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 ClH,-C6H, to naphthalene and phenanthrene ions occurs with collision energies between 40 and 160 eV. Formation efficiency for these ionadsorbate 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 (<lo0 fs residence on the surface) and thus proceed by an Eley-Rideal mechanism. More than 50 years ago, Eley and Rideal) proposed that gas-surface reactions could proceed by direct pickup of species from the surface by a gas-phase molecule. Evidence has accu- mulated that almost all gas-surface reactions follow instead the Langmuir-Hinshelwood mechanism,2 whereby the reactants are first adsorbed on and equilibrate with the surface prior to reaction followed by desorption. The question has been hotly debated whether the Eley-Rideal mechanism applies to any known gas- surface reaction. Recently, Kuipers et ale3 have reported the abstraction of a proton from a hydrogen-covered Pt( 1 1 1) surface by bombardment with a 2-9-eV beam of N(C2H4)3N. They found the time-of-flight distribution of the adduct changed with different initial precursor kinetic energies, clearly indicating an Eley-Rideal mechanism for reaction. Other more indirect evidence for an Eley-Rideal mechanism has been pre~ented.~ In this paper, we report additional examples of an Eley-Rideal reaction involving ion-adsorbate attachment. Cooks and co-worker~~-~ first demonstrated that ions can be made to react with surface-adsorbed molecules upon collision with a surface. Pickup of one to four hydrogenss-' and carbon species up to C4H?-l0 have been observed for several precursor ions colliding with a variety of surfaces. The source of these adsorbates has been attributed to vacuum pump oil present on these surfaces. More recently, well-characterized organic monolayer surfaces have been used;,12 with a perdeuterated thioalkane-gold surface, attachment of CD3 to the molecular ion of pyrazine, (C4H4N2)'+, was observed, a clear demonstration of these ion-adsorbate reactions. Ion fragmentation, called surface-induced dissociation (SID), also OCCU~S,~-~* and Cooks has pioneered its development for structural elucidation of ions by tandem mass spectrometry.19 Here, we show that species as large as C6HW4 can be attached to polycyclic aromatic hydrocarbon (PAH) ions upon surface bombardment in a tandem time-of-flight mass spectrometer. Theoretical calculations indicate that these reactions are direct (<lo0 fs residence time on the surface) and thus proceed by an Eley-Rideal mechanism. Our calculations are consistent with experimental results reported here and elsewhere and provide detailed insight into ion-adsorbate attachment reactions. A reflectron time-of-flight mass spectrometer (R. M. Jordan Co.) modified with an in-line stainless-steel collision surface was used in these experiments (Figure 1). SID spectra are measured with the surface inserted into the reflectron; ions are made to undergo collisions by reducing the potential on the surface to below that of the ion acceleration energy (-2.6 kV). Ions produced at the surface are subsequently accelerated (-2.6 kV), with mass separation taking place based on their flight times to the detector.