Low-Energy Ion Beam Analysis

Category: Characterization Tools

Detecting hydrogen is notoriously difficult for most surface analysis techniques. Chemisorbed hydrogen, for example, is invisible to x-ray photoelectron or Auger electron spectroscopies, and only subtly affects low energy electron diffraction patterns. Often changes in the substrate structure (e.g. reconstruction) overwhelm the comparatively smaller effect that hydrogen has on the detected signal. As a result, most of our understanding of the behavior of hydrogen on surfaces has been acquired through indirect methods, including temperature programmed desorption, work-function, and electron energy loss measurements.

A unique angle-resolved ion energy spectrometer (ARIES) instrument is available at Sandia/CA that enables direct detection of hydrogen on surfaces with low energy ion scattering (LEIS), as depicted in Fig. 1. In contrast to electron- and photon-based spectroscopies, element- and isotope-specific detection of hydrogen on surfaces is a unique strength of LEIS. Hydrogen is detected by scattering low energy (typically < 3 keV) He+ from chemisorbed deuterium, as described in Ref. 1. It is also possible to use heavier ions such as Ne+ to detect hydrogen isotopes recoiled from the surface using a technique known as direct recoil spectroscopy (DRS).2 In practice, both LEIS and DRS can be performed simultaneously using an electrostatic analyzer to measure scattered and recoiled ions energies. Both techniques are amenable to technical surfaces, such as pressed powders, and thin films, but samples must be compatible with the UHV environment of the chamber (i.e., materials that outgas may not be useable). Structural information (e.g. the hydrogen binding configuration) can also be measured for ordered surfaces. LEIS offers high surface specificity (typically only the first two atomic layers are probed) due to the limited range of the ions within the material and neutralization effects.

The LEIS/DRS instrument at Sandia/CA includes a clean transfer system that enables materials to be prepared in a glove box and mounted within our analysis chamber without air exposure. The instrument is also equipped with an atomic hydrogen doser, consisting of a tungsten capillary heated by electron bombardment. One can use this doser to compare, for example, atomic (H) and molecular hydrogen (H2) adsorption. The sample stage can be heated to 1000 °C by means of a button-style heater or cooled to -160 °C with liquid nitrogen.

Status: On line and available for use in collaboration with HyMARC.

leisReferences

  1. R. Bastasz, T. E. Felter, W. P. Ellis, Phys. Rev. Lett. 63 (1989) 558.
  2. R. D. Kolasinski, N. C. Bartelt, J. A. Whaley, T. E. Felter, Phys. Rev. B 85 (2012) 115422.