Dissociative scattering of hyperthermal energy CF3+ ions from modified surfaces

Abstract

Dissociative scattering of CF3+ ions in collision with a self-assembled monolayer surface of fluorinated alkyl thiol on a gold 111 crystal has been studied at low ion kinetic energies (from 29 to 159 eV) using a custom built tandem mass spectrometer with a rotatable second stage energy analyzer and mass spectrometer detectors. Energy and intensity distributions of the scattered fragment ions were measured as a function of the fragment ion mass and scattering angle. Inelastically scattered CF3+ ions were not observed even at the lowest energy studied here. All fragment ions, CF2+, CF+, F+, and C+, were observed at all energies studied with the relative intensity of the highest energy pathway, C+, increasing and that of the lowest energy pathway, CF2+, decreasing with collision energy. Also, the dissociation efficiency of CF3+ decreased significantly as the collision energy was increased to 159 eV. Energy distributions of all fragment ions from the alkyl thiol surface showed two distinct components, one corresponding to the loss of nearly all of the kinetic energy and scattered over a broad angular range while the other corresponding to smaller kinetic energy losses and scattered closer to the surface parallel. The latter process is due to delayed dissociation of collisionally excited ions after they have passed the collision region as excited parent ions. A similar study performed at 74 eV using a LiF coated surface on a titanium substrate resulted only in one process for all fragment ions; corresponding to the delayed dissociation process. The intensity maxima for these fragmentation processes were shifted farther away from the surface parallel compared to the thiol surface. A new mechanism is proposed for the delayed dissociation process as proceeding via projectile ions' neutralization to long-lived highly excited Rydberg state(s), reionization by the potential field between the collision region and entrance to the energy analyzer, and subsequent dissociation several microseconds after collisional excitation. A kinematic analysis of experimental data plotted as velocity Newton diagrams demonstrates that the delayed dissociation process results from the collisions of the ion with the bulk surface; i.e., the self-assembled monolayer surface acts as a bulk surface. A similar analysis for the highly inelastic collision processes shows that these are due to stronger collisions with a fraction of the thiol molecular chain, varying in length (mass) with the ion energy.