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

Time-of-flightrquadrupole 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 range of laser fluences 0.20–2.50 Jrcm2. and the most prominent species were monatomic Zn and O atoms and ions; this study focuses on the energetics of Zn and Znq. At laser fluences below ;0.7 Jrcm2, the Zn atoms had an average kinetic energy that depended logarithmically on the laser fluence, ranging from ;1.2 eV near the desorption threshold of 0.25 Jrcm2 to a maximum of ;4 eV near the ablation threshold at 0.7 Jrcm2. 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 energy of ;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 Jrcm2 fluence range, while the low-energy component decelerated to an average value of ;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. q1998 Elsevier Science B.V.