Positron-induced dissociation of organic molecules

We have measured positron-ionization time-of-flight mass spectra of butylbenzene, decane, tetraethylsilane, and other organic molecules as a function of the positron kinetic energy. Our data show that fragment ions are produced at onset energies above the positronium- (Ps) formation threshold, while the molecular ions are formed starting at the Ps threshold. The onset energies have been found to depend strongly on the nature of the fragment and to be equal to the dissociation energy of the fragmentation reaction. Decreases in the molecular-ion populations in the mass spectra were observed to be accompanied by increases in fragment-ion populations. A two-step process, stripping followed by unimolecular dissociation, is proposed to be responsible for the production of the fragment ions. The degree of dissociation can be controlled by selecting the positron incident energy