The dissociation of large organic ions by grazing interaction with a surface potential

Abstract

The special excitation process observed in surface scattering experiments with large organic molecules must be different from the ion-surface interactions related to classical surface induced dissociation. Excitation by deformation, charge exchange and phonon generation are not likely to play a significant role, when a large molecular ion is surfing for a short period of time on a wavy surface potential. Instead two other interaction processes are suspected to be responsible for the energy transfer into the sliding molecule: (i) Braking forces (friction) will cause the molecule to rotate and vibrate internally. If the resulting centrifugal forces exceed a certain limit, the molecule can dissociate. (ii) The peripheral dipoles of the molecule experience a periodical surface potential with frequencies depending on molecule velocity and surface lattice parameters. If those frequencies coincide with frequencies corresponding to the first vibrational excited state of the dipoles, resonance excitation can occur. The vibrational excitations have lifetimes which depend on the structure of the scattered molecule. In special cases they can be accumulated in the molecule during the interaction period until rapid dissociation of an internal bond is induced. In order to reveal the relation between molecular structure and the excitation process, the fragmentation of cyclodextrines and special derivatives thereof induced by grazing surface collisions is investigated in a TOF-mass spectrometer for different surfaces.