Dynamics of the dissociative and nondissociative scattering of hyperthermal CS2+ from a self-assembled fluoroalkyl monolayer surface on gold substrate

Abstract

Dissociative and nondissociative scattering of low energy CS2+ ions from a self-assembled monolayer surface of fluorinated alkylthiol [CF3(CF2)9CH2CH2SH] on vapor deposited gold has been studied using a modified crossed-beam instrument. Dissociation of CS2+ ions begins at ∼30 eV ion kinetic energy, much higher than the thermochemical threshold of 4.7 eV for the lowest energy dissociation channel forming S+. This product channel is dominant up to the ion energy of ∼50 eV, the highest energy accessible by this instrument. Both inelastically scattered parent ions and product ions leave the surface with very low kinetic energies, demonstrating that most of the ions’ kinetic energy is taken up by the surface rather than transferred into internal modes of recoiling ions. The scattered ion intensity maximum is found between the specular angle and the surface parallel. At all energies studied, primary ion intensity remains higher than that of fragment ions. Also, the intensity of S+ fragment ions is higher than that of CS+ fragment ions, suggesting that the distribution of internal excitation of the recoiling CS2+ ions extends only slightly above the thresholds for the two product channels. A comparison of the relative intensities of the fragment ions with those from earlier collision-induced dissociation study of the CS2+ ions with xenon suggests that only ∼6.5 and ∼7.5 eV are transferred into internal modes for 30.6 and 49.8 eV energy collisions, respectively. This is lower than the energy transferred into internal modes in the gas phase collision-induced dissociation process, for which the center-of-mass collision energy is well defined. We infer from our observations that the effective mass of the surface collision partner is much less than that of the infinite mass which would apply to scattering from the gold substrate or the mass of the monolayer surface molecule. The experimental velocity vectors of scattered primary and fragment ions reveal that their maxima follow a circle whose center falls on the ion velocity vector, away from the laboratory collision center. From this collision center, we estimate the effective mass of the surface for surface-induced dissociation to be 150 corresponding to the CF3CF2CF end group of the SAM molecular chain.