120 eV Cr(C0)6+ ions are scattered off heptafluorobutyrate and hexanethiolate monolayers adsorbed on Ag( 1 1 l), 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(C0)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(C0)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 unimolecularly. 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.

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.

The modification of a hexanethiolate self-assembled monolayer (CH3(CH2)5S) adsorbed on Ag(lll) by exposure to pyridine, thiophene, furan, and fert-butyl molecular ions during surfaceinduced dissociation (SID) experiments is probed using infrared reflection absorption and scattered ion spectra. As much as 60% of the monolayer is modified after a 2.9 X 1014 ions/cm2 exposure at an energy of 32 eV. Ion-induced modification consists of C-H and C-C bond cleavage in the hexanethiolate adsorbate. Ion-induced adsorbate modification appears at an ion kinetic energy threshold of ~20 eV and rises sharply at 25 and 32 eV but is independent of the species of the incident ion. Even after extensive ion bombardment of the hexanethi-

It is demonstrated that laser desorption ion trap mass spectrometry (LD-ITMS) can be successfully applied to the chemical analysis of a monolayer of adsorbates on a solid surface. Negative ion spectra obtained from LD-ITMS of self-assembled monolayers adsorbed from solutions of alkanethiols (CH3(CH2)nSH with n=5, 9, and 15) onto polycrystalline gold surfaces displayed clear ion peaks corresponding to the sulfonate adsorbate species. Sulfonate ions with the general formula CH3(CH2)n SO3 were detected at m/z 165, 221, and 305, respectively, and were derived from the partial oxidation of the corresponding alkanethiol self-assembled monolayers. Little fragmentation and no clustering was observed in these mass spectra. These results indicate that the sensitivity of LD-ITMS is sufficient to allow its application to a wide array of problems in surface science.

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