Field induced and surface catalyzed formation of novel ions : A pulsed-laser time-of-flight atom-probe study

Abstract

High electric field induced formation of novel ions such as H+3, AuH+2, RhHe2+, and PtHe2+2 on metal surfaces has been studied in the pulsed-laser time-of-flight atom-probe field ion microscope. From the fractional abundances, and the high resolution mass spectra and energy distributions measured for these ions, several conclusions can be drawn. Field desorption of hydrogen below the evaporation field of the substrate often results in formation of H+3 ions. This formation depends not only on the applied field strength, but surprisingly also on the material and the atomic structure of the substrate. Plenty of H+3 ions can be found from the high index planes and the lattice steps of Mo, W, and Au surfaces in the field range of 2 to 3 V/Å. Few are found from Ir and Ni surfaces, and none are found from the densely packed W(110) plane. Formation of H+3 can therefore be considered a surface catalyzed and field induced chemical reaction. Using H2–D2 mixed gases, we find little correlation between the ionic species obseved in field desorption and the chemisorption states of the gases. The field desorbed ions are formed directly from the field adsorbed state. Field adsorption occurs mostly in the diatomic molecular form. On highly protruded atomic sites of some materials, field adsorption occurs also partly in the triatomic molecular form. Field evaporation of metals in hydrogen or helium often results in the formation of metal hydride and metal helide ions. These ions are formed right at the metal surface by a polarization binding. A fraction of them dissociates in the high field region near the surface. The ‘‘dissociation zone’’ is found to be only several Å in width. From this width, the average lifetime of these complex ions in a field of 3 to 4.4 V/Å is estimated to be on the order of 5×10−13 s.