Differentiation of isomeric C 3H 4+·· ions and T → V energy partitioning in surface-induced dissociation

Abstract

Inelastic collisions of allene and propyne molecular ions at a fluorinated self-assembled monolayer (F-SAM) surface allow unambiguous differentiation of these isomers over a wide range of collision energies. Surface-induced dissociation (SID) spectra, examined at collision energies ranging from 10 eV to 100 eV, are similar only in the low collision energy region, where the experimental results suggest that isomerization between allene and propyne molecular ions proceeds more rapidly than dissociation of the CH bond(s). These results also suggest that isomerization proceeds via a common intermediate. As the collision energy is increased, more internal energy is deposited in the projectile, and the CH 3 + CH 2 +· ratios for the isomers begin to diverge owing to successful competition of direct skeletal CC bond cleavage with isomerization of the projectile ions. However, at no collision energy are the products of CH bond cleavage observed to compete successfully with isomerization. Isomeric differentiation can be achieved readily from two plots: the CH 3 + CH 2 +· abundance ratio versus collision energy and a plot of the ratio of this abundance ratio, i.e. CH 3 + CH 2 +· for propyne/CH 3 +/CH 2 +· for allene, versus collision energy. For each plot, an optimum collision energy range is identified for differentiation of the isomers by SID. The total SID signal is also measured over the entire collision energy range from 10 to 150 eV: it rises rapidly from a threshold value of 10 eV to a maximum of > 90% of the total scattered ion current at 35 eV and then slowly falls at higher energy due to competition from chemical sputtering. Besides SID, elastic scattering and chemical sputtering, products of ion/surface reactions, such as CH 2F +, are also observed. All observations are in good agreement with previous thermochemical studies on C 3 H 4 +· . They also agree with earlier results for T → V conversion for F-SAM surfaces for other ions in showing that, on average, for laboratory collision energies up to 40 eV, 20% of this energy is converted to internal energy in the collision.