demonstration of the Eley-Rideal Mechanism

Surface collisions of polycyclic aromatic hydrocarbon ions can be made to produce abundant pickup of large, surface-adsorbed species in a tandem time-of-flight mass spectrometer. Attachment of C₁H_n-C₆H_n to naphthalene and phenanthrene ions occurs with collision energies between 40 and 160 eV. Formation efficiency for these ion-adsorbate attachment reactions can be as high as 0.8%. Surface collisions produce no measurable shift in our flight times nor distortion in peak shapes for these species; this indicates the reaction time on the surface must be less than 160 ns. Theoretical calculations show that these reactions are direct (<100 fs residence on the surface) and thus proceed by an Eley-Rideal mechanism.