Collisions of slow polyatomic ions with surfaces: the scattering method and results

Surface-induced dissociation (SID) and reactions following impact of well-defined ion beams of polyatomic cations C2H5OH+, CH4+, and CH5+ (and its deuterated variants) at several incident angles and energies with self-assembled monolayers (SAM), carbon surfaces, and hydrocarbon covered stainless steel were investigated by the scattering method. Energy transfer and partitioning of the incident projectile energy into internal excitation of the projectile, translational energy of products, and energy transferred into the surface were deduced from the mass spectra and the translational energy and angular distributions of the product ions. Conversion of ion impact energy into internal energy of the recoiling ions peaked at about 17% of the incident energy for the perfluoro-hydrocarbon SAM, and at about 6% for the other surfaces investigated. Ion survival probability is about 30-50 times higher for closed-shell ions than for open-shell radical cations (e.g., 12% for CD5+ versus 0.3% for CD4+, at the incident angle of 60 degrees with respect to the surface normal). Contour velocity plots for inelastic scattering of CD5+ from hydrocarbon-coated and hydrocarbon-free highly oriented pyrolytic graphite (HOPG) surfaces gave effective masses of the surface involved in the scattering event, approximately matching that of an ethyl group (or two methyl groups) and four to five carbon atoms, respectively. Internal energy effects in impacting ions on SID were investigated by comparing collision energy resolved mass spectra (CERMS) of methane ions generated in a low pressure Nier-type electron impact source versus those generated in a Colutron source in which ions undergo many collisions prior to extraction and are essentially vibrationally relaxed. This comparison supports the hypothesis that internal energy of incident projectile ions is fully available to drive their dissociation following surface impact.