Surface-Induced Dissociation of Polyatomic Hydrocarbon Projectile Ions with Different Initial Internal Energy Content

Abstract

The specific effect of initial internal energy of CH 5 +, C 2 H 4 +, C 2 H 5 +, and C 2 H 6 + ions on the extent of surface-induced fragmentation was investigated. The ions were prepared either in a gas discharge Colutron type (using a hydrogen-methane mixture at 0.2-0.5 Torr) or in a low-pressure Nier-type electron impact ion source using various target gases. Whereas projectile ions from the Colutron source are thermalized due to collisions in the high pressure environment and thus contained a negligible amount of internal energy, projectile ions from the Nier-type ion source resulting from direct electron impact ionization reactions have internal energies up to several eV. Their internal energy content was estimated using break-down curves and photoelectron spectra from literature. Results obtained here show that their different initial internal energy content had a considerable effect on the extent of fragmentation of the surface-excited projectile ions: ions with initial internal energy fragmented at much lower collision energies than internally relaxed projectile ions. It appears that the initial internal energy content of the projectile ions is entirely preserved in the projectile ion during the ion/surface collision, and thus is available in the subsequent dissociative processes as additional energy to the internal energy acquired by the projectile ion in the surface-excitation process.