PNNL

Abstract

Dissociative and inelastic scattering of low energy CS2+ ions colliding with a self assembled monolayer surface of fluorinated alkylthiol [CF3 (CF2)9 CH2 CH2SH ] on gold at 45o with respect to the surface normal has been studied using a modified crossed-beam instrument. Dissociation of CS2+ ions begins at about 30 eV collision 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 ion energy of ~ 50 eV, the highest energy accessible with the present apparatus. Both inelastically scattered parent ions and product ions leave the surface with modest 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, consistent with kinematics of an ion-surface collision process that absorbs so much of the kinetic energy of the projectile ion. At all energies studied, projectile ion intensity remains higher than that of fragment ions. Further, the intensity of S+ 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 relative intensities with those found for earlier collision-induced dissociation study of the CS2+ ions with xenon suggests that only ~ 6.5 eV and ~7.5 eV are transferred into internal modes for 30.6 eV and 49.8 eV energy collisions, respectively.