Dynamics of hyperthermal energy ion–surface collisions: dissociative and non-dissociative scattering of ethanol cations from a self-assembled monolayer surface of fluorinated alkyl thiol on Au (1 1 1)

Abstract

Dissociation and inelastic scattering of ethanol molecular ions from self-assembled monolayer (SAM) surface of fluorinated alkyl thiol on an Au (111) monocrystal have been studied at 28.9 and 52.9eV collision energies. A single dynamics mechanism for quasi-inelastic scattering was found at both energies. Ions recoil nearly parallel to the surface with very small kinetic energy losses of the order of ≤2eV. Dissociation dynamics features for the main dissociation channel, loss of methyl radical, are dramatically different from that of inelastically scattered primary ions and are different at the two collision energies studied. At 28.9eV two energetically and angularly resolved features are observed, one corresponding to the loss of very large amounts (nearly all) of ion’s translational energy and the other appearing to gain energy (superelastic scattering). This dynamics feature is interpreted as delayed dissociation of ions transmitted through the energy analyzer as molecular ions. This implies a lifetime of such excited ions of more than 5μs. The same dynamics features are observed at 52.9eV ion energy except that a second inelastic process begins to compete with the nearly fully inelastic process. Moreover, at this energy the delayed ion dissociation mechanism is the dominant mechanism. The hypothesis that collision of ethanol cations with the SAM surface initially involves collision of the ion with a single end group of the SAM polymer chain provides a useful rationale for the observed dynamics. Support for this hypothesis is provided by Newton diagrams, which summarize momentum conservation relationships in terms of a common center-of-mass, CMeff, which provides a basis for describing scattering mechanisms for this system. Observation of three energetically distinct scattering processes suggests uniquely different ion–surface interactions contributing to surface-induced dissociation of ethanol ions. Preliminary experiments with Ar+ scattered from the same surface exhibit very similar dynamics features to that observed for ethanol cations. Finally, we note that intensities of scattered primary or fragment ions never approach the specular angle at the energies investigated here.