Mechanisms of laser desorption of positive ions and plume formation on ionic crystals

In the use of high-intensity lasers in chemical analysis and materials processing, the formation of an ablation plume is of high importance. For wide bandgap insulators (e.g., oxides, halides, nitrides, carbides) irradiated with sub-bandgap photon energies, the route to plume formation is not well understood. For example, contrary to metals and semiconductors, inverse bremsstrahlung (IB) is not possible for a wide range of laser intensities on these materials due to insufficient photon and electron densities. We present an alternative path to plume formation on nominally transparent materials. We first examine the interaction of photo-and thermally-emitted particles from exposure to pulsed laser irradiation of surfaces that include photoelectrons, energetic positive ions, and neutral metal atoms and present a model for the energetic ion emission. We establish experimentally that there is overlap in space and time of significant portions of the distributions of these particles in the near surface region. We present a model for the collected motion of these particles, and show that as laser fluence is increased, we achieve sufficient densities, overlap and kinetic energies to result in the onset of plume luminescence and eventually ionization at fluences far below any IB or catastrophic breakdown process. (C) 1998 Elsevier Science B.V.