Mass spectrometry and photoionization studies of the ablation of ZnO: ions, neutrals, and Rydbergs

Time-of-flight/quadrupole mass spectrometry (TOFQMS) and photoionization studies have been performed on the ablation plume from a ZnO target to determine the mass, charge, and kinetic energies of the species ejected. Results were collected for ions and neutrals over a ranee of laser fluences (0.20-2.50 J/cm(2)) and the most prominent species were monatomic Zn and O atoms and ions; this study focuses on the energetics of Zn and Zn+. At laser fluences below similar to 0.7 J/cm(2), the Zn atoms had an average kinetic energy that depended logarithmically on the laser fluence, ranging from similar to 1.2 eV near the desorption threshold of 0.25 J/cm(2) to a maximum of similar to 4 eV near the ablation threshold at 0.7 J/cm(2). These kinetic energies were modeled in terms of a three-photon process at the surface. The Zn ions had similar kinetic-energy distributions to the neutrals, but of much lower intensity. At the ablation threshold, the ion signal rapidly increased and attained a peak kinetic enery of similar to 17 eV. Slightly above this threshold, the ion kinetic energy broadened and separated into two distinct components centered about 17 eV. The high-energy species rapidly accelerated to over 100 eV over the 0.7-2.5 J/cm(2) fluence range, while the low-energy component decelerated to an average value of similar to 4 eV, limited by the presence of the slower-moving neutrals. Electron photodetachment experiments performed over this fluence range showed significant gas-phase electron-ion recombination and were used to reconcile some of the energetics observed. (C) 1998 Elsevier Science B.V.