IMPULSIVE EXCITATION OF CR(CO)(6)(+) DURING SURFACE-INDUCED DISSOCIATION AT ORGANIC MONOLAYERS

20 to 120 eV Cr(CO)(6)(+) ions are scattered off heptafluorobutyrate and hexanethiolate monolayers adsorbed on Ag(111), and the fragment ions formed by surface-induced dissociation (SID) are detected. The relative fragment ion intensities are monitored to produce SID breakdown curves, which are subsequently compared with threshold photoelectron photoion coincidence (TPEPICO) data to estimate the internal energy of Cr(CO)(6)(+) following the surface collision. The kinetic to internal energy transfer is more efficient for the fluorocarbon than the hydrocarbon monolayer. The scattered ion velocities are centered near 3000, 4000, and 5000 m/s for initial Cr(CO)(6)(+) velocities of 5100, 7800, and 9800 m/s, respectively (30, 70, and 110 eV). A three-step mechanism for SID is proposed in which ions initially undergo impulsive excitation by collision with the surface, inelastically reflect off the surface, and finally dissociate unimolecular. The experimental kinetic to internal energy efficiencies are fit to an impulsive excitation model which quantitatively duplicates the dependence on both the initial kinetic energy and the effective mass of the adsorbate, at the lower collision energies.