Space-Filling Fractal Description of Ion-induced Local Thermal Spikes in Molecular Solid of ZnO

Anions of the molecules ZnO, O2 and atomic Zn and O constitute mass spectra of the species sputtered from pellets of molecular solid of ZnO under Cs+ irradiation. Their normalized yields are independent of energy of the irradiating Cs+. Collision cascades cannot explain the simultaneous sputtering of atoms and molecules. We propose that the origin of the molecular sublimation, dissociation and subsequent emission is the result of localized thermal spikes induced by individual Cs+ ions. The fractal dimension of binary collision cascades of atomic recoils in the irradiated ZnO solid increases with reduction in the energy of recoils. Upon reaching the collision diameters of atomic dimensions, the space-filling fractal-like transition occurs where cascades transform into thermal spikes. These localized thermal spikes induce sublimation, dissociation and sputtering from the region. The calculated rates of the subliming and dissociating species due to localized thermal spikes agree well with the experimental results.