Impulsive excitation of FeCp+2 and SiMe+3 during surface-induced dissociation at organic multilayers

Abstract

These experiments scattered 20–90 eV FeCp+2 (Cp=cyclopentadiene) and SiMe+3 ions off multilayers of propylene (C3H6), hexafluoropropylene (C3F6), and tetrachloroethylene (C2Cl4) adsorbed on Ni(111). The ion internal energies following the surface collisions were estimated from the relative fragment ion intensities. The kinetic to internal energy transfer efficiencies varied as follows: C2Cl4 (∼15%)≥C3F6 (∼14%)≳C3H6 (∼10%). The scattered ion kinetic energies were less than 10% of the incident ion energy and did not depend upon the incident ion energy or angles, but did depend upon the chemical identity of the multilayer. The experimental data was analyzed in terms of a three step model of surface-induced dissociation which was previously described to explain the scattering of Cr(CO)+6 off organic monolayers [J. A. Burroughs, S. B. Wainhaus, and L. Hanley, J. Phys. Chem. 98, 10 913 (1994)]. Impulsive excitation is the first step of this model and it semiquantitatively predicted (within a factor of 2) the experimental kinetic to internal energy transfer efficiencies. Impulsive excitation was estimated to occur within 10–35 fs of the initial ion impact. Inelastic scattering and unimolecular dissociation were the latter steps of this model. Both FeCp+2 and SiMe+3 underwent the inelastic scattering step, with the surface dissipating ∼70% of the incident ion kinetic energy. Unlike the unimolecular dissociation observed for Cr(CO)+6 scattering off organic monolayers, both FeCp+2 and SiMe+3 were crudely estimated to dissociate at the surface within 3 ps of the initial ion impact.